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& T I I H.
PHILOSOPHICAL LIBRARY
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PROFESSOR GEORGE S. MORRIS, :
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Presented to the University of Michigan.
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SPACE AND WISION.

77Tºl
SPACE AND VISION:
AN ATTEMPT TO DEDUCE ALL OUR KNOWLEDGE OF SPACE FROM THE
SENSE OF SIGHT,
WITH
A NOTE ON THE ASSOCIATION PSYCHOLOGY.
wº º
W. H. S. MONCK, M.A.
Author of “An Examination of Cousin's Psychology;”
“An Essay on The Christian Miracles,” &c.
DUBLIN :
WILLIAM McGEE, 18 NASS Au STREET,
coLLEGE, scHool, AND MEDICAL BOOKSELLER.
1872.
P. R. E. F. A C E .
THE theory advocated in the following pages first occurred
to me about ten years ago, when reading the counter-theory
of Brown. Its leading features were then communicated
by me to one or two friends, who are still connected with
the University of Dublin. The subsequent publication of
Professor Abbott's work on Sight and Touch, led me to
entertain my own opinions more strongly, though it will be
seen that in some important respects my conclusions differ
from his. The reader will see that I have borrowed several
of his facts and arguments in refuting the common theory;
but I think he will also recognise something of my own.
As a repository of facts, indeed, Mr. Abbott’s book will be
indispensable to any student of the subject, however widely
he may differ from the author in his explanation of them;
and I venture to think that had these facts been studied
with equal care by Messrs. Mill and Bain, they might have
been led to cancel a good many pages of their exposition.
At all events, the question has now reached a stage which
renders a minute examination of the actual phenomena
absolutely necessary. There are two or more theories
before the world which sufficiently explain the broad facts
of the case. It is by the accounts they render of the less
obvious and striking phenomena that such theories must be
tested. It is not in ordinary reflexion and refraction that
we need expect to find an eaſperimentum crucis between the
ii.
Emission and Wave Theories of Light, nor in a thunder-
storm that we need look for a test of the Single and
Double-fluid hypotheses in Electricity. The same observa-
tion will apply to theories in Psychology; and the writer
who confines himself to the explanation of mere generali-
ties will never establish his theory so long as there is another
in the field. I should, therefore, have desired to make my
own explanations more special than they are; but finding I
could not devote enough time to the subject to enable me to
reconsider and rewrite it completely, I have contented myself
with making a few corrections and insertions in a manuscript
that has been lying by me for some years. The question is one
with which Trinity College, Dublin, is peculiarly identified,
through Bishop Berkeley and Professor Abbott; and I
trust she will not suffer it silently to fall into the hands of
inquirers of other nations. Berkeley’s theory of vision led
directly to his now-celebrated Idealism; and if there is any
mode of escaping from that subtle doctrine, I believe it must
be discovered in the same field.
There is a current doctrine on this topic which is so
closely related to the subject of this Essay, that I may be
excused for touching briefly on it here, viz.: That tactual
sensations and resistance are the great tests of material
existence and reality. I find this doctrine maintained by
two thinkers of such opposite schools as Messrs. Mill and
Mansel. It seems to have originated in a confusion between
vulgar and the philosophical conception of matter. The
vulgar include in their idea cohesion of the particles, and
can hardly be persuaded to regard liquids and gases as
material, until they see them enclosed in some solid body
which gives them a kind of artificial coherence. Philosophers
took up the same idea with little examination, and thought
the Newtonian theories of gravitation and inertia afforded a
iii.
strong confirmation of it. But if by matter we mean the
Real in Space—that which exists therein and affects our
senses—it is now certain that a great part of the material
universe consists of imponderable and penetrable fluids,
which would probably offer no resistance to the finest
sensitive or muscular apparatus. This I take to be the
proper philosophical idea of matter, and, in this sense of the
term, sight is quite as competent to reveal to us the existence
of matter as touch or the motor nerves. In this respect I
think Kant, whose tests of material substance are permanence
and action, is considerably in advance of more recent writers.
Visible unresisting objects are no doubt often regarded as
unreal; but this is not the case when they continue to exist
permanently, and affect our senses of hearing or smell:
otherwise smoke would appear as unreal as a ghost. How-
ever, if anyone desires to confine the term “matter” to
tangible bodies, he is at liberty to do so. I only contend
that the eye reveals to us one spacial reality, namely, the
nervous organism connected with it, and that we can thence
reach other spacial realities outside us by a legitimate in-
ference. If so, whether the existence of matter is proved
or no, Berkeley's Idealism is refuted.
I hope the concluding note on Idealistic Associationism
will not be considered out of place.
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SPACE AND VISION.
For some years past the leading problems of Psychology
have been tending towards a single point—our idea of space
and the judgments relating to it. Is the idea a priori or
a posteriori 2–if a posteriori, is it derived from sense directly,
or by a slow process of combination and association ?—if
directly from sense, is it from sight or touch, or both 2–are
samples of one class of these problems. Are our spacial
judgments necessary or contingent P – are they synthetical
or analytical?—and if necessary, is this necessity original
or derived 2–are samples of another. Is the Associationist
asked to produce an extreme instance of the potency of his
favourite principle P He appeals, without hesitation, to the
space-perceptions of sight. Is the Intellectualist asked to
produce an extreme case of an original and irreversible
necessity of thought 2 He is almost certain to appeal to the
truths of Geometry. The battle-ground on which the two
great psychological schools must meet and contend is thus
clearly marked out; it becomes manifest that a careful
investigation of the phenomena of space and vision must
throw a flood of light over the whole range of the mental
sciences; and he who directs the attention of earnest meta-
physical inquirers to this central question may have cause
to congratulate himself on having done so, though every
portion of his own building should ultimately be uprooted.
I shall for the present set aside the consideration of the
a priori origin of the idea of space. Such an assumption is
only admissible if it be impossible to derive that idea, with
10 WARIATIONS IN THE BERKELEIAN THEORY.
all its actual characteristics, from sense, or from sense com-
bined and associated with other elements; and even if the
a priori origin of the idea were granted, it is not easy to see
how by means of it we could arrive at a knowledge of the
particular figures, magnitudes, and situations of particular
objects.” Turning, therefore, to the sensitive perception of
space or extension, our attention will be, in the first place,
directed to Berkeley's theory of vision. Let me remark,
however, at the outset, that leaving out of account Mr.
Bailey, Professor Abbott, and other avowed opponents of
this theory, its professed defenders are by no means at one.
This will become evident, if we recount briefly the various
ideas which are popularly ascribed to sight, and observe the
manner in which this rough catalogue would have been
retrenched by different disciples of Berkeley. The ideas
commonly referred to sight are—
1. Colour.
2. Two dimensions of space—plane superficial extension.
3. Three dimensions of space—solid figures and relief.
4. Direction from the eye.
5. Distance, and especially distance from the eye.
All Berkeleians (and indeed all philosophers) admit that
the perception of colour is original to the sense of sight;
and all Berkeleians deny that distance from the eye is so.
But here their unity is at an end. Berkeley himself seems
doubtful whether anything beside colour is originally per-
ceived, and this doubt becomes a distinct negative in the
hands of Condillac, Brown, Bain, and J. S. Mill in his
Examination of Hamilton. (In his criticism of Mr. Bailey,
Mr. Mill's doctrine is different.) Most Berkeleians, however,
admit the originality of our visual perception of two dimen-
* This difficulty affords a satisfactory reason for Sir William Hamilton's
supplementing his a priori origin of space by an a posterior, one, which
has been so often stigmatized as a flagrant violation of his own law of
Pºngy.
:
;
VARIATIONS IN THE BERKELEIAN THEORY. 11
sions of space; among whom I may particularly mention
D'Alembert and Stewart. Many of these philosophers,
indeed, virtually admit an immediate perception of trinal
space, though, in consequence of confounding superficial
with plane extension, they express themselves erroneously.
Such, for instance, must have been the doctrine of Reid ;
for he maintains that persons who had no sense but that
of sight, would take all objects to be portions of spherical
surfaces. Sir William Hamilton and others have added
direction from the eye and even outness, perceptions which
clearly involve trinal space. Thus, while Condillac and
Brown admit but one of the five present perceptions of
sight to be original, Sir William Hamilton accepts four;
and yet he, no less than the others, professes an almost
unreserved acceptance of the Berkeleian theory.
If we inquire of these theorists, how these original defi-
ciencies of sight are made good, we shall find an almost
equal diversity of opinion. Time, perhaps, enters as an
element into every solution, (even that of Berkeley him-
self,) but at this point the agreement ceases. One says,
simple touch ; another, muscular sensibility; a third, the
active faculty of locomotion; and some, (as Dean Mansel,
and perhaps Sir W. Hamilton,) even bring in a supposed
a priori idea of space. Others combine two or more of
these in various proportions. There is, moreover, another
solution of the problem open to all who hold that colour is
not the only direct perception of sight, viz., that this sense
supplies its own deficiencies, by forming associations be-
tween different visual perceptions—associating, for example,
the faintness and indistinctness of a seen object with the
idea of a great number of intervening visible objects, which,
though at present overlooked, would become visible before
the object in question attained its maximum degree of
brightness and distinctness. That such associations might
be formed by experience, and even that they actually are
formed, few persons will probably feel disposed to deny ;
12 WE PERCEIVE EXTENSION BY SIGHT.
but I do not know whether a theorist who explained by
their means the whole difference between our original and
our present visual space-cognitions would be regarded as a
Berkeleian.” But without taking account of this doctrine,
it is evident that there is a very great difference of opinion
among professed followers of Berkeley, both as regards the
direct perceptions of sight and the sources of our present
improved conceptions; and consequently, even if the argu-
ments of Mr. Bailey and Professor Abbott should be totally
discarded, there remains abundant reason for a new and
thorough investigation of the entire problem. There is, in
truth, no theory of vision which has met with exclusive
or even general acceptance; but, on the contrary, there
have been (and still are) a number of distinct theories
advocated by different persons, and agreeing only in one, or
at most in two points. If these differences have been little
noticed hitherto, it is a further proof that the subject has
not been studied with sufficient care. No one can seriously
contend that the theory of Brown, who held that sight
does not give us any idea of extension at all, is identical
with that of Stewart, who contends that if the mechanism
of vision were explained to a blind man, he could infer for
himself the necessity of our (direct) perception of visible
figure. (Dissertation, note, p. 134, Hamilton's Ed.)
Is sight, then, immediately perceptive of extension ? This
is the first important question which meets the inquirer, and
his answer to it will possibly determine his solution of the en-
tire problem. The prima facie case is certainly in favour of
the affirmative. The great majority of mankind believe that
they perceive extension by sight, and even chiefly by means
of that sense. The analogy of the lower animals tends to the
same conclusion. Their organs of touch and powers of loco-
motion are endlessly varied, but the vast majority of them
* He might however claim an earlier, and I think a higher, authority
in his favour in Locke. That philosopher evidently holds that sight
supplies its own deficiencies. Essay, book ii., cap. ix. 9.
SIR WILLIAM HAMILTON's ARGUMENT. 13
possess two eyes situated similarly to those of the human
subject. They all appear to possess a distinct knowledge of
extension by sight, and exhibit it at a period so early as to
lead even eminent Berkeleians to doubt whether their space-
perceptions could have been formed by the complicated pro-
cess of association which they insisted on in the case of man.
The intimate association of colour and extension—or rather, to
express the facts accurately, the belief of ordinary men that
the extension which they see is coloured, and the colour
which they see extended—is further evidence on the same
side ; and other indications might easily be added. The
philosopher who maintains that we have a direct perception
of extension by sight, is therefore in possession of the field,
and his adversary must dislodge him before erecting his own
superstructure. 13ut the Visionist, if I may use the term,
need not confine himself to the defensive. He can appeal
to positive and powerful arguments in favour of his thesis.
The first and strongest of these has been urged by D'Alem-
bert and Stewart, but is, perhaps, best stated in the following
passage of Sir W. Hamilton. (Lectures, ii. p. 165:—)
“It is admitted by all that we have by sight a perception of colours,
consequently a perception of the difference of colours. But a perception
of the distinction of colours necessarily involves the perception of a dis-
criminating line; for if one colour be laid beside or upon another, we can
only distinguish them as different by perceiving that they limit each other,
which limitation necessarily affords a breadthless line—a line of demarca-
tion. One colour laidi upon another, in fact, gives a line returning upon
itself, that is a figure. Ibut a line and a figure are modifications of
extension. The perception of extension, therefore, is necessarily given
in the perception of colours.”
Mr. Mill, who is one of the ablest advocates of the contrary
doctrine, deals with this argument in his “ Examination of
* It must not be supposed, however, that we are conscious of visual
extension only when and where the colours are different. If I look, for
example, at a green square depicted on white paper, I see the length not
only of the sides which are lines of demarcation, but of the diagonal which
lies wholly within the green square.
14 MILL's REPLY TO HAMILTON.
Sir William Hamilton's Philosophy.” His reply is too long
to quote in extenso, but I think the purport of it is fairly
given in the following extracts:—
“First, it must be observed that when the eye at present takes cogni-
zance of a visible figure, it does not cognise it by means of colour alone,
but by means of all those motions and modifications of the muscles con-
nected with the eye, which have so great a share in giving us our acquired
perceptions of sight. To determine what can be cognised by sight alone
we must suppose an eye incapable of these changes. . . . Now there is
nothing more certain than that an eye with its axis immovably fixed in
one direction gives a full and clear vision of but a small portion of space,
that to which the axis directly points, and only a faint and indistinct one
of the other points surrounding it. . . This fact seems to warrant the
conclusion, that if the axis of the eye were immovable, and we were
without the muscular sensations which accompany and guide its move-
ments, the impression we would have of a boundary between two colours
would be so vague and indistinct as to be merely rudimentary. A rudi-
mentary conception must be allowed. . . . But to confer on these
discriminative impressions the name which denotes our matured and
perfected cognition of extension, or even to assume that they have in their
nature anything in common with it, seems to be going beyond the evidence.
. . . I cannot admit that we could have what is meant by a perception
of superficial space, unless we conceived it as something which the hand
could be moved across.” (Examination of Hamilton, pp. 285-7, 3rd Ed.)
He goes on to adopt Mr. Bain's statement, that “we
never consider that we have a notion of space unless we
distinctly recognise this possibility;” (of muscular motion;)
“but how a vision of the eye can reveal beforehand what would
be the experience of the hand or the other moving members I am
unable to understand.” Further, he quotes with approbation
the following remarks from a note of Mr. Bain's :—
“The essential import of visible form is something not attainable with-
out the experience of moving the eye. If we looked at a little round spot,
we should know an optical difference between it and a triangular spot, . . .
but this would not be to recognise form, because by form we never mean
so little as a mere change of colour. S. . . We can never cross the chasm
which separates an optical meaning from an effect combining light and
movement in any other way than by bringing in experience of movement.”
(Examination of Hamilton, p. 291, and Bain's Senses and Intellect,
pp. 366-7, 2nd edition.)
MILL's ARGUMENTS EXAMINED. 15
This is all the argument I can find, if I except a statement
of Mr. Bain’s that “our notions of form are manifestly ob-
tained by working on the large scale;” not on the small
space to which the field of distinct vision is confined : an
assertion which needs no refutation, as it rests solely on Mr.
Bain's ipse divit, and appears to be opposed to all evidence.*
We may note then, as Mr. Mill's first argument, that
the perception of diversities of colouring (at least to the
extent requisite to give a clear idea of a bounding line or a
figure) requires muscular motion as well as simple vision.
This argument, however, assumes that the only way in
which muscular motion contributes to the clearness and dis-
tinctness of our perception of space, is by becoming associated
with our ideas of colour and its varieties. But it is evident,
from Mr. Mill's own statement, that the muscular motions
and adaptations of the eye contribute to the perfection of
our visual perceptions in a very different way. The proper
adjustment of the eye, and convergence of the axes, are
* I have already intimated that Mr. Mill advocates a very different
doctrine in his criticism of Mr. Bailey. Indeed the following passage
might almost be taken for a statement of my own theory:—“We cannot
see anything which is not painted on our retina, and we see things like or
unlike according as they are painted on the retina like or unlike. The
distance between an object to our right and an object to our left is a line
presented sideways, and is therefore painted on our retina as a line. The
distance of an object from us is a line presented endways, and is repre-
sented on the retina by a point. It seems obvious, therefore, that we
must be able by the eye alone to discriminate between unequal distances
of the former kind, but not of the latter. Unequal lines drawn across
our sphere of vision we can see to be unequal, because the lines which
image them on the eye are also unequal. But the distances of objects
from us are represented on our retina in all cases by single points, and
all points being equal, all such distances must appear equal, or rather, we
must be unable to see them in their character of distances at all. This
argument proves that distances from us cannot be seen in the way in
which we see the distances (or rather the apparent distances) of objects
from one another—namely, by the original powers of the sense of sight.”
—(Dissertations, by J. S. Mill, vol. ii. p. 95.)
16 HAMILTON's ARGUMENT WINDICATED.
necessary to see with distinctness the particular object
looked at, and motions of the eye or the head are needed to
bring the surrounding objects into the field of distinct
vision. If these muscular motions contribute to the perfec-
tion of visual perceptions by altering the character of the
visual sensations and perceptions themselves, the demand
that the eye should be deprived of their aid when we are
endeavouring to ascertain the original perceptions of that
sense is manifestly unreasonable; and no one with the small-
est knowledge of optics can deny that these motions affect
our visual perceptions directly, as well as through the
channel of association. Put the truth is, that by sight we
have a perception of diversity of colouring quite sufficient
for the purpose of Sir W. Hamilton's argument, without in-
troducing these muscular motions at all; and the two philo-
sophers with whom we are arguing seem to have become
indistinctly aware of this, when they admit an “optical
difference,” “discriminative impressions,” a “rudimentary
conception,” &c. The fact is proved by superabundant
evidence. Whenever sensations of vision are produced
otherwise than by the action of light reflected from external
objects, we are manifestly deprived of all these aids; and it
is a well-known fact that these sensations may be produced
by electricity, by a blow, by derangement of the stomach,
by inflammation of the optic nerve, and other causes.”
Will Mr. Mill contend that in such cases there is either no
perception of extension or one dependent on the accompani-
ment of muscular feelings? This, however, is not all. In
cases of couching for cataract adjustment and convergence
are completely eliminated from our visual perceptions; and
yet, as Mr. Abbott has clearly established, there are percep-
* I believe, as I shall have occasion to notice hereafter, that if philo-
sophers would observe with care their ocular sensations when in the
dark, or with their eyes shut, they would find that they are never with-
out some perception of colour and visual extension. These perceptions,
however, are often very faint.
HAMILTON's ARGUMENT WINDICATEp. 17
tions of visible extension from the first. If the blind from
cataract have any knowledge of coloured extension, (and a
knowledge of coloured extension is a perfectly distinct
thing from a knowledge of the form and situation of ea-
ternal objects,) they must likewise have it from vision alone.
There is another experiment still more decisive. We can
distinctly see the extension and even the figure of objects
when illuminated by the electric spark or a flash of light-
ning; but it admits of demonstration that no muscular
motion could take place during the time that this illumina-
tion lasts. I would add to this another fact, of which
Mr. Mill is, doubtless, aware—the persistence of visual im-
pressions. If Mr. Mill or Mr. Bain will gaze steadily on a
brilliant object of any kind, and then close his eyes, he will
See a similar object still, which, however, usually changes
colour several times before its final disappearance. What
has motion of any kind to do with this perception ? The
argument of Mr. Mill is, therefore, inconsistent with a
number of known facts. And the reason is obvious. Small
as the portion of an object is, which can be distinctly seen
without muscular motion of any kind, it contains hundreds,
or probably thousands, of sensitive minima, each of which is
capable of being distinctly seen. The extreme minuteness
of the minimum visibile when the eye is in good condition,
and the arrangement of brilliancy and contrast of colouring
favourable, is truly marvellous. The fixed stars, perhaps,
afford the most remarkable example; and the effect of con-
trast is finely illustrated by the fact that they are rendered
invisible in the day-time by an increased quantity of light—
increased even in their own particular direction;” while, if
* I may here notice the prevalent mistake that the minimum visibile
is a quantity of extension. It depends much more on the brightness of
the object, and its contrast to the surrounding objects in brilliancy and
colour, than on its magnitude, whether real or angular. It also depends
on the state of the organ whose sensibility may be augmented as well as
diminished by nervous excitement or other disorders.
18 MILL's EVASION OF THE QUESTION.
we cut off the light laterally diffused by looking up a tall
chimney, or from the bottom of a deep well, the stars be-
come again visible, even at noon. The fibres of a gos-
samer's web are often visible at a distance of several feet.
Mr. Mill seems to have some perception of the strength
of his opponent's case here, and, accordingly, he takes
refuge in admitting a “ rudimentary conception,” or certain
“ discriminative impressions.” Discriminative of what?
Do these impressions merely discriminate colours, or do
they discriminate figures and forms also P. If Mr. Mill
accepts the first alternative, the admission removes no part
of the difficulty. Sir W. Hamilton says that a discrimina-
tion of colours necessarily involves a discrimination of lines
and figures; and Mr. Mill replies, I admit the justice of
the argument to this extent only, that we do, as a matter of
fact, discriminate colours by unaided vision. The futility
of this reply is obvious. But if Mr. Mill concedes that
these impressions discriminate figures and forms he con-
cedes the whole question at issue. From the whole tone of
the passage, it would appear as if Mr. Mill desired to admit
more than a discrimination of colours and less than a dis-
crimination of figures and forms. But what is the third
idea intermediate between colour and visible figure which is
thus referred to sight? Has any one ever experienced it P
Has it got a name? Or is there any evidence whatever of
its existence P Moreover, if Mr. Mill means to concede the
existence of this hitherto unobserved tertium quid, Mr. Bain
does not. He seems expressly to identify “change of
colour” with all observable “optical differences” in objects.
The expression, “change of colour,” I may remark, in the
first place, is inaccurate. There is no change—no succes-
sion—of colours in our perception of variety, but the two
colours are perceived at the same time as distinct from and
limited by each other. Waiving this objection, when Mr.
Bain identifies “change of colour” with “optical differ-
ence,” he assumes that diversity of colour exists by itself
HIS ARGUMENT AGAINST WISUAL EXTENSION. 19
alone, and has no necessary connection with visible figure and
form. He forgets that his antagonist has advanced forcible
argument to prove that such a necessary connection exists and
takes the very unphilosophical liberty of assuming the con-
trary, and leaving the argument in question unanswered.
The second part of Messrs. Mill and Bain's reply appears
to me to consist in defining space so as to include muscular
motion in the definition, and then arguing that because
mere sight cannot give us the idea of muscular motion, it
cannot give us any direct perception of space. The question
then would appear to turn on the correctness of this de-
finition; but it must be remarked, that even were it correct,
it would not follow that sight could not give us all the
other elements of the idea of space, or that these other
elements do not constitute the greater and more characteristic
part of it; and if this were granted I believe Sir William
Hamilton would have gained everything worth contending
for. Mr. Mill maintains that nothing can be properly de-
signated space unless we conceive our hand capable of being
moved across it; and although Mr. Bain is more liberal and
allows that a motion of some other part of our bodies might
answer equally well, he too includes muscular motion of
some kind in his definition. I think I can here fairly appeal
to the reader whether he is conscious of any such element
in the idea which he attaches to the term P requesting him
to bear in mind that the question is not, whether the idea
of motion or mobility is intimately associated with that of
space, (which from the nature of the case it must be,) but
whether the former idea forms a constituent part of the
latter? But even supposing this question decided against
me, my general thesis is by no means overthrown. We have
a visual idea of motion,” and it is very doubtful if we have any
other. It will, I think, appear highly probable from what
will be said hereafter, that a man destitute of the sense of
* We see two objects approaching each other without any change in
our own position. This, I think, may be fairly called visible motion. It
is quite distinct from muscular motion, (or rather muscular action,) which
can only be perceived in ourselves and not in external objects.
20 NATURE OF OUR PRESENT IDEA OF SPACE.
sight would perceive his voluntary motions only in their
character of voluntary efforts or muscular actions. At all
events, the idea of motion derived from touch or the muscular
sense is not the only one we possess; and supposing motion
or mobility to be a necessary ingredient in our idea of space,
the question arises whether this motion be not visible motion?
And here I appeal to the consciousness of my readers with
much greater confidence; for in my mind at least it is
visible motion which is intimately associated with, or forms
an element of my idea of space. And I think I can appeal
to Mr. Mill himself in corroboration of this truth; for
he unequivocally admits that our present pictures of spaces
and extensions are exclusively “eye-pictures.” “The visual
imagery,” he says, “effaces from our minds any distinct
consciousness of the series of muscular sensations, of which
it has become representative. The simultaneous visual
sensations are to us symbols of tactual and muscular ones,
which were slowly successive;” (Examination of Hamilton,
p. 283;) and he goes on to adopt a paragraph from Mr.
Herbert Spencer, in which the latter says that “these
symbols occupy the mind to the entire exclusion of the ideas
symbolised.” (Ibid.) It is thus conceded that if there is
any representation of motion in this picture of space,
it must be a representation of visible motion; and in-
deed there is a further concession here made that when
we employ spacial terms the sole object of consciousness is
generally, if not always, a collection of visual sensations.
Messrs. Mill and Spencer, indeed, speak of these visual
sensations as merely the symbols of other ideas, seldom or
never realised, but which nevertheless constitute our true no-
tion of space. Now, in the first place, it appears to me that
the idea which every one has in his mind when he employs
spacial terms, is the true idea of space:” and in the second
* Mr. Mill, after observing that the blind man's idea of space is some-
times wanting in the element of the simultaneity of the parts, proceeds to
confess that “what is thus wanting to him is the chief feature of the
conception as it is in us.” (Exam. of Hamilton, p. 278.) If anything is
essential to a conception one would imagine the “chief feature” of it is so.
REFUTATION OF MILL AND BAIN. 21
place these visual ideas are themselves intuitive cognitions,
and if the philosophical phraseology of Messrs. Mill, Bain,
and Spencer will not permit us to apply our ordinary space-
terms to them, their existence and importance ought to be
admitted, and they ought to receive proper designations of
their own. I may add, that in the idea of space described
by Messrs. Mill and Bain, an element which I believe most
persons will think much more important than that of
mobility, namely, the simultaneous existence of its parts is
suffered almost entirely to evaporate. As regards the
prophetic power supposed to be ascribed to the sense of sight
by the Visionist, it has, I think, never been claimed by any
advocate of that theory; unless it merely means that nothing
can change its situation from one place to another at a distance
from it without passing through an uninterrupted series of
intervening positions—a proposition whose self-evidence
will, I think, be generally admitted.
When Mr. Bain says that the “essential import” of visible
form is something not attainable without experience of
motion, he overlooks the fact that the principal use of a
thing is not the thing itself. The principal use of the sense
of hearing is to assist us in holding communication with our
fellow-creatures; but no one would on this account propose
to limit the meaning of the word “sound” to articulate speech,
and deny that the “essential import” of the perceptions
of the ear could be known to one who had never heard men
speak, but who had heard other noises. Besides, if the
guidance of muscular motion is one of the chief uses of our
visual space-perceptions, it is by no means the only one.
In reading a book, for example—and few uses of sight are
more important than this—muscular motion plays a very
subordinate part, and much the greater portion of the employ-
ment of the sense consists in the simple perception of visible
figure. I am therefore unable to see any ground on which
Messrs. Mill and Bain's objection to Hamilton's argument
can be sustained.
22 PROOF THAT EXTENSION IS WISIBLE.
This, however, is not the only argument in favour of the
perception of extension by sight. Let us suppose sight inca-
pable of presenting to us anything besides colour, and inquire
what means we have for ascertaining the extension and figure
of external objects on this supposition? Touch and locomo-
tion; of which modern writers lay most stress on the latter.
But it must be noted, that unless muscular motion be guided
by tactual differences, it can give us no information of the
figure and form of objects. This is evident; for one object
can only be discerned from another by sight or by touch;
and sight, (on our present hypothesis,) presenting diver-
sities of colour only, can afford no guidance to the hand (or
other member) in moving round an object which presents
no tangible peculiarities to distinguish it from those that
surround it. Hence we find a limit to the association of
visual impressions with tactual and muscular ones. If, in
the great majority of the objects which come within the
reach of a child's senses, diversities of colour are attended
with tactual differences, the association will become very
close and intimate; if, on the other hand, most of the
objects present exhibit differences of colour without any
corresponding diversities of tactual sensations, the associa-
tion between these impressions will be very feeble, and
diversities of colour will suggest diversities of form either
very slightly or not at all. Now, let us take a child who
is born in the higher ranks of life, and inquire how the fact
stands in his case? In how many of the objects which are
presented to him are diversities of colouring attended by
any other perceptible diversity ? Almost every article of
clothing which he sees, (including the carpet, the table-
cover, and all similar objects,) the room-paper, pictures,
books, newspapers, and even the looking-glass when he
looks at it, present varieties of colouring where there are no
tangible differences, and where, if sight merely presents us
with colour, there is nothing whatever to guide the hand in
ascertaining the shape. Years roll on. Reared amid these
REFUTATION OF ASSOCIATION THEORY. 23
sights, our child becomes a student, who spends the greater
part of his waking hours in deciphering letters in various
languages, not one of which presents any tangible pecu-
liarity, and yet the very meaning of a word or a passage
often depends on a very slight difference in the shape of a
letter—a difference too minute to distinguish clearly by
touch or locomotion, even if there was anything to be felt
along its outlines; and it may be added, that the clearest
perception of the figure and form of these objects coexists
with the fullest conviction that touch and locomotion would
find no distinguishable characteristics in them. What be-
comes of the Association theory here 2 It would, indeed,
be almost a sufficient refutation of it to say, (as Mr.
Abbott does,) that we do not handle one object out of a
thousand we see, and that consequently we cannot form by
this means an association so intimate that figure is always
and necessarily suggested to us by diversity of colour.
When we add, that objects too minute to be distinctly felt
nevertheless give us the clearest impressions of their figure
and magnitude, the argument is much strengthened; but
when to this we can still add, that objects large enough to
exercise our tactual and muscular senses, and on which we
do exercise them, do not present to these senses any dis-
tinguishable trait whatever; and that, notwithstanding this,
we recognise their figure by sight with a clearness and
accuracy to which unaided touch and locomotion never
approximate—and this not merely now and then, but at
every instant that the diversities of colour are presented to
us—I think we have a second and hardly less powerful
argument in favour of the direct perception of extension by
sight.
These two arguments are, I think, sufficient, and if they
were not they might be added to.” But what is it that
* It may not, perhaps, be amiss to state an argument virtually the
same with Sir W. Hamilton's in a different form. If colour be a mere
sensation, presentations of a single colour can differ only in intensity,
24 EXTERNAL OBJECTS NOT IMMEDIATELY SEEN.
sight informs us of the extension of P Does it give us (as
the vulgar seem to think) the real magnitude and figure of
external objects? Or, does it give us their projection on a
plane? Or, does it give us their projection on the sensitive
surface of the eye—the well-known picture on the retina P
There is, I think, abundant proof that we do not see the
real external objects. Every illusion of sight is evidence of
the fact; and these illusions are of such a character as,
under certain circumstances, to reverse every spacial rela-
tion of these objects. Reflexion, refraction, and aberration
alter the apparent direction of external objects from us, and
so does the time-propagation of light. The latter in parti-
cular is worth attending to, as in the case of a fixed star
the real direction of the object may be directly the reverse
of that in which it is seen, and the star may even have
ceased to exist thousands of years ago, and is nevertheless
still perceived. The shape of the external object is altered by
refraction and perspective. It may be converted into two
or more objects by using a multiplying glass, by a proper
direction of the axes of the eyes, or by placing it close to
the eye and looking at it through a number of pin-holes.
Paintings and the stereoscope afford sufficient evidence that
and presentations of two simultaneous colours can differ only in the
absolute and relative intensity of the two. Hence, when we see two
colours at once, (red and blue suppose,) the impression received de-
pends only on the quantity of each kind of light that falls on the retina,
and the sensitiveness of the place on which it falls; and consequently
if we take care to make the resultant or joint effect of these two factors
constant, we may vary the distribution of the two colours in any way
we please without altering the resulting sensation. This is contrary to
the fact. Hence we conclude that there is a perceptible difference in
our sensations of colours quite distinct from their absolute or relative
intensity; and as this is not a difference in intensive quantity, those
who deny it to be a difference in extensive quantity (or rather in spacial
relations) are bound to tell us what it is. To call it a “discriminative
impression ” is to throw no light on its character. The real question is,
what does it (originally) discriminate 2
MR. ABBOTT's THEORY CRITICIZED. * 25
the dimensions of an object can be altered; and every one
who has looked through both ends of a telescope knows that
an object can easily be made to appear farther off or nearer.
This latter effect can also be produced by looking at any
uniformly-coloured objects, such as clouds or water, through
a long narrow tube. Objects, moreover, continue to be
seen after the light from them has ceased to impinge upon
the eye, and visual extension is often perceived when no
objects are present, and even when the eye is closed. The
variations in the apparent magnitude of external objects
produced by distance and other circumstances, are known to
every one. If it be objected, that in some of these cases the
medium is artificially modified by glasses, &c., the reply is
that what we see is not the sum of the objects and the
glasses, but an illusive representation of the objects. Ex-
ternal objects, therefore, cannot be immediately perceived
by sight unless we deny the veracity of direct consciousness.
A doctrine not very dissimilar to the vulgar error just
refuted is advocated by Mr. Abbott in his able work on
“Sight and Touch.” This author maintains, that distance
from the eye and relief are original perceptions of sight,
dependent on certain visual sensations connected with con-
vergence and adjustment, and so completely determined by
these sensations, that even when the latter occur without the
presence of external objects, the corresponding perceptions
follow. That this is no misrepresentation of the doctrine
alluded to is evident from the fact, that Mr. Abbott quotes
in confirmation of it Funke's assertion that “we place with-
out us in space not only the entoptic appearances in the
open eye, but also the subjective appearances in the closed
eye;” (Sight and Touch, p. 79;) and from his insisting that
if we at first saw objects in contact with (or in) the eye,
“ erect vision as well as single vision is simply inexplicable.”
(Id. p. 78.) Now, I cannot see any difference between an
original law or natural suggestion which leads us into
error in certain cases, (and this not by consequence but
C
26 THEORIES OF PLANE AND SPHERICAL PERCEPTION.
directly,) and a consciousness whose direct deliverance is
sometimes untrue; and, consequently, if Philosophy should
ever adopt Mr. Abbott’s theory that a subjective appearance
in the closed eye is immediately perceived as external, (and
not judged to be so in consequence of early association,) she
will thereby commit suicide. But I do not think that the
facts to which Mr. Abbott appeals justify such a conclu-
sion. They will be considered hereafter.
Nor can I see any valid reason for believing that objects
are originally seen as if projected upon a plane. The only
reason to think so is that the appearance of solidity can be
imitated by a plane figure, as in a picture or the stereoscope;
but I think there is no doubt that a figure of three dimen-
sions could be equally made to represent a plane, as indeed
actually occurs in the case of the sun and moon. Neither
the external object nor the sensitive surface of the retina are
planes, and consequently the supposed plane of projection
does not exist anywhere. How then are we to reconcile the
theory of plane-perception with the veracity of consciousness?
The chief source of this doctrine seems to be a confusion of
plane with superficial extension. Philosophers, however,
have by no means uniformly adopted it. Reid, in his chapter
on the Geometry of Visibles, maintains that in immediate
vision all objects appear as portions of spheres, and proceeds
to show how completely Geometry would be metamorphosed
in the case of persons possessing the sense of sight only. In
fact, if we prolong to infinity the pencil of rays, proceeding
from any distant object to the eye, and make a section of it
at a sufficient distance from the eye to render the effects of
convergence and adjustment insensible, it is evident that this
section may be substituted for the real source of light without
any change in the resulting visual impression; but as any
one of an infinite number of sections will produce the same
effect, we cannot say that this effect is that of a plane or of a
spherical section rather than that of any other we choose to
name. Mr. Abbott, too, mentions facts which seem to me
NO PERCEPTION OF DISTANCE FROM THE EYE. 27
quite inconsistent with the plane-theory, as I shall have
occasion to notice hereafter. I may further observe, that
Reid's doctrine, by apparently denying the necessity of
geometrical truths, proves so attractive to Mr. Mill that he
accepts it completely, (Examination of Hamilton, p. 87,) for-
getting that he is about to advocate a doctrine which confines
the sense of sight to the perception of colour, and of course
excludes it from perceiving either plane or spherical ex-
tension.
This is, perhaps, the best place to discuss the question,
whether sight is or is not originally perceptive of distance,
or rather of distance from the eye. Against this doctrine the
chief arguments of Berkeley were directed, and the passage
already quoted from Mr. Mill's criticism on Mr. Bailey
states one of them in a clear and precise form. The argu-
ment in question rests on the facts that lateral distance is
represented on the retina, and that longitudinal distance, or
distance from the eye, is not ; while in all other respects the
perception is known to correspond with the picture on the
retina. This argument seems to me a powerful one, but
Berkeley could not state it with sufficient clearness, because
he evidently hesitated about allowing to the eye any percep-
tion of lateral distance; while Mr. Mill, by his later adoption
of the views of Brown and Bain, precludes himself from em-
ploying it at all. There are, however, many other reasons
for denying this perception of distance from the eye. Dis-
tance is always measured between two things, and hence to
see distance from the eye, we must see both the eye and the
object. If I do not know where my eye is, I cannot know
how far a visible object is distant from it; and if I do not
see the external object, I cannot know how far it is from my
eye. Now, I have already adduced reasons for holding that
the immediate object of sight is not anything external to us;
but in addition to this, those who hold that we see the exter-
nal objects themselves, are positive that we do not see the eye.
This statement is clearly made by Mr. Abbott:—“Sight
28 NO PERCEPTION OF DISTANCE FROM THE EYE.
can plainly give only a vague and negative notion of the
place of the seeing organ.” (Sight and Touch, p. 77.) Nor
can touch and locomotion remedy this defect; for, “ in
the first place, it is only by rare accident that we touch the
eye-ball ; the sensitive organ which is not external cannot
be touched at all; and, secondly, touching either the eye-lid
or the eye produces no visual sensation. The hand, then,
cannot by contact produce vision, but it can when not in
contact, interrupt the sensation;” and, consequently, “touch
cannot discover to us the place of the eye, as it does that
of the tactile surface of the body—namely, by compelling us
to associate a certain part as felt with a certain organ, as
feeling.” (Ibid.) The organ, he adds, moreover, does not,
properly speaking, feel at all; there is no consciousness of
the locality of the sensorial affection; of which we have a
“demonstrative proof” in the fact that, “notwithstanding all
our experience, we cannot with the utmost attention distin-
guish the impressions on one retina from those on the other.”
(Id. p. 78.)* Mr. Abbott seems quite unconscious of the
havoc which these admissions make in his own system.
Indeed he goes on to maintain that this perception of distance
from the eye takes place even when the object is in the eye,
nay, actually on the retina; for in proof of his doctrine he
quotes Funke, who says that subjective appearances in the
closed eye naturally appear to be in space without us, “ and
this to such a degree, that even when the subjective image of
a flame is observed in the closed eye we involuntarily form
a judgment of its distance.” (Sight and Touch, p. 79.)
But neither his denial that we have any perception of the
* On the same rock the doctrine of Sir W. Hamilton, that we are
directly conscious of the outness and direction of the objects of vision, is
shipwrecked. To know that a thing is in a particular direction, I must
know the position of the thing from which this direction is measured, and
to know that A is outside B, I must know that B is outside A. If I do
not perceive B, I cannot perceive the direction of A from B, nor even
know that A is outside B at all.
NO PERCEPTION OF DISTANCE FROM THE EYE. 29
position of the eye—from which, nevertheless, we measure
not only directions but distances—nor the consequence that
we see in direct and original vision the distance of the eye
from objects that are in it, seem in the least to disconcert
Mr. Abbott. He endeavours to prove that we have no per-
ception of distance by any other means than vision, which I
concede. He overlooks altogether the fact that vision can
educate itself, though quoting passages from other authors”
which establish this truth; and he then proceeds to deter-
mine the physical antecedents of the perception of distance,
as if there was no doubt that the perception was an original
one. But these antecedents, I think, afford a new argument
against his doctrine.
Mr. Abbott's antecedents are the visual sensations which
(in his opinion) determine the convergence and adjustment
of the eyes, and which he thinks consist in the relative
brightness, diffusion, and multiplicity of the pictures on the
central and lateral parts of the retina, together with the
duplicity and separation of the images in binocular vision.
Now, in the first place, it is evident that these antecedents
consist of a large number of sensations, and the resulting
perception supposes a comparison between them. This is
admitted by Mr. Abbott himself, who concedes the necessity
of a varied lateral field, and gives a good instance of its
necessity in the apparent nearness of the stars, as seen by the
sweeper of a tall chimney. This already looks sufficiently
like association, and unlike direct perception. Butlet us next
observe, that all these sensations vanish as soon as the proper
convergence and adjustment are attained, or rather that they
are replaced by different sensations of the same description,
which must, upon Mr. Abbott's theory, suggest a different
distance. Our perception of distance will therefore be ac-
curate only for the moment that the eye is first turned on the
object, and will cease to be so if we continue to look at it.
But even this is not all: for these sensations of the first
* See Sight and Touch, p. 38.
30 LONGITUDINAL DISTANCE PRESUPPOSES LATERAL.
instant which, according to Mr. Abbott, determine the correct
adjustment and convergence, as well as our perception of
distance from the eye, are themselves not invariable when
the distance is the same, but depend also upon the previous
convergence and adjustment. Nor would Mr. Abbott come
off much better if he had said (which he does not) that the
sensations which follow the proper adjustment and conver-
gence determine our perception of distance; while if he called
on both classes of sensations the perception of distance would
result from observation of a series of successive phenomena,
and therefore could not upon his own showing be an original
Oſle.
Mr. Abbott holds that longitudinal distance is a perception
equally direct and immediate with lateral: yet he makes a
comparison of many sensations a necessary antecedent of the
former. This comparison again is made between the pictures
corresponding to the central and lateral parts of the retina with
respect to their relative brightness, distinctness, &c. Now,
in the first places, to observe the rapidity” of this loss of dis-
tinctness as we proceed outwards from the centre, it is
necessary not only to observe the actual brightness or diffusion
of the various images, but also their lateral distances; and
thus the perception of lateral distance becomes, on Mr.
Abbott's own showing, a necessary antecedent to the percep-
tion of longitudinal. Further, how are we to distinguish
between the central and lateral parts of the retina P. There
is no absolute line of demarcation, and consequently this
distinction itself presupposes the perception of lateral dis-
tance, since by this means alone is the distinction in question
rendered possible. This presupposition of many sensations,
* “Lateral objects,” says Mr. Abbott, “supply a sort of base from
which distances are measured; this base including, as its most important
part, the body itself, whether actually seen or not. In such estimation of
distance, it is clearly of some consequence whether the loss of distinctness
in the parts near the centre of the field is slow or rapid.” (Sight and
Touch, p. 99. See also p. 125.)
IBINOCULAR PERCEPTION OF RELIEF. 31
a comparison between them and a perception of their relative
and absolute lateral distances, is anything but favorable to the
theory of direct perception; but in the case of the binocular
perception of relief, (which is clearly a perception of the same
description with that of distance,) Mr. Abbott is moreover at
pains to point out that the conditioning or antecedent impres-
sions on the two eyes need not be simultaneous—a fact, which
to my mind is perfectly subversive of the theory of direct
perception of distance from the eye.
In arguing that the perceptions of distance and relief are
original, Mr. Abbott lays great stress on binocular vision,
and regards the stereoscope as an experimentum crucis of the
correctness of his theory. (See in particular Sight and Touch,
p. 117.) But it must be recollected that in the stereoscope
the relief is not real but illusory, and that the instrument
is constructed with the object of making the illusion as
striking as possible. In the perception of real relief, the
difference between single and binocular vision appears to be
trifling, and this trifling difference is easily accounted for by
association of visual impressions. In ordinary vision we are
quite unaware how much of the landscape is seen by two eyes
and how much by one eye only—a clear proof that there is
no abrupt change as we pass from the one to the other.
Instances, too, are recorded of men who have lost the sight
of one eye without knowing it, which would be impossible
if this loss involved that of so remarkable a perception as
relief. Relief can be realised to a single eye to a very con-
siderable extent in paintings; and perhaps, if painters were
willing to sacrifice the background entirely, they might
bring the foreground into stronger relief. But we have a
demonstrative proof that this perception of one-eyed relief is
not original in the case of Cheselden's patient, since he did
not perceive it for two months after he was couched. Mr.
Abbott thinks that a certain perception of distance and relief
is original, even in vision with one eye: chiefly because an
object A presents different appearances to the eye when it is
32 PERCEPTION OF RELIEF NOT ORIGINAL.
placed at two different distances, a and b, its direction from
the eye remaining unaltered. And this is true: but it
appears from Mr. Abbott's own plates, (Sight and Touch,
p. 90,) that an object B at the distance a, will produce the
same impression as the object A at the distance b. This
seems to me to show that we can have no immediate
perception of the distance of either object from the eye.
Another fact, too, seems to me to go far towards proving
that our perception of relief is not original—that a figure can
be seen at pleasure either as of two or of three dimensions, and
that a plane figure can be made to appear as of three dimen-
sions without any aid from colouring, brightness, confusion,
&c., whatever. Every one who has studied Geometry of Three
Dimensions is aware of this fact. Three lines for instance
are drawn upon a plane, no two of which are mutually at
right angles, and without any artificial assistance whatever
they are regarded throughout some long problem as three
rectangular lines in space : and it often happens that we can
at pleasure see all three lines in the plane of the paper, or
any two of them in that plane, and the third at right angles
to it. The fact is a remarkable one, and deserves more
attention than it has hitherto received. I would explain it
by saying that the ideas suggestive of (geometrical) solidity
are here borrowed from imagination, (by an act of will,) and
not (as usually happens) directly perceived : but however
explained, it seems difficult to reconcile it with the doctrine
that relief is an immediate visual perception.
The facts relative to the space-perceptions of the lower
animals, &c., on which Mr. Abbott places a good deal of re-
liance, are, I think, very little to his purpose. What he has to
prove is an immediate or direct perception of distance from the
eye; but the instances in question only tend to prove a per-
ception of distance from some other member of the body—e.g.,
from the beak of a bird or the claws and mouth of a quadruped.
Animals never grasp anything with the eye, and when they
wish to avoid contact between the eye and an external object,
EVIDENCE OF THE LOWER ANIMALS. 33
it is the distance of the object from the anterior surface of
cornea and not from the real visual organ which it concerns
them to know. If the animal originally perceived the dis-
tance of objects from the retinal surface, this knowledge
would not of itself be of any use, and before it could apply
it to any practical purpose it would have to solve an intricate
problem in Natural Geometry. Then what are we to say of
binocular vision ? From which eye is distance originally
perceived P Mr. Abbott replies from neither, but from a
point situated between the two, which he terms “ the
binocular centre.” (Sight and Touch, p. 109.) A perception
of distance or direction from this point would be of still less
utility, for it would not enable us to ward off an external ob-
ject from either cornea. Let us add, that the nose is always
imaged (though not very distinctly) on both retinae, and hence
this perception of distance from the binocular centre may be
resolved into a corrected estimation of lateral distance. Mr.
Abbott further holds, that when we see a difference of distance
between two objects not in the same direction, “the distance
thus measured or perceived is not the difference of distance
from the eye as centre, but the projection of this on the line
of sight.” (Sight and Touch, p. 99.) This knowledge if
real (and it appears to me that this doctrine is contrary to all
present experience) would be of still less use in guiding
our voluntary motions. I have already quoted his remark
that lateral objects, and especially our own bodies (whether
actually seen or not) afford a kind of base from which the
distances of objects are measured. What need of such a
base if distance from the eye is a direct perception of sight?
On the whole, I cannot see that Mr. Abbott has advanced
far in proving that longitudinal distance and relief (as well
as externality and direction) are original perceptions of vision.
He has advanced arguments, which I think of great weight
to prove that if these were not original perceptions of vision,
touch and locomotion could not directly supply their place;
(the theory of Mill and Bain which denies space to be a
34 THE RETINAL IMAGES ARE DIRECTLY PERCEIVED.
direct perception at all, and regards it as a compound made
out of many feelings by association, he seems scarcely to have
contemplated;) and he has brought forward reasons which
are not to be despised, to prove that the signs which suggest
to us longitudinal distance are not faintness, indistinctness,
and the muscular sensations connected with the eye, &c.,
but certain other visual sensations which determine con-
vergence and adjustment. But the first part of this argument
neither refutes those who hold that lateral visual impressions
become associated and correct each other, nor even Mr. Mill:
while, in the second part, Mr. Abbott has merely substituted
one set of suggesting ideas for another. That the connexion
between these visual sensations which determine convergence
and adjustment, and the judgment of longitudinal distance
which follows them is anything more than one of intimate
association and ready suggestion, Mr. Abbott has hardly
attempted to prove.”
If, then, the object of vision be nothing external to the
eye, the natural conclusion is that it is the picture, or collec-
tion of pictures, on the retina, the exact conformity of which
to the accompanying visual impression has been so frequently
remarked. This doctrine will at first sight appear no novelty.
Mr. Mill, in a passage already quoted from his Disserta-
tions and Discussions, implicitly adopts it. Reid, in his
Geometry of Visibles, seems to entertain the same view.
Dr. Nunneley says, “In seeing, it is not the tangible and
large distant landscape, but the very different and small
picture thrown on the retina itself, that is the immediate and
only object of perception.” (Nunneley on the Organs of
Vision, p. 7; see also p. 20.) Brown represents this as the
doctrine of all Berkeleians prior to his time, (Lects. xxviii.,
xxix.,) and Dean Mansel tells us that in certain cases we can
* The theory of the line of visible direction is exposed to all of the fore-
going objections which apply to direction as well as distance, and seems
moreover to be inapplicable to the lower animals, whose retinae not being
spherical, perpendiculars to their surfaces do not pass through a point.
THE RETINAL IMAGES ARE DIRECTLY PERCEIVED. 35
even still bring the question to the test of direct experience,
the sensation of darkness being merely the consciousness of
the retina in a state of repose; (Metaphysics, p. 78;) and
still more decisively, “The presented object is on the surface
of the retina.” (Metaphysics, p. 80.) It would be easy to
make further quotations to the same effect. Yet, paradoxical
as it may appear, the doctrine in question has never, I
believe, been really maintained. Mr. Mill, as early as the
Essay I alluded to, adopted almost in its entirety the hostile
theory of Brown. Reid stopped short at the sphericity of
the presented objects. Dr. Nunneley, speaking of the vari-
ations of brightness, distinctness, &c., in the images on the
retina, says—“Not only is the mind unconscious of these
variations, but it is in point of fact unaware of the existence
of the figure itself, much less of differences in it;” (Organs
of Vision, p. 27;) and Dean Mansel observes, that “when
theory declares that the object we really see is in the organ
of sight, it does not follow that the infant has ever actually
seen it there.” (Metaphysics, p. 117.) Sir W. Hamilton
and M. Cousin, I may notice, are no less than Messrs. Bailey
and Abbott openly opposed to this doctrine; and a similar
remark will, of course, apply to Messrs. Brown, Mill, and
|Bain.”
The mere existence of this picture is to me a strong argu-
ment in favour of its being the immediate object of visual
perception, because it does not appear to answer any other
purpose; and this argument, of course, becomes stronger as
the arrangements for producing the picture are more univer-
sally met with, more complicated, and more minutely accurate.
Let any one, for example, take up the description of our
visual apparatus in Paley's Natural Theology, (which is
* Berkeley himself expressly denies that the pictures on the retina are
the objects of sight. (Theory of Vision Windicated and Explained,
Sect. 1.) It will be seen how widely this differs from the passage already
quoted from Mr. Mill's answer to Mr. Samuel Bailey, in which the former
believes that he is vindicating the Berkeleian doctrine.
36 PROOF THAT THE RETINA IS PERCEIVED.
there given as a proof of design in nature,) and ask himself
at the end of it whether he can seriously believe that Nature
(or the God of Nature) took all this trouble to produce an
invisible picture of a visible object P But while this picture
serves no other end, (and ends are guiding principles in all our
physiological researches,) it is confessed to be the universal
and necessary condition of vision.” Further, we have seen, on
the one hand, that if we make any other spacial object theim-
mediate object of vision, we are almost necessarily led to hold
that consciousness is in some instances deceptive; while, on
the other hand, consciousness appears to indicate clearly that
the immediate object of vision is spacial : and hence we are
almost driven to believe in the immediate perception of the
retinal picture, in order to save the veracity of immediate
consciousness. But the strongest evidence on the subject is
afforded by the exact correspondence between the picture
and the accompanying or resulting visual perception—a cor-
respondence so precise and accurate, that the optician never
doubts for a moment that the same change will take place
in the vision of his patient, that he proposes to produce in the
picture by means of his glasses. He only inquires what kind
of glass will render the picture distinct, and when that is
done he is satisfied. Conclusions, too, as to the nature of
the lenses of the eye as optical instruments, are constantly
drawn from the facts of vision without the slightest hesitation.
It was, indeed, the facts of vision that first suggested to
Dollond the falsity of Newton’s conclusion as to the impos-
sibility of an achromatic combination; and it has since been
concluded that these lenses are not perfectly achromatic, in
consequence of the discovery of some small symptoms of
chromatic aberration in the visual phenomena. No writer
* In certain cases, however, the retinal affection cannot be strictly
called a picture, as it represents nothing without us, but is determined by
internal (though not purely mental) causes. The retinal affection, how-
ever, could even here be represented in a picture or drawing, and may,
therefore, receive the same name.
PROOF THAT THE RETINA IS PERCEIVED. 37
on Optics, in fact, imagines that he is only describing the
unseen retinal picture, and not the real object of visual per-
ception, when he explains, from the laws of reflection and
refraction, why a particular external object must be seen in a
particular direction, situation, &c. Objects disappearing
while within the limits of the visual field are considered to
afford full proof of the existence of insensible points on the
retina, and even to determine their precise position. The
correspondence in all these cases is assumed to be exact, and
all experience justifies the assumption. It is, moreover,
worthy of remark, that this correspondence is not supposed
to exist between the pictures on the retina and our first
uneducated visual impressions, but between these pictures
and our present educated vision—a fact which would seem
to indicate that association has much less influence on our
present perceptions of sight than is commonly supposed.
Entoptic appearances and visual phenomena in the closed
eye strongly bear out the same conclusion; and I think that
if carefully attended to, many more phenomena of this class
would be observed than have been hitherto. It is hardly
necessary to remark, that no one suspected that vision was
affected by the blood-vessels of the retina, till the fact was
brought to light by the observations (they can hardly be
called experiments) of Purkinje. I believe that these
blood-vessels are also perpetually visible under other cir-
cumstances, which, perhaps, are now noticed for the first
time. When I look at any not very brilliant and pretty
uniformly-coloured object, I can almost always see some of
them on its surface traced in a peculiar manner, somewhat
resembling water-marks on paper. Thus seen, they seldom
exhibit any trace of red colour. They will, perhaps, be
best seen soon after waking in the morning, when the eye is
fresh, on looking at a cloudy sky; but I have sometimes
seen them under very different circumstances—sometimes
even projected against the side of my nose. They seem
quite as far off as the object looked at. On nearly closing
38 RETINAL IMPRESSIONS PERCEIVED IN THE DARK.
the eye, so as to make the outlines of external objects more
or less indistinct, I can frequently see the field of vision
covered with a kind of minute net-work, with roundish
meshes apparently corresponding to the cones and bulbs of
Kolliker. The conditions most favourable for seeing these
appear to be the same as for Purkinje's blood-vessels, which
are likewise often rendered more visible by a partial closing
of the eye. There is also a collection of brilliant round
spots about the centre of the visual field, which, though
perhaps not always capable of being rendered visible, very
frequently are so. They can be seen in the dusk, or in a
darkened room, projected against the objects looked at ; and
they are also often visible with the eyes shut, which does
not seem to be the case with the phenomena hitherto noticed.
I have made further observations with my eyes shut, with
results which surprised me. At first, probably from habitual
inattention, I could only say of the visual perception with
light excluded from the eye, that it was a spacial one, and
not uniformly black, several ill-defined grayish or slightly
luminous spots appearing in it. But I now never close my
eyes for a minute without a distinct perception of colours,
resting, of course, on a dark ground. Blue was the first I
noticed, and I have often since observed green and red—very
rarely yellow, but this perhaps arises from some peculiarity
in my own eyes. They generally appear in moving patches,
not well defined, but sometimes by no means wanting in
brilliancy. As I never observed these before I paid special
attention to the subject, and have frequently seen them
when in perfect health, they are evidently different from
the vision of colours produced by derangement of the stomach
or other disorders, and already well known. Persons accus-
tomed to make delicate experiments in Optics are well aware
that subjective influences in vision extend much beyond what
is generally supposed. There are experiments of importance
which cannot be made after long fasting, after a hearty
dinner, or after drinking a very moderate quantity of ardent
OBJECTIONS TO THE THEORY OF RETINAL PERCEPTION. 39
spirits. A remarkable instance of the effects of bodily
health, and especially the state of the nervous system, (of
which the retina forms a part,) on our visual perceptions,
occurs in a letter of Shelley's, written at the time that he
was composing his famous Prometheus Winctus: — “My
health,” he says, “ has been materially worse. My feelings
at intervals are of a deadly and torpid kind, or awakened to
such a state of unnatural and keen excitement, that only to
instance the organ of sight, I find the very blades of grass
and the boughs of distant trees present themselves to me
with microscopic distinctness.” It deserves to be noticed
(though the observation can hardly be called an entoptic
one) that most of the phenomena of diffraction can be pro-
duced, under favourable circumstances, by the hairs of the
eye-lash. This proves that the interference of the luminous
waves on the retina produces the common visual effects of
interference. There are, again, some very remarkable dif-
ferences between interference experiments, when the light
is received on a screen, and when it is received directly on
the eye, which are easily explicable if the pictures on the
retina are the objects of vision, but which appear otherwise
wholly unaccountable.
A host of objections to the doctrine that the pictures on
the retina are the immediate and only objects of visual per-
ception will at once occur to the reader. First, perhaps, it
will be said that we are not at all aware of the existence of
such a picture until science discovers it to us at a late
period of life. But I apprehend that the true state of the
case is merely that we are until then ignorant of the
locality of this collection of pictures—which on my prin-
ciples we must be. For of the tangible parts of the head
which surround these pictures we see nothing, and their
existence and locality is not, therefore, revealed to us by
direct consciousness. We see nothing and perceive nothing
in space, except the collection of pictures on our retina, and
for this very reason we can have no direct perception of
40 ILLUSIONS OF SIGHT AND OF TOUCH.
their locality or spacial relations to anything else. And
that this is the true explanation of the fact is strongly con-
firmed by some cases of couching for cataract. Thus,
Cheselden's patient “ was never able to imagine any lines
beyond the bounds he saw. The room he was in, he said,
he knew to be part of the house, yet he could not conceive
that the whole house could look bigger.” (See Theory of
Vision Vindicated and Explained, sec. lxxi.) If space is
originally presented only as a collection of retinal pictures,
our inability to conceive spacial objects outside of this
collection (and, consequently, to determine the position in
'space of this collection) needs no explanation.
The illusions of sight will, perhaps, be adduced as afford-
ing further evidence against the view here advocated. But
on this theory these illusions become at once explicable.
We have, in the first place, the large invariable external
object, and, in the second, the much smaller variable pic-
ture of it; and all illusions arise from attributing to one of
these the properties of the other. Mr. Abbott has clearly
shown that touch is subject to similar illusions. This is
particularly remarkable in the case of tactual magnitude.
In Weber's experiments two compass points were pressed
on the skin at short distances from each other, and moved
apart until separate impressions were received from them.
It was found that at different parts of the body the dis-
tances of these points varied in the ratio of sixty to one,
and the intervals were felt to be greatest where they were
really least. Illusions with respect to shape also occur in
this sense. But how are these illusions possible, if the
modes of extension are original perceptions of touch The
impressing object and the impressed organ have the same
extension and the same figure, and, therefore, no illusion
can arise from transferring the magnitude and shape of the
one to the other. The illusions of sight are, therefore, per-
fectly consistent with our thesis, that there is a direct per-
ception of extension (viz., that of the retinal pictures) by
SPACE TERMS USED WITH DIFFERENT MEANINGS. 41
means of that sense. But the illusions of touch are not
consistent with the supposition, that we have an immediate
perception of tactual extension and figure; because in this
case, unlike that of sight, the extension and figure of the
affected organ is identical with that of the external body.
This argument, with others adduced by Mr. Abbott and
other writers, would, perhaps, justify me in accepting the
conclusion of Platner, that our empirical knowledge of ex-
tension is derived from sight only, and that when a man
completely blind from birth employs our space-terms he
uses them with a totally different connotation (though with
the same denotation) from that of the seeing man. The
possibility of this may be thus proved. We will for a
moment suppose the theory of Messrs. Mill and Bain to be
fully established. Let us then imagine a man possessed of
the sense of sight, but wholly destitute of that of touch and
of the power of locomotion. We will further suppose some
mechanical apparatus affixed to his eyes, and worked by a
friend—without any consciousness on his part—in such a
manner as to produce the succession of visual sensations
which usually arise in the seeing man from the combination
of muscular motions with the exercise of vision. Such a
person would manifestly see visual differences between what
others called circles, squares, cubes, right lines, up, down,
far, near, &c., because it is admitted on all hands, that it is
mainly the perceptions of these visual differences that suggest
to us the tactual and muscular sensations which are sup-
posed to be connoted by these terms; and it is further evident,
that our Idomenian (as Reid calls him) would associate
these visual distinctions with the terms which he usually
heard applied to the objects in which they existed. He
would, therefore, employ all our spacial terms, and employ
them all correctly as regards denotation, though he could
not have the slightest idea of the connotation attached to
them by Mr. Bain. And here a similar concession must be
made to Mr. Bain’s blind man. He could certainly form
D
42 OBSERVATIONS OF PLATNER,
ideas, or collections of ideas, having the same denotation (or
extension) as our space-ideas. But then the question arises,
whether the Idomenian's idea of a square would be, at
bottom, the same with that of the blind man? or, suppos-
ing them different, which of them agrees best with the idea
of the ordinary man who can both see and move 2 To this last
question Messrs. Mill and Bain reply that of the blind man,
while Platner and Mr. Abbott reply that of the Idomenian. I
agree with the latter; and I think Platner most worthy of
credit, because he instituted careful observations with a
view to decide the question, and was at first deceived by
the appearances which must always be presented on the sur-
face in such cases. It would be well to know whether the
observations of Mr. Kinghan, (referred to by Dr. M'Cosh
and Mr. Mill,) got beyond this first superficial stage in
which the correct employment of space-terms is supposed to
indicate the possession of space-ideas whose intension (or
comprehension) corresponds with that of our own.
I need not discuss the question of the perception of space
by other senses—including muscular effort—at greater
length. I refer the reader to Mr. Abbott's work, Sight
and Touch, for a fuller investigation of it. I will assume
during the rest of my remarks, that these other senses either
give us no direct knowledge of extension and magnitude, or
else one so vague and rudimentary as to be incapable of
correcting the space-perceptions of sight.” I will, there-
* I venture, however, to state, in a note, a few of my reasons for
denying the perception of space to any other sense than that of vision.
1st. Let us observe that no conflict between sight and the other senses,
in reference to the magnitude, unity, and position of objects, has ever
been observed; nor has any reason been assigned why, in case of such a
conflict, sight should give way to the other senses at all, much less at
such an early period as to leave no trace of this conflict in the memory.
We find sight very obstinate in some cases where it is evidently in the
wrong, e.g., the celestial motions and some cases of double vision. 2nd.
The organs of feeling and locomotion, as already observed, are subject
to much greater variations in different animals than those of vision.
EXTENSION DERIVED FROM SIGHT ONLY. 43
fore, endeavour to deduce all the actual phenomena of
spacial perception from sight only. The readiness with
which these ideas seem now to be suggested by touch and
locomotion is, in my opinion, the result of associations be-
tween them and certain visual impressions—associations
which are very easily formed. If all the actual phenomena
of space-perception are deducible from our visual sensations
only, my theory is at least entitled to a fair hearing. It is
quite as scientific as that of Messrs. Bain and Mill, and
possesses certain advantages over theirs at the outset;
while I shall endeavour to give, upon its basis, a much
more complete explanation of the actual facts than they
have attempted.
Starting, then, with the immediate perception of the pic-
Organs of feeling, or ascertaining shape by touch, many animals seem to
have literally none. Yet, so far as we can judge, their space-perceptions
are very accurate. 3rd. It is difficult to understand how we could
ascertain the shape of any tolerably extensive object by touch and
motion without sight; for in feeling it over (not to mention the difficulty
of accurately retaining the series of feelings in their proper order in the
memory) we do not feel some parts at all, and we feel others twice or
even oftener, especially when deprived of the aid of the eye. But sup-
posing all the feelings retained and put together in the proper order, it is
evident that to attain a complete knowledge of the shape, we must have
felt every part once, and no part twice. This seems to me to indicate
strongly that our knowledge of tactual forms arises from visual associa-
tions; which is confirmed by the extraordinary difficulty experienced in
ascertaining them by touch alone, when the form of the object handled is
unusual and unfamiliar, and, therefore, but slightly associated with
visual impressions. But sight takes in the shape at once, however
unfamiliar; and, it may be added, that it reveals diversities of shape
and figure too minute to be grasped by touch or sensible locomo-
tion. 4th. Many men who judge of space with accuracy have not yet
learned some of the equivalences which Messrs. Mill and Bain consider
necessary for that purpose. A very simple experiment, suggested by
Brown, suffices to prove this. Let us pass the hand over a book or desk,
first rapidly and then slowly, with our eyes open. No difference in size
is perceived. Perform the same operation with the eyes shut. The
difference is startling, the object appearing enormously larger when
44 ORIGINAL AND PRESENT PERCEPTIONS.
tures on the retina, and of these only, what progress can we
make in explaining the actual facts of vision ? The optician's
principle that the character of the lenses &c. of the eye, as
optical instruments, can be inferred from the actual pheno-
mena, implies, as I have observed, that the change from the
original to the actual percept is but a slight one. On the
other hand, however, a formidable list of differences is made
out even by those who admit that the retinal picture is
originally perceived. Thus Dean Mansel says they differ in
five important respects.” 1. The presented object is double,
the represented (actually perceived at present) is single.
2. The presented object is inverted, the represented is erect.
3. The presented is very much smaller than the represented.
4. The represented object exists in three dimensions of space,
while the presented exists only in two. 5. The presented
handled very slowly. We have not, in fact, learned (in the words of Mr.
Bain) “to feel that a slow motion for a long time is the same as a
quicker motion with less duration;” which he thinks we do by observing,
“that they both produce the same effect in exhausting the full range of
a limb.” See also on this point Mr. Abbott's remarks, Sight and
Touch, p. 34. In conclusion he observes, “A blind man will distin-
guish better by touch, because his touch is practised; therefore, clearly,
ours is not. Is not this a demonstration that what we distinguish are
not representations of tactual perceptions?” Lastly, Men have been
born almost without the sense of touch and power of locomotion, and
others have almost wholly lost these senses through paralysis; but I have
never heard that the space-perceptions of these men appeared to be un-
usually deficient. Paralysis, indeed, seems to interfere with a man's
power of localising his sensations when the cause which produces them
is invisible, but it does not appear (except when the optic nerve is
paralysed) to affect his perceptions of extension and figure in the slightest
degree. I ought, perhaps, to have mentioned before that Berkeley him-
self remarks, that when we examine an object in the microscope we
receive from it visual impressions, which are almost entirely free from
associations with touch and locomotion; but extension and figure are,
nevertheless, suggested to us as forcibly as when we view the same ob-
ject with the naked eye.
* Mansel's Metaphysics, p. 80.
SINGLE AND DOUBLE VISION. 45
object is on the retina, and the represented is external to it.
To this we might add that the presented object is bent in the
concavity of the retina, which Sir William Hamilton urges
as an argument to prove that the pictures on the retina are not
the immediate objects of perception: for he says if they
were so all objects must appear similarly bent.
1. To the first of these I reply, that the original percept
or presented object is not always double, and that in many
cases the present percept or represented object is double;
In fact, I believe that whenever the one is double so is the
other, and that, in this respect, education makes no change.
It is evident that as the space which separates the two retinae
is not an object of visual perception, and, in fact, is not
perceived at all, we cannot originally perceive these retinae
as separated by the space which really intervenes between
them. It cannot, therefore, be determined a priori whether
the two collections of pictures would be seen as wholly
separate, wholly superposed, or partially overlapping; and
in the first of these cases it does not appear to me that we
could represent to ourselves the unity of space at all. But
the bifurcation of the optic nerve at once suggests to us the
probability that the two fields will partially overlap; for
there is no reason to suppose that when two portions of the
same nerve are drawn apart and separated by a piece of
insensible matter, they lose their natural power of acting in
concert, and suggesting the same, or at least immediately
contiguous localities. The arguments that single and double
vision, as actually experienced, depend upon original laws and
not on education or association, are very strong, and the fact
is by no means inconsistent with my theory. Nor is it easy
to see how the unity of objects could be given us by touch
or locomotion, while there are many cases in which, after
all the information that these can give us has been obtained,
we still see the objects double as before. I cannot, there-
fore, admit that there is in this instance any difference
between the presented and the represented object.
46 VISION IS INVERSE, NOT DIRECT.
2. To the second alleged difference a similar reply can be
made. As long as it was held that touch and locomotion gave
us accurate information as to the situation, &c. of external
objects, the fact that no conflict arose between their pre-
sentations and those of sight, was supposed to establish de-
cisively that we saw objects erect: but if, as I have con-
tended, sight alone is directly percipient of the situation of
objects, (or rather of the retinal pictures of objects,) and touch
and locomotion only suggest it in consequence of previous
associations with sight, there is absolutely no evidence for
the universally assumed fact of erect vision. The whole
problem, How can inverted pictures give rise to erect im-
pressions of objects? seems thus analogous to King Charles's
problem of Why a live fish does not add to the weight of the
water in which it is placed, while a dead fish does P and the
answer in both cases is the same, viz., that the assumed fact
has not been proved. Indeed, even if touch could give us
some idea of situation directly, it is not probable that it could
do so with anything resembling the distinctness and minute-
ness of sight, and the conflict between the senses would
naturally be decided in favor of that which spoke most
clearly and authoritatively. But, in fact, touch does not
seem to give us any such information. We are directly
conscious of effort only, not motion ; and that our voluntary
efforts are accompanied or followed by motions we learn from
sight, which, in presenting this motion to us, reverses its
true direction. Vision, therefore, always continues to be
inverse; a fact which is strongly borne out by some experi-
ments of Scheiner with pinholes placed near the eye, which
were successfully repeated by Porterfield and Reid.” Setting
aside the line of visible direction, which I have already re-
jected, the only explanation that seems to me applicable to
these phenomena is that our present vision is actually inverse.
3. Some of these remarks will also apply to the magnitude
of objects. If the only space presented to us be on the
* See Hamilton's Reid, pp. 160-62.
REAL MAGNITUDE, whAT 2 47
.*
retina, we can only compare the magnitude of the retinal
pictures, and not that of the objects themselves. Hence no
visible object can be larger than the whole retina, unless
indeed we infer this by observing that the whole of it can-
not be seen at once. When an object is distant, its image is
small, and it looks small. When it approaches nearer, the
image is larger, and it looks larger; and when it approaches
to the nearest distance compatible with distinct vision, it
looks largest, and it is largest. It is this last magnitude, I
believe, that is usually meant when we speak of the real or
absolute magnitude of objects; and if the object be brought
nearer than this, it becomes so confused that its identity is
no longer discernible. It can, however, be still seen dis-
tinctly, if we look at it through a pin-hole; and in this case,
as was to be expected, the object looks larger than what is
called its real magnitude. The visual image can then, under
favourable circumstances, enlarge as well as diminish the
real size of an external object. The mistake of supposing
the presented object to be much smaller than the actually
seen object, arises from the fact, that when we look at another
retina, (or the reflection of our own,) the secondary pictures
thus seen are very much smaller than the objects (pictures)
which they represent; but in this case it is evident, that
what is really seen is not the pictures on the retina looked
at, but the pictures of those pictures imprinted on our own
eyes by reflection from them. I can also prove by measure-
ment, that the surface of the retina is confined to a square
inch, whereas I can see yards and even miles; but what I
in fact see are pictures of yards and miles on a very di-
minished scale; and when I measure another's retina and
find it an inch square, what I really see is only the picture
of that inch, on a very reduced scale also. If we distin-
guish between the presented and the represented retinae—
the pictures of external objects and the pictures of those
pictures—we shall find no reason to believe that the
pictures originally presented undergo any considerable
48 ORIGINAL MAGNITUDE NOT ALTERED.
augmentation of size in consequenee of experience and
education. They remain in fact, I believe, of almost the
same apparent magnitude as at first. Cases of couching for
cataract strongly confirm this view. The patients never
observed that the objects pointed out to them and named by
their friends, appeared much smaller to the eye than they
had appeared to touch and the muscular sense. There is,
in fact, some evidence that, on the contrary, they appeared
larger. (See Abbott, Sight and Touch, p. 145.) I am
inclined to think, however, that by these expressions they
only meant that objects appeared larger after couching than
they had done to the eye before couching (for none of the
patients had been totally blind.) Franz's patient, for example,
on whom Mr. Abbott relies a good deal, thought he could
see (previous to couching) “as much as one-third of a square
inch of a bright object held within an inch or an inch and a-
half of the eye, and in a certain direction.” (Sight and Touch,
p. 157.) This, of course, formed a limit to his distinct idea of
the magnitude of a visible object; and when, after couching,
he could see much more of the object, he naturally thought
it larger than it had appeared hitherto. Those, however,
who hold that we perceive distance, &c., by touch, may see
in this fact an extension of the law that the least perceivable
tactual interval appears greatest at those parts of the body
where it is really least. Nor in these cases of couching
does the object appear larger after comparison of our visual
perceptions with those of touch than it did at first. Chesel-
den's case proves the contrary. “Being lately couched of
his other eye,” writes that physician, “he says that objects
at first appeared large to this eye, but not so large as they
did at first to the other.” Their appearing large to the
second-couched eye must, of course, mean larger than they
did then to the other; and indeed the very words “not so
large as they did at first to the other,” can only be understood
as implying a diminution of apparent magnitude during the
interval that elapsed between the two operations. The
APPARENT MAGNITUDE, How DETERMINED. 49
reader will further observe that the words “objects at first
appeared large to this eye,” seem to indicate that even in
this second eye a diminution had set in at the time that
Cheselden made his inquiries. I have dwelt on this point
not merely in refutation of the Association Theory, but
because I think Mr. Mill draws a wrong inference from
Cheselden's remark, that “objects when seen with both eyes
appeared about twice as large as when seen with the first-
couched eye only.” (Examination of Hamilton, p. 293.) Mr.
Mill puts in place of the words I have italicised, “with one
eye only,” a substitution which the context proves to be
erroneous, but on which his inference depends.
The perception of visual magnitude, however, requires a
somewhat fuller explanation. The retina limits the per-
ceivable magnitude of any object; and of objects larger than
this we can only form a vague idea by observing that the
whole of them cannot be all seen at once—an observation
which could not be made until after considerable experience.
But the real limit is, in fact, much less than this. There are
always a number of confused pictures on the exterior parts of
the retina, of which we are totally or almost totally uncon-
scious. Thus many persons have never observed double
vision, though double pictures are constantly in the field;
and he who reads this page is scarcely conscious of seeing
anything beyond it, though unquestionably many of the
surrounding objects are painted on his retina at the same
moment. What is really seen is, in fact, only the field of
distinct vision, and the visible magnitude of any object will
necessarily be judged of by the proportion which it bears to
this field. But the extent of the field of distinct vision
varies with the adjustment of the eye : and hence the actual
magnitude of the picture of an object on the retina does not
by itself determine its visible magnitude. For instance, let
an object be placed at the nearest distance of distinct vision,
and then slowly moved away. The continuity and simi-
larity of the pictures seen lead us to regard them as a
50 APPARENT MAGNITUDE, HOW DETERMINED.
single visible object, though, in fact, there are several dif-
ferent pictures, successive in time. Let us go on to inquire
what change takes place in the magnitude of this object,
which is thus judged to be one and the same P. At first, as
the image or picture diminishes, the field of distinct vision
diminishes in almost exactly the same proportion. The
magnitude of the object is, therefore, sensibly constant until
it has moved for several yards—a fact fully borne out by
experience, and which justifies my statement that the real
magnitude of an object means the magnitude of its image or
picture when placed at the nearest distance of distinct vision.
Let us continue the motion. The field of distinct vision
begins to contract more slowly than the distance of the
object, and it now begins to appear of smaller size. But
there is here another difference to be noticed. The picture
becomes smaller continuously, while the size of the field of
distinct vision alters per saltum, in consequence of the changes
of adjustment and convergence which do not take place con-
tinuously. Hence the diminution in the seen size of the
object (or its increase, if it be approaching from a distance)
is not quite continuous. It sometimes appears almost con-
stant for a time, and then undergoes an abrupt change.
Experience again fully confirms this conclusion. Let me
draw another. The confusion which leads us to pay no
attention to the lateral parts of the retina, necessarily sup-
poses a diversity of colours. If the whole retina was covered
by one uniform colour, it would make no difference in the
resulting perception whether the light were focussed on the
retina or not ; and, in like manner, when a considerable part
of the retina is covered by a picture of uniform colour, the
confusion is transferred to its boundaries, though these may
be far outside what is, properly speaking, the field of distinct
vision. A large uniformly-coloured object, therefore, prac-
tically increases the field of distinct vision, and consequently
appears less than its proper magnitude. Wheatstone proved
experimentally that the apparent magnitude of an object
LOCALITY OF OUR PERCEPTS NOT ALTERED. 51
is altered by a change of convergence, the magnitude of
the eye-pictures remaining the same. The field of distinct
vision diminishes with near convergence. The pupil also
contracts in a strong light. This has often the effect of
making a bright object appear larger.” Thus by this theory,
and I think by this theory alone, can all the anomalous
variations of visible magnitude be completely accounted for.
4. If Dean Mansel means that the presented object is
presented as on the surface of the retina, I have already
given reasons for holding that the local situation of these
pictures relatively to the unseen parts of the face could not
possibly be given in the original visual intuition: while if he
means that it is in fact there, the same thing is true of what
he calls the represented object. The real state of the case
I take to be this:—The retina is originally presented to
us, only in its character of a collection of visible figures
and not in that of visual organ. The continuity and simi-
larity of some of these figures convince us that, notwith-
standing the changes they undergo, they represent (or rather
are) the same objects. Some of these are either constantly
in the field of vision, or can be made to appear in it at any
instant by a voluntary effort. We find that changes in the
space-relations between these and the other pictures are at-
tended by sensations, and, above all, that we can produce (vis-
ible) motions in them by our own voluntary efforts. These
are therefore in a peculiar sense our pictures, and they are, in
fact, soon mistaken for our bodies. Part of the retina is there-
fore taken for the body, and as the eye is not pictured on
itself, no part of it is taken for the eye. We soon learn
from our perceptions of other men, and the reflection of
our own form, to apprehend the space-relations which exist
between our eyes and the visible members of our bodies; and
when this stage is reached, we place (in imagination) an eye
behind the collection of pictures which we have mistaken for
* On this whole question see Abbott, “Sight and Touch,” pp. 57,
99, 100, 124.
52 THE THIRD DIMENSION OF SPACE.
our own bodies; and as we are conscious of no second set of
pictures, we naturally enough conclude, when we learn the
existence of the pictures on our retina, that these are not
objects we were conscious of already. We judge our own
eyes to have about the same visible magnitude as those of
another man, (reflected on our retina,) or as seen by reflection
in a mirror at the usual distance. This process, it will be
seen, is the very reverse of that sometimes described. Ex-
perience does not teach us to project the pictures on the
retina forward into external space, but to project the retina
(if I may so speak) backward into an imaginary space situated
behind the pictures which are really upon it.
5. The last difference noted by Dean Mansel is, that the
presented object is extended in two dimensions, and the
represented object in three. If this be meant literally, it is
evidently not true. The retina, being a portion of a spherical
surface, is necessarily extended in three spacial dimensions.
But probably he means no more than that it is a regular
surface with no hills and hollows in it—nothing correspond-
ing to what we term relief. And here I think is, in fact,
the greatest change that association and education effects.
But then space, once presented in three dimensions, affords
all the requisite elements for our perception of relief; and I
believe that the education which vision undergoes in this
respect is effected for it by itself.
In the first place, all that we see, or fancy we see, con-
sists of surfaces in three dimensions, though no longer con-
fined to a single spherical surface. We see surfaces at a
distance from us, but it cannot I think be said in strictness
that we see the distances themselves. Further, we see no
surfaces from which rays do not come to the eye. No
amount of experience will enable us to see round a corner;
nor will the most perfect conviction of what would be seen
behind an opaque object enable us to see it till the object is
removed. Even in imagination we cannot represent our-
selves as seeing more than the laws of vision permit, e.g., as
concBPTION OF DISTANCE, How AcquireD. 53
seeing the entire surface of a closed solid of any kind. The
difference here is evidently not so essential as to require the
intervention of elements supplied by other senses, or by the
a priori reason. But, secondly, sight affords everything
necessary for its own education. I already remarked, that
we observe, at a very early age, that contact between the
pictures of our bodily members and those of other objects is
constantly accompanied by tactual sensation. But this is not
true of every contact. Apparent contacts are perpetually
occurring which produce no such effect. Can we, then, find
any criterion to distinguish by sight between contact attended
by tactual sensation and contact not so attended? Certainly.
Where contact is not attended by sensation it will almost
always be found that we cannot at once see our limb and
the object in apparent contact with it with perfect distinct-
ness. When one is seen clearly, the other, if not double,
is always more or less confused. The two eyes, in fact, do
not refer the contact to the same precise locality. This is
evident from the accompaning sketch. Let the two eyes be
#
§
*

*B
ºE.
placed at A and B, respectively, and let an object of any
kind OP be partially obscured by the finger at D. Then
to the eye at A the finger-point D will appear in contact
with the point F, while to the eye at B it will appear in
contact with the point E. Whenever this occurs the hand
at D will experience no sensation of touch; while on the
54 PERCEPTION of DISTANCE, How AcquiBED.
other hand, if we can make the points E and F coincide, a
tactual sensation immediately follows. Here then we
have a visual distinction between contact attended with
and contact unattended with tactual sensation, or as we
afterwards call them, real and apparent contact. Now,
is there any other mode of visually representing this
distinction ? Move the eyes to H, G and the separating
lines DE and DF become visible. Visual contact,
unaccompanied by tactual sensation, thus becomes asso-
ciated with a visible distance which, though not seen at
the moment, can be rendered visible by a change in
the position of the eyes. E and F then come to be
represented as farther off than D. The visual pheno-
mena which distinguish apparent from real contact are,
moreover, found to vary with the length of the lines DE
and D.F, becoming more marked as those lines are longer.
In a word, these visual distinctions become marks of dis-
tance from the eye. The various pictures can now be no
longer regarded as lying on the same spherical surface.
We are obliged, if I may use the expression, to push some
parts of this surface back and to pull other parts of it for-
ward. Another circumstance which aids considerably in
the education of vision is the disappearance of objects that
are clearly within the field of vision. For example: I stand
over a cup or jug, or a hollow in a field, seeing the interior
very distinctly. I then move away, and almost instantly lose
sight of it. But this object has not disappeared by getting
beyond the limits of my visual field in the usual way, for
I can still see the objects which surrounded it on every side.
I am, therefore, compelled to regard it as still within my
field of vision, though no longer visible, which I can only
do by means of the representation of relief. A still more
remarkable case is when I look at, perhaps, an extensive
prospect over a wall or mound of nearly my own height, and
then lose sight of it the moment I begin to move back-
wards. In all these cases it is easy to ascertain, by return-
NATURE OF OUR PERCEPTION OF RELIEF. 55
ing, that the object is still there, and has not vanished like
a dream. It often happens, too, that there is an inconsis-
tency between the apparent distance of an object (judged of
by confusion and duplicity) and the number and size of the
intervening objects which are visible. This would, of
course, occur where our position caused us to overlook a
broad valley or chasm. But, perhaps, the most potent in-
strument in the improvement or education of vision is the
different appearances which the same object presents when
viewed from different directions. To take a familiar in-
stance, a man appears quite differently when seen in front,
from behind, and from one side. After a little time there
can here be no doubt of the identity of the object seen;
and by the ordinary laws of association, the visual appear-
ance of the man from one point of view immediately sug-
gests the visual impressions received from him when seen
from other directions; but it is evident that these conflict-
ing appearances cannot be attributed simultaneously to the
same object, without supposing unseen spaces contiguous to
those actually seen—a contiguity which would be impossible
in most cases if these unseen spaces were outside the limits
of the visual field—and it is these spaces (vividly depicted to
imagination, with a full belief in their reality) which, in my
opinion, constitute our perceptions of relief or solidity.
Why these perceptions are most vivid in binocular vision
will be gathered from the diagram. The true distance of an
object is, I apprehend, estimated by enlarging all the objects
which intervene between it and some visible member of the
body to their real magnitude, (i.e., their sizes when at the
nearest distance of distinct vision,) with a correction for ob-
jects overlooked, which is suggested by the presence of more
confusion and duplicity in the object looked at, or in those
close at hand, than the magnitude of the seen intervening ob-
jects will account for. Hence, if the distant object be uniform
in colour so as to disguise these appearances, or if the inter-
vening objects be cut off by a tube or otherwise, distance
56 THE IMAGES ARE BENT.
from the eye is hardly suggested, and a distant cloud or
piece of blue sky or the waters of a distant lake appear to
be drawn across the end of our tube. This is the reason
why (as noticed by Mr. Abbott) the presence of a varied
lateral field is necessary to enable us to judge distance.
The whole difference between our original and present
visual perceptions can therefore, I think, be accounted for
by associations between different visual impressions. It is
evident that, as the law of Resemblance will here operate in
addition to that of Contiguity, the association will naturally
become closer than that assumed in the common theory;
and as, on the other hand, I attribute much less to associa-
tion than is commonly done, I think Mr. Bailey and Mr.
Abbott can hardly accuse me to overrating the force of that
principle. Of course I maintain further, that association
connects tactual sensations and muscular efforts with the
visual sensations which usually accompany them, and thus
(after a time) enable these sensations and efforts to suggest
to us the varieties of (visible) extension. But these asso-
ciations are not, I believe, of a very close or powerful
character.
Sir William Hamilton’s objection with regard to the
images being bent in the concavity of the retina is easily
disposed of. If the fact that these pictures are bent ren-
dered it impossible for us to see planes, his own theory that
we originally see objects as planes would render it equally im-
possible for us to see concave surfaces. But that the bend-
ing of these pictures does, in fact, affect vision is proved by
the unsuspicious authority of Mr. Abbott. “One or two
circumstances,” says he, (Sight and Touch, p. 82,) “de-
serve to be mentioned, which act as motives, so to speak, of
the appearance of solidity. One is the spherical form of the
retina, owing to which the images received on it cannot be
projected or represented on a plane without distortion. One
result of this is, that angles less than a right angle usually
appear too large;” and the writer goes on to trace some
EVIDENCE OF THE LOWER ANIMALS. 57
remarkable optical delusions to this source, for particulars
of which I must refer the reader to the figures exhibited in
his book. (p. 83.) They appear to me to constitute a deci-
sive refutation of the plane-theory. As to Sir W. Hamilton's
second objection, that if the retinal pictures were the im-
mediate objects of vision we would require a second eye
behind the first, I can only reply that I do not understand
it. The light being focussed on the retina there is evi-
dently no further need of lenses to concentrate it on the
required points; and if the first retina cannot be an object
of immediate perception the same remark would evidently
apply to the second.
I do not consider the case of the lower animals, so often
referred to, at all inconsistent with this theory. The dis-
tances which they know, (or at least prove that they know,)
as has been already remarked, are not those of objects from
the eye, but of objects from other members of the body
which are painted on the retina; nor could the latter be
deduced from the former without supposing a very compli-
cated Natural Geometry. It is probable, I think, that in
many of them the increase of confusion and duplicity with
distance is more rapid than in mankind, and this gives rise
to earlier and finer perceptions of the variations of near dis-
tances. Their superior powers of locomotion also afford
more abundant and early experiences of the kind necessary
to instruct them than is possible in the case of the human
infant; and without at all contending that the lower animals
are comparable to man in respect of intellect, it may be
doubted whether the partridge of a week old is not a more
teachable animal than the human infant of the same age.
There is probably an implanted or instinctive desire in
those birds that feed themselves to see the objects destined
for their food with their greatest magnitude and distinct-
ness. This wish directs them onward in a right line, (any
deviation from this direction altering the appearance more
or less,) and the moment confusion begins to result from its
E
58 CASES OF COUCHING FOR GATARACT.
close proximity, the object is within reach of the beak or
talons. In fact, so far as I can judge, the mouths of all
animals are placed at about the best distance from the eye
for accurate vision, and also at the very verge of the visual
field; so that passing the beak beyond the food which the
animal attempts to seize is always followed by a confusion in
its appearance and often by its total disappearance. How
favourable to the instruction of the animal these circum-
stances are it is unnecessary to insist upon. I am convinced
also that the cases of couching for cataract (which I cannot
examine in detail) will be found quite in accordance with
my theory. One peculiarity observed in many of them I
may, perhaps, offer an explanation of the fact that objects
appeared to be in contact with the organ. To suppose that
sight alone could reveal this fact is to suppose that the
patient saw both the organ and the object, and made a mis-
take as to their relative positions, which I suppose no one
will maintain. The truth, I apprehend, is this. The
patients had previously judged of distance by time. A
distant object was to them an object that could not be felt
at once, but only after going through a series of successive
tactual or muscular sensations. Hence they naturally ex-
pected that in the vision of the distant, the object would
not be seen at the first instant, but that there would be a
successive series of visual sensations terminating in a per-
ception of the object. When this was found not to be the
case—when the pictures which were said to be near objects,
and those which were said to be distant objects, were per-
ceived at once and in precisely the same manner—in what
other way could the patient express this fact than by say-
ing that they appeared to touch his eyes” ” And as distant
* Another explanation however may also be offered, and one which is
considerably favoured by some observations on couching. Prior to the
operation the patients had not been totally blind. They had been able to see
the colour, and somewhat of the shape of objects held close to the eye
(i.e. to the cornea), and these perceptions had become more vivid and
WHY SPACE CANNOT BE ANNIHILATED. 59
objects appeared just like those which were near enough to
come in contact with his cornea if he made any sudden
motion, it is not surprising that he feared to move, and held
up his hands before his eyes to guard them. But it was with
his cornea, not his retina, that he apprehended a collision.
In this point of view all that the observations prove is that
distance from the eye is not an immediate perception of
sight, which is an essential part of my theory of vision.
In conclusion, I shall endeavour briefly to reconcile my
theory with the necessary properties of space and the
elementary laws of geometry, which Messrs. Mill and Bain
have, in my opinion, utterly failed to do. The necessary
existence of space appears to me to result from the fact
that space is always seen, even when our eyes are closed
and all light excluded from the retina. In consequence
of this, when we attempt to annihilate space we are at-
tempting to annihilate an actual perception, which con-
tinues to exist during the attempt. Our belief in the infinity
of space requires some further explanation. Cheselden's
patient, who could not imagine space outside the room in
which he sat,” could hardly have been at that time a firm
believer in its infinity. But it is, nevertheless, probable
that he could not imagine a limit to space; for the field of
distinct vision, of which alone we are conscious, never pos-
sesses a precise and definite boundary. At a certain dis-
tance from the centre of the field, consciousness and uncon-
clear the nearer the objects were held. On being couched these patients
saw forms and colours more distinctly than they had ever done before,
and they naturally inferred that the visible objects were in very close
proximity to the cornea—which is all they seem to have meant by saying
that they appeared to touch the eye.
* This fact affords strong evidence, that even at this early stage space
was to Cheselden's patient a visible object. His previous experience of
touch and locomotion ought surely, according to Mr. Bain, to have pre-
cluded any such inability.
60 INFINITY OF SPACE EXPLAINED.
sciousness seem to melt into one another insensibly, and the
mere effort to look at the limit of the field suffices of itself
to make that limit shift farther off. Of course we may after-
wards come to know that a picture which is near the centre
of the retinal field is really farther from us than one nearer
to the edge; but I am speaking of visual limits, and the
ground at my feet is often nearer to the visual limit than
the sun in the horizon.” The fact, then, that the field of
distinct vision is always surrounded by other spaces, of
which we have a kind of obscure semi-consciousness, renders
it impossible for us to see or imagine a limit to space; and
when to this we add our positive knowledge of its vastness
as something surrounding and containing all that we see,
(which experience soon teaches us,) these two elements seem
to me to afford everything that is necessary to account for
our belief in space infinite. As to its infinite divisibility,
what I have already said as to the minimum visibile not
being a quantity of space but a quantity of light, depen-
dent moreover on the quality of this light and of that
which surrounds it, as well as the condition of the visual
organ itself, will explain a great part of the facts; and when
* This knowledge and representation of the greater distance of ob-
jects near the centre of the field than those near the edge is however of
no use towards enabling us to imagine a limit to space; because it arises
from the imagined appearance of the object, when seen from a different
point of view in which we imagine ourselves seeing space beyond it.
Indeed, these representations have the very reverse effect. When I look at
what seems the most distant visible object—the blue sky, whose curvature
is turned towards me—I am irresistibly led to imagine another point of
view in which the curvature is turned away from me, and thus to sup-
pose space beyond it. It should also be noted, that our only idea of a
visual limit (of bodies or particular spaces) is a bounding line between
two colours. Colour and space are thus supposed to exist at both sides
of this limit; and this expérience, therefore, renders it still more difficult
for us to imagine a limit to all space. (See Hodgson, Space and Time,
pp. 128, 139.)
INFINITE DIVISIBILITY OF SPACE. 61
to this we add, that very minute objects can be made by
proper arrangement to exhibit a multiplicity of parts to
vision, there is little left to account for. This little, I
think, will be cleared up if we bear in mind, that in the
minima sensibilia we do not drop at once from clear con-
sciousness into total unconsciousness, but that objects pass
from the one to the other by insensible degrees, just as day-
light melts into darkness. The infinite divisibility of the
intensity of sensation, or the possibility of reducing it by
insensible degrees from clear consciousness to absolute zero,
thus appears to me to carry with it the infinite divisibility
of space.” The divisibility of time, perhaps, also assists us
here; and still more assistance is afforded us by the fact of
visual motion.
But why are the laws of geometry irreversible even by
the imagination+ 2 For similar reasons. The imagination
* It will appear from what has been said, p. 7, &c., of this Essay, that
the visible line of demarcation which separates two coloured spaces is a
breadthless one; and this divisibility by breadthless lines is, I think, closely
connected with the phenomena of infinite divisibility.
f While philosophers are generally agreed in ascribing the most abso-
lute necessity to the laws of Geometry, geometers usually admit that
Euclid makes an assumption with regard to parallel lines, which is not
self-evident, and of which no satisfactory proof has hitherto been given.
The consideration of the infinity of space I think, however, really enables
us to overcome the difficulty, and affords a proof which, if scarcely
elementary enough for a schoolboy, may be found sufficiently satisfactory
to those who have really felt the difficulty. It turns on the following prin-
ciples: 1st. That if the legs of any angle be produced to infinity in one
direction, the space that lies between them will bear a finite ratio to the
whole plane infinitely extended in all directions; and 2ndly. That the
space lying between a pair of parallel lines, produced to infinity in one
direction, and terminated by a finite intersecting line at the other ex-
tremity, bears an infinitely small ratio to the same plane. (By parallel
lines I mean lines which make internal angles together equal to two right
angles with the particular intersecting line already referred to). Both prin-
ciples can be proved by repeated superposition. For example, if A O B be
62 NECESSITY OF GEOMETRICAL AXIOMS.
in representing figures and forms employs the organ of
sight, and what cannot be presented on that organ cannot
the angle in question, make B O C equal to it, and produce O C (fig. 1) to
Fig. 1.
infinity, and then the space lying in the angle B O C can be shown to be
equal to that lying in the angle A. OB by superposition. Repeating the
same process for COD, &c., we must, plainly, at length arrive at angle
terminated by a line On falling within our original angle. When this
has been done, the sum of all the angular spaces will exceed the whole
plane infinitely extended by the angular space AQn ; and since they are all
equal, if there be n of them, each will exceed; of the whole infinite
plane. To take the second case, (fig. 2.) let O, O, A B be the space re-
Fig. 2.
o/ A—
Oz / 8
% C—
•y D–
ferred to. Make O, O, = O, O, and draw O, C, so as to make the angle
O,OAC = O, O.B. Similarly draw. O.D., making O, O, - O,Oz, and
the angle O, O, D = O, O.B. Then, by superposition, the spaces
O, O, AB, O, O, B C, O, O, CD, &c., are all equal, and we can evi-
dently cut off as many of them as we please without exhausting the
whole plane. Each of them, therefore, is less than is of the whole

NECESSITY OF GEOMETRICAL AXIOMS. 63
be represented by the imagination. The supposed visual
appearances which we call two right lines enclosing a space,
a plane triangle with one side greater than the other two
&c., cannot take place on the retina, and therefore the
imagination which uses that retina cannot represent them
as taking place on it. When we endeavour to imagine two
right lines enclosing a space, we (as Mr. Mill remarks) re-
peat the scientific experiment which proves the contrary. I
must here deduce the subjective law of the imagination
from the objective conditions of the nervous organism.
But, on the other hand, I hold these laws to be necessary in
the strictest sense; for it is not in consequence of early
association that these combinations of lines and figures
have become unrepresentable. It is the original constitu-
tion of our nervous organism that renders the requisite in-
tuitions impossible; and so far as I can judge, the impossi-
bility in question arises solely from the fact, that the
organism is extended or exists in space. On this theory,
therefore, we are able to maintain the objective no less than
the subjective truth of geometrical laws, which on the a
priori theory can only be done in connexion with Idealism.
plane infinitely extended. Now, to prove Euclid's famous principle let
Fig. 3.
O 3C
/ Sºy
02 2.
the lines Oly, O, z make the angles y O, O, O, Ozz together less than
two right angles. Draw Ola, so that the angles a O,Oz, O, O,2 shall
be together equal to two right angles, and, if it be possible, let Oy not
intersect O.2. Produce the lines Ola, Oly, Ozz to infinity, and if
possible let the lines Oly, Ozz not intersect. Then the space lying be-
tween Ola and Oly bears a finite ratio to the whole plane infinitely
extended. It is therefore greater than the space a O, O,2, which
bears an infinitely small ratio to the same plane; but it is also a part
of it, which is absurd. Consequently the lines Oly, Ozz intersect. Q.E.D.
64 MAGNITUDE OF THE HEAVENLY BODIES.
The necessary laws of number are I think also con-
sequences of the laws of visual intuitions in space. For
example, the intuition necessary to prove that a x b = b x a
(an axiom which no mere time-intuition could prove) is of
this description (for a-4 b-5); . . . . . where
the number of the points is evidently the same, whether we
add them up in rows or in columns.”
I conclude with a few remarks on the apparent magni-
tudes of the heavenly bodies. Everything that we see
above us–clouds, birds, the tops of trees, houses, &c., ap-
pear largest when nearest to the zenith, for the simple reason
that they are then nearest to us, and occupy the largest pro-
portion of our visual field. Hence we come generally to
consider object near the horizon (at least if above the level
of the eye) as farther off than those near the zenith, and we
alter their apparent size to correspond with their judged
distance. Moreover the sun and moon seem fixed in the
sky, and there is no deception in thinking that the hori-
zontal sky is farther off than the zenithal. The blue sky is
in fact determined in position by the points of mean diffu-
sion of the blue sunlight which falls upon them, and these
points are much farther off near the horizon, where perhaps
there are four hundred miles of air dense enough to diffuse
a sensible quantity of light than near the zenith, where
there are not forty. This is the obvious reason why sky-
intervals look larger for the same angular measurement,
* Arithmetic is a mere art, the theory of whose processes has gene-
rally to be derived from the science of Algebra: of which the rules for
the extraction of square and cube roots afford a striking example. If the
view here maintained be correct, the axioms of geometry and algebra could
not possess immediate evidence for one who had always been totally blind,
since he could not have the kind of intuition required to prove them.
But he would, of course, believe in them as truths generalised from ex-
perience. A blind metaphysician might be able to throw some light on
this question; but most men confound familiarity with immediate evi-
dence to a degree that renders observation almost fruitless.
INSEPARABILITY OF COLOUR AND EXTENSION. 65
when near the horizon, and why the whole visible sky does
not appear to be a hemisphere, but a much smaller segment
of a sphere. In all this there is no deception, or if any, the
deception is in the opposite direction from what is commonly
supposed. The sun and moon appearing fixed in this sky
are supposed to vary their distances from us as the sky
does.
I need not here discuss the (so-called) association between
colour and extension, which has puzzled so many inquirers.
I do not regard it as an instance of association at all. The
colour and the extension which I see are both in the same
place, namely, on the surface of the retina. I cannot see
colour without extension, because colour is only presented to
me in an intuition of this extended retinal surface. I cannot
see extension without colour, because extension is only pre-
sented me in an intuition of the coloured surface of the re-
tina. I cannot imagine either of them without the other,
because imagination is, equally with perception, a veritable
affection of the retina in which colour is presented to me as
extended, and extension is presented to me as coloured.
Here again there is no delusion. On the contrary, the in-
separability of the two perceptions becomes obvious as soon
as we recognise the retinal images as the only direct objects
of sight.
This theory, then, appears to explain all the ordinary facts
in a much more complete manner than the common or
Berkeleian one. It is free from several objections which
can be urged against the former, and it is supported by some
arguments which seem applicable to it alone. On all these
grounds it appears to me entitled to a fair hearing, and
must, I think, be refuted before either the Berkeleian, the
a priorist, or the disciple of Mr. Abbott can submit his to
the public as exclusively capable of accounting for all the
phenomena. It is very far from being perfect, and very
far from being demonstrated—as, indeed, every new theory
when first submitted to the public must be—but, on the
F
66 CONCLUSION.
other hand, it seems to me quite sufficiently advanced to be
considered as a scientific hypothesis equally admissible with
that usually accepted, and equally deserving of investiga-
tion by scientific men. An analogy from the corresponding
branch of physical science may perhaps illustrate its pos-
sible importance. The emission theory of Light, like the
Berkeleian theory of Wision, reigned supreme for almost a
century after Newton proposed it. It explained most of
the known facts, and led to the discovery of others. Yet,
after all this, another theory was started—at first cautiously
and without full proof–in opposition to it. The new
theory, though supported by less famous names, soon made
progress. First, its explanation of the facts was found to
be more complete than the Newtonian; then it led to the
discovery of new facts, and the explanation of others dis-
covered by chance; and finally, experimenta crucis were dis-
covered by scientific inquirers, and the result was the
complete overthrow of the once-famous optical theory of
Newton, and the installation of the Wave-theory of Light—
now considered one of the most beautiful and far-reaching
discoveries in the physical sciences—in room of the de-
throned monarch. Whether a similar fate awaits the theory
advocated in these pages time alone can determine.
S U PPL E MENT A R Y NOT E S.
1. On the Association Psychology.
As their theory of Space and Vision is perhaps the greatest strong-
hold of the Association School, it may not he out of place to add a
few general criticisms on a philosophical system which is certainly
the most pretentious, if not the most popular, of those now current.
These observations could not have been introduced into the text
without interrupting the thread of the argument.
The Association Theory as at first advocated, was, I think, a con-
sistent and intelligible one. It pre-supposed nothing in the human
mind except a capacity for receiving sensations, and a power of
associating these sensations; from which it undertook to explain all
the mental phenomena. Its explanations however were, I think,
frequently defective. It despised what Mr. Mill terms Introspection,
which, if not the chief mode of advancing Psychology, is at all
events an indispensable preliminary step. It is absolutely necessary
to ascertain, by careful introspection, the actual characteristics of
the phenomenon we are going to explain, or else instead of account-
ing for the facts we shall account for a part of them, or perhaps for
some fiction of our own. This is the great fault of Hobbes' account
of our active principles, as Bishop Butler pointed out long ago; and
Hobbes may be almost regarded as the founder of the Association
School. It is also, according to Mr. Mill, the chief defect in Brown's
Theory of Space; and that Mr. Mill's own theory is not exempt
from it will, I hope, appear from the preceding pages. In particular,
I cannot find either in his theory, or in that of Mr. Bain, any ex-
planation of why all spaces are regarded as parts of one vast space,
or why this one vast space has three dimensions and no more.
There was also another difficulty always attached to the Association
Theory, namely, How are we to determine what elements are to be
assumed as (at least provisionally) ultimate in order to explain the
68 SUPPLEMENTARY NOTES,
the others? Suppose, for example, that a, b, c are three ultimate
elements, and that associations among them produce a fourth ele-
ment, d. In such circumstances it is pretty evident that an ingenious
theorist who started with the assumption that a, b, and d were ulti-
mate elements would have little difficulty in giving a very plausible
explanation of c; especially if he did not investigate the actual
characteristics of c too minutely. This, if I mistake not, has been
frequently done. Those who explain space by means of motion
afford an extreme instance of it, and it is still a matter of dispute
whether space is to be explained by simultaneity or vice versá.”
Again, a distinction between the “mass or volume” of two sensa-
tions has been introduced for the same purpose. But it seems to me
very probable that this distinction is dependent on our space-per-
ceptions instead of the contrary. I doubt if a blind man could say
whether the sensation arising from putting his foot into hot water up
to the ankle was more or less voluminous, (if he could attach any
meaning to this phrase,) than that which arose from submerging his
arm to the elbow. My knowledge of spacial magnitude enables me
to ascertain that the immersed portion of my body is greater or
smaller in the one case than in the other, and I judge of the mass
or volume of the sensation accordingly.
But ordinary Association was found insufficient to accomplish the
task set before it, and a new kind, designated Chemical Association,
was called in to supply the deficiency. It was now assumed that
a collection of ideas intimately associated with each other ulti-
mately became fused together, and gave rise to a product, in which
the separate elements could no longer be discerned. The analogy
to chemistry, however, was evidently incomplete. The chemical
elements are permanently-existing substances which continue to
exist in the compound, and into which the compound can again be
resolved; but our sensationsan d ideas are not permanent entities,
but fleeting existences, whose esse is percipi, and which cease to
exist when they cease to be perceived. The resultant idea could
not, therefore, be regarded as composed of the simple ideas in the
same sense, that the compound body is made up of the simple
* Mr. Mill's arguments on this latter question would not appear so formi-
dable, if he had distinguished between a complex sensation whose elements
are known, and two or more distinct simultaneous sensations.
ON THE ASSOCIATION PSYCHOLOGY. 69
chemical elements. This difference is manifest in one most im-
portant particular. The (supposed) compound cannot be again
resolved into its (supposed) elements; while this is precisely the
characteristic that distinguishes chemical composition from ordi-
nary causation. The chemical compound, too, may be formed
the first time that the elements are brought together, as easily as
"on any subsequent occasion, but the resultant idea is not supposed
to arise until repeated juxta-position has produced an intimate
association among its elements. The asserted analogy is therefore
confined to one or two subordinate features, and fails in what is
most characteristic of the chemical phenomena. This may be con-
sidered an objection rather to an illustration of the theory than to
the theory itself; but the truth is that what enables us to prove the
existence of chemical composition are precisely the features that are
wanting in this alleged mental chemistry. We believe that water is
composed of oxygen and hydrogen, because we can mix these gases
and convert them into water at once by the passage of an electric
spark, and because we can again convertwater into oxygen and hydro-
gen by sending an electric current through it. If there was no possi-
bility of performing either experiment we should never have
suspected that water was a compound, still less could we have
ascertained that it is made up of oxygen and hydrogen. Now this
is the position in which the Associationist finds himself. He does
not know what ideas are elementary, and even if he has reason to
suspect some of them to be compound, he has no means of ascer-
taining their composition. In this exigency he resorts to conjecture
and hypothesis. I am not certain how far Mr. Mill and Mr. Bain
regard the idea of space as analogous to a chemical compound,”
(rather than a mere mechanical juxta-position of the elements;) but
I may be permitted to make that supposition for the sake of
illustration. How is space shown to be a compound 2 For if it is
admitted that the elements are not separately perceptible in it, this
cannot be discovered by introspection. If it is a compound, how are
we to ascertain the elements? Only by hypothesis, so far as I can
see. Again, how are these elements proved to be simple 2 for the
chemical theory supposes that an idea may appear simple and yet be
really compound. Lastly, how is it proved that space does not form
* I think, however, that Mr. Mill has said so in some of his works.
70 SUPPLEMENTARY NOTES.
an element in their composition? for on this theory it might form
an element in them, without being detected, (not to mention that the
opponents of the theory profess to have actually detected it.) In this
particular case, too, the elements are not proved to have been in the
mind before the idea of space, which latter idea seems to be in the
mind of the infant as soon as it is accessible to observation. Nor
are the supposed elements proved to have been in the infant's mind,
and to have been subsequently lost. On the contrary, all the
sensation-elements continue to be found in the adult man. The
elements, then, do not disappear into the resultant as in chemistry.
The whole theory seems to have been invented, in order to make out
an explanation of all the phenomena of mind by the single principle
of Association. But in reality, even granting its truth, it fails to
accomplish this purpose; for this (so-called) Chemical Association
is manifestly a totally distinct principle from ordinary Association,
and ought, therefore, to have a distinct name of its own. There is
no extension of the principle, but only an extension of the name.
And, indeed, this remark will be found to apply to a large propor-
tion of these professed discoveries. When, for example, we are
told that a concept or a judgment has been proved to be a “case
of association,” it will usually be found that these and the like
operations have merely been analysed in two or more elements
and a relation or connexion found between them; which every one
knew before. And in the same category I should be disposed to
place the great modern discovery of Relativity. No one who has
read Bacon ought to imagine that Psychology can be converted
into a science by means of this kind of aſciomata generalissima.
I, therefore, hold the Chemical Association theory to be un-
proved, and that the proof of it is surrounded by difficulties, which
its advocates have in a great measure failed, not merely to overcome,
but even to apprehend. But I do not mean to reject it as either re-
futed or as unprovable.” There is one way in which I think it might
* Mr. Mill would, I suppose, contend that this Chemic il Association was
proved by the instances produced by him, in which the elements of cogni-
tions disappear in the wholes, provided the latter alone interest us. (Exam. of
Hamilton, p. 315.) His chief example is the act of reading a book in which
we begin with the letters and syllables, which almost entirely drop out of
subsequent consciousness in the representation of the “general purport of the
discourse;” just as we lose the individual leaves in retiring from a forest. But
ON THE ASSOCIATION PSYCHOLOGY. 71
be proved. We might be able to detect the elements first in mere
juxta-position, then slightly associated, then more and more closely
connected, and at last, in a kind of incipient fusion, which left no
doubt that the resultant idea was the final stage of the process we
had been tracing. It might also attain a high degree of probability,
if it were ascertained that there were certain sensations associated
with each other in the child's consciousness before the idea we
sought to explain was found there, and that these sensation-groups
had disappeared as soon as the (supposed) resultant was ascertained
to exist. But I do not think that this has hitherto been done; and
the process, if proved to exist, would rather resemble the growth of
a plant or animal than chemical composition. And moreover, it
might be found that the development of the ideas depended on
laws quite distinct from those of Association.*
But the old Association Theory was only a consistent theory of
the mental phenomena, on the condition that the sensations with
which it started needed no Psychological explanation. Assuming
these, it derived the laws of belief and imagination from them at
once. We believed this because we had always experienced it to be
so: we could not conceive that because we had no experience of it.
But here the Associationist really takes for granted more than the
mere sensations. In fact, he assumes what Mr. Mill does in his
explanation of our belief in the existence of an external world,
namely, “1st, sensations; 2nd, succession and simultaneousness of
sensations; and 3rdly, a uniform order in their succession and
this example seems to me to involve two errors. For, first, we do not
begin with the individual letters, but on the contrary, we limit our attention
to them by a special cffort; indeed, we have seen Mr. Mill himself adopting
Mr. Bain's statement that “our notions of form are manifestly obtained by
working on the large scale.” (Id. p. 291.) And secondly, if the “general pur-
port of the discourse” is a whole, (rather than a selection of the more im-
portant parts.) it is certainly not the whole, of which the written characters
are parts; and my idea of this whole does not seem to be clearer than my
ideas of the individual characters. Nor can I see any force in his analogous
case of sensations. (Id. p. 308.) For in this case of the revolving cylinder,
though the causes of the various sensations act in rapid succession, the sensa-
tions themselves (in consequence of the well-known persistence of visual
impressions) are rigorously co-existent; and therefore the effect is the same
as that which would be produced by the co-existence of their causes.
* See on this theory, Mansel’s Metaphysics, p. 247.
72 SUPPLEMENTARY NOTES.
simultaneousness.” (Exam. of Hamilton, p. 249.) Here a doubt
arises. An uniformity of succession or co-existence among sensa-
tions is in other words a law of sensations. Ought you not, then, to
assign a place to this law in your Psychological system 2 To this
the older Associationist replied—No. It is not a law of mind at all.
There is a real world existing without me, which acts on me, and
produces these sensations; and the uniformities of succession and
co-existence which they exhibit are the results of the laws of
this external world, They are, therefore, out of the province of
Psychology. In the mind there are but two things, a capacity for
receiving sensations, and a power of associating them. What par-
ticular sensations will be received, and what uniformities these
sensations will exhibit, depend on something quite distinct from the
mind; and the necessities of thought, which our opponents represent
as mental laws, are produced by these laws of the external world
through the medium of Association. From the realistic point of
view, this answer was quite satisfactory, and moreover, it seemed to
to afford a mode of escaping from the idealistic conclusions of Kant.
That philosopher had inferred from the necessity of geometrical
judgments that they depended on laws of mind, and that therefore
space, and everything which it contained, existed in the mind only.
But the disciple of Hartley could answer—The necessity of the
judgment only proves the uniformity of the experience on which
it rests, and this uniformity of experience is the surest indication
of a law of the outer universe. Geometry, therefore, is univer-
sally true of objects independent of the mind, and space is a
property of these objects.
But when Associationists began to investigate how Association
operating on mere sensations might give rise to the belief in a real
world independent of us, the conditions of the whole problem were
altered. The existence of such a world could no longer be assumed.
Indeed the explanation showed that we should equally believe in
it if it did not exist. But the theorist was still obliged to
assume uniformities of succession and co-existence among his sen-
sations; and as these sensations existed nowhere but in the mind,
and did not depend on any non-mental causes, their uniformities or
laws plainly became the subject-matter of Psychology. Nothing
exists but actual and possible sensations, and their existence consists
ON THE ASSOCIATION PSYCHOLOGY. 73
in their being actually or possibly perceived. They and their laws
belong exclusively to mind. This consequence the Idealistic As-
sociationists seem to me to have entirely lost sight of. They
laboriously seek to generate from Association operating on these
uniformities the very laws which form the scientific expression of
the generating uniformities themselves. Thus, for example, we
meet with a long discussion, endeavouring to deduce the necessity
of geometrical laws from Association. I cannot conceive two right
lines enclosing a space. Why? Because I have always experienced
them diverging, and hence the idea of divergence becomes in-
separably associated with that of two intersecting right lines; and,
in consequence, I cannot now conceive them as approaching again.
But why is my experience uniform 2 or rather, what does this
uniformity imply? Simply this, that my mind is so constituted
that I cannot (or, at all events, do not) perceive two right lines
enclosing a space; and this being granted as a law of sense, it is
much more philosophical to extend this law to explain the similar
phenomena of imagination, than to suppose the latter governed by a
different law, generated by a slow process of Association operating
on the former. When the motions of the earth round the sun were
traced to the combination of a centripetal and centrifugal force, it
was surely more philosophical to trace the similar motions of the
moon round the earth to the same sources, than to suppose that
the latter did not gravitate, but had a tendency to imitate the
motions of surrounding bodies, which, as its imitative faculty grew
stronger, gradually became more accurate, until at length it came
to pursue the precise path which it would follow if subject to the
law of gravitation. In this particular case, moreover, Mr. Mill sets
up an explanation which I think inconsistent with the Association
Theory; namely, “the exact resemblance of our ideas of form to
the sensations which suggest them.” (Logic, vol. i. p. 262.) If this
explanation be correct, we could never have imagined two right
lines enclosing a space, and the impossibility was not, therefore, a
gradually-acquired one. But here again the resemblance is as-
sumed as a fact, without referring it to its proper place in a system
of Psychology. Kant accounts for it. Space, says he, is a law
of our faculty of intuition, and imagination being a branch of
this faculty, is, of course, subject to the same laws. Hence geo-
74 SUPPLEMENTARY NOTES.
metrical truths are equally binding in perception and imagination,
and in both cases for the same reason. Is not this, at all events,
simpler than assuming separate uniformities of co-existence among
our sensations for each of the fundamental laws of geometry; and
then either a long associating process to generate similar uniformities
of co-existence among the corresponding imaginations, or else a new
law of resemblance between them limited (at least in this degree) to
this particular instance? It is the office of Philosophy not to mul-
tiply laws, but to reduce them to the smallest number. Imagina-
tion has often been described, even by Associationists, as a kind of
weak sensation or decaying sense, while it is now believed that its
representations take place in or by means of the sensitive organ.
What is more natural, then, than to suppose that the laws of our
sensations embrace the products of imagination also 2 But if the
reader wishes to see a complete theory of the subject, he should
take up Kant's Transcendental Analytic, where he will find the
laws of sensations assumed by Mr. Mill reduced to a few general
heads, and connected with the laws of thought recognised by
logicians in a manner that, in point of systematic connection,
completeness, and simplicity, is as far superior to the Association
Psychology as the Copernican Astronomy is to the Ptolemaic. I
do not mean, of course, to affirm the truth of Kant's conclusions.
Those of the Associationist may ultimately prove more nearly
correct. But the Association school boasts loudly of the scientific
character of its method and its results, and these are exactly the
points in which it seems to me most glaringly deficient. It may,
perhaps, have got hold of a sound theory. But its nomenclature
is faulty; it fails to see the conditions of proof, assumes what ought
to be explained, leaves the consequences half thought out, and is
wholly wanting in system. It professes to adopt the method of
Physics, but presents us with speculations that no natural philo-
sopher would tolerate, even if he did find it necessary to cut the
theorist short with some such remark as-“You undertook to give
me a theory of dew, and you are accounting for rain.”
Mr. Mill thinks that (with perhaps the exception of the principle
of contradiction) all our present mental necessities or inconceiva-
bilities would cease, “if our minds were the same, but our ex-
perience different.” (Exam. of Hamilton, p. 84.) What does this
ON THE ASSOCIATION PSYCHOLOGY. 75
mean? Have our “minds” nothing to do with our “experience?” Is
not our experience, on his own showing, merely so many actual or
possible mental sensations, whose laws are laws of mind? He gives
us, however, some specimens of the experiences that would be
required to enable us to conceive the reverse of truths commonly
regarded as necessary. Suppose that a minimum visibile was not
increased in magnitude, when we used a powerful microscope to
examine the object, we should be able to conceive a limit to the
divisibility of matter. (Id., p. 102.) The minimum visibile is, I
think, a quantity of light not of extension, so that Mr. Mill starts
with an erroneous assumption. But waiving this objection, let us
examine the case he puts. He takes an object containing 100 minima
visibilia, and puts it into a microscope magnifying fifty times, and
then he tells us that if each minimum visibile looked the same size
as before, and if the whole object made up of the 100 minima visibilia
looked fifty times larger, we should believe in a limit to the divisi-
bility of matter. Certainly; and if the same minimum visibile was
presented to me as all black and all white at the same instant, (see
Exam. of Hamilton, p. 84,) I could go a step further, and dis-
believe the principle of contradiction (for I fear I must always re-
gard black as not-white.) But it seems to me that so long as “my
mind is the same,” both these “experiences” are simply impossible.
My experience, I believe, is in part determined by my mind; and
Mr. Mill, on his idealistic principles, is bound to maintain that
it is so entirely. The -barrister, I think, makes a similar blunder
when endeavouringgo prove that the reverse of arithmetical truths
might be made conceivable by a different experience. He supposes
that whenever I attempt to put two and two together, a new object
is “created and brought within the contemplation of my mind,”
(Id., p. 86,) and argues that in this case I would believe that 2 and
2 make 5. But while my mind is the same, I cannot be com-
pelled to add the new object, thus brought within its contemplation,
to the 2 and 2 which I am seeking to put together; and even if my
mind was so altered as to make this addition compulsory, I
could not fail to see that what I had added was not 2 + 2, but
2 + 2 + the new-created unit; so that I would have really arrived
at the harmless result, 2 + 2 + 1 = 5. Will Mr. Mill or his
barrister maintain that in a world, where the two pairs were anni-
76 SUPPLEMENTARY NOTES.
hilated whenever I attempted to add them, 2 + 2 would be = 0 °
Some of Mr. Mill's other instances are mere tricks with words. If I
hold a circle sideways it appears to be an ellipse, and if a
man had never felt or seen it in any other position, he would
use the term “circle” with the same connotation as my term
“ellipse,” and he would, of course, believe that the radii of a
circle are unequal. Translated into my language, this would
amount to conceiving that the radii of an ellipse are unequal,
which I can conceive with equal ease, though my experience
has been different from his ; and as he would not have my idea of .
a circle at all, he could not conceive anything about the equality or
inequality of its radii. This is exactly the trick that the barrister
performs with his right lines, (Id., p. 86,) and that Reid enlarges on
in his Geometry of Visibles (cited, p. 87.) If right lines looked
like circles, we should ascribe to them the properties of circles, at
least if we had no means of correcting the mistake. Accordingly, to
Reid's Idomenian, it would be a “self-evident truth,” that “any
two right lines, being produced, will meet in two points;” but the
Idomenian would mean by this proposition exactly the same thing
that I mean by the proposition, that “any two great circles described
on a spherical surface will meet in two points;” which is to me a
self-evident truth likewise. Reid never supposes that the Ido-
menian would ascribe any property to his right lines that I do not
ascribe to circles on the sphere, and thereby he recognises the
necessity of geometrical truths, if we only take care that the same
term shall have the same meaning. Had Mr. Mill forgotten
his own doctrine that the meaning of a word resides in the conno-
tation, not in the denotation ? I fear Helmholtz has here fallen
into the same mistake.
I may further observe, that I am not yet satisfied that Association
can generate necessary beliefs. Mr. Mill's examples (Exam. of
Hamilton, chap. xiv.) are not examples of necessary beliefs or judg-
ments at all, and it is only in judgments that necessity is generally
insisted on as establishing a mental law. They are, moreover, in
general instances of a “could not,” not of a “must,” and it is, I
think, conceded that a present inability need not be a priori. For
instance, whenever there is a series of acts to be performed, which
requires memory, anything that confuses the memory may produce
ON THE ASSOCTATION PSYCHOLOGY. 77
an inability to perform it; and as memory is admitted, on all hands,
to depend chiefly on association, a change in the usual associations
will naturally produce an inability here.” There is something
similar to this in positive necessity also. One idea will recall
another which is associated with it, in spite of all our efforts to
resist it. In fact, forgetfulness is even less in the power of the will
than memory. But though the suggestion is involuntary, I am not
compelled to predicate one of these ideas of another. I may be
unable to think of Mr. Mill without thinking of Mr. Bain, but that
does not compel to judge that Mr. Mill is Mr. Bain, or even that
the two philosophers are inseparable in existence. What the Asso-
ciationist is called on to explain is the characteristic of necessity in
judgments, and this is exactly the kind of necessity which he avoids
giving instances of. It may be that in every familiar judgment
the subject and predicate are associated; but the problem which
Mr. Mill and his followers have to solve is not why they become
associated, but why the association between them takes this parti-
cular shape. There are pairs of phenomena, between which the
association is most intimate, but which we never dream of combin-
ing as the subject and predicate of a proposition. Consequently,
if this be a “case of association,” it is, at all events, a peculiar
case, and it is just this peculiarity which requires to be accounted
for.
In conclusion, I may remark, that the earlier Berkeleians main-
tained that our immediate visual sensations were at once re-
placed by the suggested tactual ones, and were themselves either
unperceived or instantly forgotten; and they were quite satisfied
with the explanation of this substitution, which the principle of
Association afforded. The later advocates of the theory, as we have
seen, admit that the present representation is wholly an “eye-
picture;” and they call in the same potent principle to explain why
* I may remark that Mr. Mill is in error in regarding necessity and incon-
ceivability as correlatives, for under the law of the conditioned, two contra-
dictories may be both inconceivable. The necessity of so thinking, of course,
involves the inconceivableness of the contrary, but the inconceivableness of
a judgment does not involve the necessity of the opposite. This confusion
leads Mr. Mill into several unfounded criticisms on Hamilton, in whose sys-
tem the distinction of positive and negative necessity (inconceivability) is all-
important. (See Lect, vol. ii. p. 366, seqq. and 527.)
78 SUPPLEMENTARY NOTES.
the suggested tactual sensations either do not rise into conscious-
ness at all, or else leave no trace in the memory. When we find
the same principle invoked with equal confidence to explain two
directly opposite (alleged) phenomena, the natural presumption is
that it really explains neither; while, on the theory of Messrs. Mill
and Spencer, the Natural Geometry, which Berkeley treated so
contemptuously, has to be replaced by a still more complicated
Natural Algebra. Such explanations are, to say the least, eminently
unscientific.
2. On Colour-Blindness.
Some persons may perhaps consider the case of the colour-blind a
strong argument against the theory that the perception of a variety
of colours involves that of extension; since it may be said, here are
men who can see no variety in colours, and whose space perceptions
nevertheless appear quite as accurate as those of persons who do.
But all observations on colour-blindness lead to the conclusion that
the patients do perceive a diversity of colours (and this not mere
gradations of the clare-obscure, as Sir W. Hamilton supposes)
though they are unable to discriminate between some particular
colours which ordinary men distinguish with ease. Thus the man
who confounds red with green will often distinguish yellow and
blue with much accuracy. It is not my intention here to make any
hypothesis as to the nature of colour-blindness, (which, however, I
have done in the numbers of Nature for July 28, and October 6,
1870,) but merely to notice that we here possess what Mr. Mill
wishes to obtain in the case of the blind—a colour-blind metaphy-
sician. Dugald Stewart could see no difference in colour between
the green leaves and the scarlet fruit of the Siberian crab, though
he could perceive the difference of shape perfectly. (Nunnely, on
Organs of Vision, p. 336.) Yet Stewart stated in one of his
earliest letters, and repeated it in his latest work, that he could
not subscribe to the doctrine of Reid on this subject, “because it
appeared to him evident that the varieties in our perception of
colour are the means of our perception of visible figure.” (Disser-
tation, note, p. 134.) It is strange that with this letter before him,
Sir W. Hamilton should have in his Lectures credited Stewart with
the doctrine that we have no perception of extension by sight,
ON COLOUR-BLINDNESS. 79
(Lectures, vol. ii. p. 161,) a statement which he qualifies in his edi-
tion of Reid, (p. 920,) by saying that it was tacitly withdrawn in
his later works; whereas, in the note already referred to, Stewart
is professedly quoting a letter which he had written to Reid “more
than forty years before.” It is true, indeed, that Stewart was
of opinion that the inseparability of colour and extension in the
imagination was a mereresult of association; but then this association
arose from the fact, that the varieties in our perceptions of colour
were the means of our perception of visible figure. If he had been
of opinion that the imagination employed the sensible organ, he
would probably have dispensed with association altogether. But
at all events it is evident that he thought the perception of a variety
of colours necessarily led to the perception of extension, and that
he found the two ideas of colour and extension as inseparable as
anyone else. I think his testimony on these two points affords a
strong confirmation of the theory which I have been advocating.
3. Observations on Couching.
I venture here to make a few remarks on Observations on
Couching, because I think they have not hitherto been made with
sufficient care. For example, when the newly acquired sense is
tested with a cube or a pyramid, it is forgotten that such an object
will look differently when it is seen from different points of view,
and that the patient might very possibly fail to recognize its identity
if successively presented to sight under a diversity of circumstances.
It would be much better to employ discs than solid bodies, though
even here the visual appearance would vary with the inclination of
its sides (if a rectilineal figure) to the line of sight. A circle and a
square should therefore be substituted for the sphere and the cube,
and it should be ascertained that the patient recognised the identity
of the square when presented to him at various angles. We should
next endeavour to make him generalize these perceptions, and
abstract from the particular colour and magnitude of the presented
objects, for which purpose sveral circular and square discs of
different sizes and colours should be successively (or simultaneously)
presented to him. It would also be desirable to quicken his faculty
of generalization by giving them general names; and in so doing,
names should be selected which were quite free from all previous
associations. Suppose, for example, (as the patient would not be
80 SUPPLEMENTARY NOTES.
likely to know Greek,) the square discs were called Alphas, and the
circular discs Betas. Care having been likewise taken to secure
the formation of the requisite touch-abstractions, (by handling
several circular and square discs of various sizes, weights, and de-
grees of hardness, roughness, &c. without seeing any of them,) he
might then be asked whether an Alpha or a Beta was like anything
he had been accustomed to feel? and if he failed to recognize
the identity, he might be asked if either of them was like a circle
or a square 2 If after several trials the patient was found incap-
able of recognizing the identity of the objects seen and the objects
felt, it would prove that the perceptions of the two senses were dis-
similar in kind; but it would still remain undecided which sense-
perception was identical with that which ordinary men designated
space or figure. If the patient was asked some months afterwards
whether his ideas of circles and squares had been altered since he
obtained his sight, or whether the meaning that he now attached to
these terms was a visual or a tactual one, it would throw some light
on this subject. The meaning which he attached to these terms
immediately after couching, would of course be the same that he
had attached to them before couching, viz., a tactual one; conse-
quently no inference could be drawn from his at first applying
these terms to tactual, and denying them of the visual objects,
except that the two senses did not convey the same kind of percep-
tions. It is too often forgotten that different men may employ the
same terms with the same denotation, but with a totally different
connotation;” and this at all events may be the case with the space-
* Nothing, perhaps, has led to more error and confusion in Psychology
than this oversight. Instances of its ill effects have, I think, been pointed
out in the preceding pages. Thus, if Mr. Mill means by space something
which the hand could be moved across, and if by moving the hand he means
a succession of muscular (not visual) sensations, to say that sight cannot
give us a direct perception of space is an identical proposition, and it is a
mere waste of time to spend more than three lines in proving it. But then
I do not think that these are the meanings which ordinary men attach to the
terms Space and Movement. To take another instance. We often meet
with an argument to prove that some proposition—the principle of Causality
suppose—is not a necessary truth, because some one has disbelieved it, when
it will be found on examination that the believer and the disbeliever merely
attach different meanings to the same form of words. This, as we have seen,
is the explanation of the fact (if it be one) that Reid's Idomenian would find
no difficulty in believing that two right lines enclosed a finite space. Again;
OBSERVATIONS ON COUCHING. 8].
terms of the blind man and the seeing man. What I mean by
“circle” and “square” might very well be what the newly-couched
man would mean by “Alpha” and “Beta” in the case I have put,
though he would also use the words “circle” and “square,” and
apply them to the same objects that I did. I should also prefer a
triangle and a square to a circle and a square. The uniformity of
the circular outline would give considerable scope for guessing if
the patient was at all quick. The whole subject requires much
more attention than it has hitherto received.
in the controversy as to whether children commence with universals or sin-
gulars, philosophers have confined themselves almost entirely to inquiring
whether they begin by using terms which are to us universal or singular;
which probably depends altogether on their instructors, and not on any pecu-
liarity of the infant mind. But perhaps the most remarkable example of
this confusion occurs in Mr. Lewes's criticism of the Kantian distinction
between analytic and synthetic judgments. This writer concludes that there
is no real difference between them, (synthetical judgments being only “ana-
lytical judgments in the making,”) because the same proposition—that is,
the same form of words—that originally expressed a synthetical judgment
(sometimes) comes afterwards to express an analytical one, in consequence of
a change in the meaning of one of the terms employed. Thus, that “All air is
heavy” was originally (and I think still continues to be) a synthetical pro-
position; but in the course of time the word Air may come to include Weight
in its meaning, (connotation,) and whenever it does, the above proposition
will express an analytical judgment. This analytical judgment translated
into my former language would be “All heavy-air is heavy,” and my former
assertion that “All air is heavy,” translated into my new dialect, would be
“Whatever possesses the other attributes of air possesses the attribute of
weight also.” At the basis of the whole theory, therefore, there lies nothing
more than that men may attach different meanings to the same form of
words at different times. When Weight becomes a part of the meaning of
the term Air, of course an opponent could not deny that the air was heavy; but
Mr. Lewes seems to have forgotten that he might still deny that any such
thing as air existed. He might very well argue that the thing that blew in
his face was not Air, since it was not heavy; and if Mr. Lewes wished to
convince him of his error, he could only do so by proving that that which
blew in his face was heavy—a synthetical proposition. Mr. Mill has done
good service in refuting similar doctrines advanced by Brown (Logic, vol. i.,
p. 224) and Hamilton. (Exam. of Hamilton, pp. 411, seqq.) Even Mr. Mansel
is not exempt from this confusion, (Prolegomena, p. 213,) in spite of his
distinctionibetween intuition and thought. Let me add, that if any such
complex motion is formed, it does not become the subject, but the pre-
dicate of the judgment. I do not judge that All heavy-air is heavy, but that
All air is heavy-air—a judgment which is not only synthetical, but is merely
the fuller expression of what I originally judged.
F
82 SUPPLEMENTARY NOTES.
4. Some Remarks on Mr. Abbott's Theory.
Mr. Abbott would, perhaps, rather maintain that we see (origin-
ally) the difference of the distances from the eye of two objects than
the actual distance of either of them. (See p. 27 of this Essay.)
But this supposes that the real external objects are immediately
perceived, since I presume it will be granted that we cannot see the
differences of distance of two things, unless we see the things them-
selves. This doctrine has, therefore, I think, been sufficiently re-
futed in the foregoing pages. Indeed, the mere change of size in
the apparent object, as we move away from it, affords a sufficient
refutation, as Hume observed long ago. If Mr. Abbott thinks that
what we see is not the difference of the two distances, but its
“projection on the line of sight,” this is only to make the theory
more complicated without removing any of the difficulties.
Our author might also defend the veracity of our intuitive con-
sciousness by asserting that the entoptic appearances, &c., whose
apparent externality he insists upon, (Sight and Touch, p. 79.)
were really external to the sensitive surface of the eye. But I do
not think he could maintain that “the subjective appearances in
the closed eye,” or the “persistent impressions,” (whose externality
is confessed to be equally apparent,) are really external to the sensi-
tive organ. Further, the same apparent externality, and even
distance evidently belongs to the visual images presented in a dream,
and I think also to those produced by other causes, such as pressure,
electricity, inflammation of the nerve, derangement of the stomach,
&c. In these latter instances there is, at all events, no visible
object external to the retina, and there is no reason to believe that
the visible appearance exists anywhere except on the sensitive sur-
face. That it has a spacial cause I do not doubt; but since this
cause is situated rather behind than before the retina, and bears no
resemblance to the impression, it is impossible to regard it as the
object seen. And here I may notice, that there is a certain am-
biguity in the word “seen;” sometimes it is applied to the cause of
vision, and sometimes of the visual appearance itself. If we take
it in the former sense, the eye sees other things, but does not see
itself; whereas, if we take it in the latter (which I think is the proper)
sense, on my theory, the eye sees itself and nothing else. Nothing
oN MR. ABBOTT's THEORY. 83
I hold is seen perceived or intuited, except the retinal affection.
But I do not think that mankind speak of all causes of vision as
objects seen. They apply the term rather to one particular kind
of cause—namely, that which causes vision by throwing light on
the retina; and this for the reason that they usually believe that
this object (and not the affection produced by it) is what we im-
mediately perceive. But when the cause produces vision other-
wise, (as is the case with electricity, disease, &c.;) and when there is
no similarity between cause and effect, no one speaks of such a cause
as seen. Even the vulgar would here apply the word “seen" to
the affection itself, as I would do in all cases. To resume, then,
the affection possesses the same apparent outness, and even distance
in this case as in others; and, therefore, if outness and distance are
equivalent to being outside of, and distant from, the retina, we are
obliged (at least if we do not call in the Association Principle
which would be fatal to Mr. Abbott) to regard direct consciousness
as in this instance delusive. But on the explanations of outness
and distance that I have given the delusion disappears; or rather
it is resolved into another illusion, whose origin I have accounted
for consistently with the veracity of our direct perceptions, viz., the
illusion that there is an eye behind the collection of visual pictures.
But this eye is not supposed to be immediately perceived—it is
inferred in consequence of our mistaking the pictures of objects
(especially of the members of our body) for the objects themselves.
We know that the eye is behind these objects, and thence infer
that it is behind the pictures of them ; which inference we extend
to all visual pictures, (or rather affections,) and are thus occasionally
led into error.
Mr. Abbott lays much stress on the perception of depth (or relief)
which arises when we combine two dissimilar pictures, and regard
the object as one. It must be remembered, however, that the two
pictures must be very nearly equal, similar, and similarly situated,
or it will be ſound impossible to combine them. Further, I think
that those who endeavour to combine them (especially when near
the limit) will be conscious of a considerable effort in doing so—
the two outlines having a tendency to separate from each other. This
is more evident with the naked eye, as the reader will find, if he tries
it on Mr. Abbott's figures. (Sight and Touch, p. 113.) It is not
unreasonable to suppose that some change is effected in the shape
84 SUPPLEMENTARY NOTES.
of one or both eyes by this effort, in consequence of which the two
pictures really fall upon identical points, and that single vision in
this case is therefore referable to the ordinary law. Why such
double images suggest relief is evident. We have been ascertaining
the existence of relief in all objects which require this effort to com-
bine them from our earliest infancy, while we have never perceived
it in those which are seen as one without any effort. I do not say
that relief is suggested by the duplicity of these objects, since they
are but rarely seen as double; but that is suggested by the effort
which is required in order to see them single, and by their tendency
to become confused at the boundaries.
The object of my quotation from Mr. Abbott, at p. 56 of the Essay,
will, I think, be evident to the reader; but to avoid misapprehen-
sion, I may observe that my language, in describing the same facts,
would be almost the converse of his. He accounts for angles less
than a right angle appearing too large, because, in consequence of
the spherical shape of the retina, (in passing I may remark that if
we perceive the external objects and not the retina, I do not see
how the shape of the latter could affect vision at all,) “the images
received on it cannot be projected on a plane without distortion;”
(Sight and Touch, p. 82;) while, in accounting for the same fact, I
would say, that in consequence of the spherical shape of the retina,
lines drawn on a plane cannot be projected on it without distortion.
Without examining the mathematical phenomena of projection in
detail, I may observe, that the three angles of a spherical triangle
(taken together) are always greater than the three angles of a plane
triangle; and, therefore, if (as I hold) the retinal picture is im-
mediately perceived, angles in general will appear too large. I think
a geometrical reason could also be given why this enlargement is
greater in the case of acute angles. The area of a spherical tri-
angle being proportional to the excess of its three angles over two
right angles, the sum of the three angles of the external plane tri-
angle, will be least augmented when it is so held that its projection
on the retina shall be as small as possible. An extension of this
remark will show why the illusions exhibited at p. 83 of Mr. Abbott's
book disappear when we look along the lines. The retinal projec-
tion of any line exhibits fewest traces of sphericity when it is
shortest, and most when it is longest.
The erectness or inverseness of vision, if satisfactorily ascertained,
ON MR. ABBOTT's THEORY. 85
would afford an eaſperimentum crucis between my theory and that of
Mr. Abbott. The reader will not confound my views with the
current explanation of the (assumed) fact of erect vision, on the
ground that when everything is inverted relative position remains
the same. This, it has been justly observed, does not account
for our seeing things erect, but merely for the inversion of the
pictures (which is requisite for erect vision) escaping our notice.
But instead of attempting to explain the universally assumed fact of
erect vision, I deny it altogether; and as Mr. Abbott's theory
affirms it, we are here directly at issue. I hold that if we could
intuite the real external objects, we would find them turned in the
opposite direction from what we now suppose, and likewise enor-
mously larger. In proof of the inversion, I think Scheiner's experi-
ments are conclusive. An illuminated point being held so near
the eye that it produced the impression of a confused circle was
looked at through three pin-holes in a piece of card. Three
images of the point (the head of a pin held near a candle will do)
were thus seen, and when the left pin-hole was covered, the right
image or picture disappeared, and vice versa. A similar experiment
may be made with one pin-hole, when, if we move the card up or
down to right or left, the luminous point seems to move in the
opposite direction. Another experiment, I think, confirms the same
conclusion. If, with my eye open, I press the outside of the eye-
ball, (next the cheek-bone,) a luminous ring appears which un-
doubtedly belongs to the impressed part; but it seems to be at the
inside of the field of vision next the nose. On pressing the inside
of the eye-ball the effect is reversed. If these observations are
conclusive, Mr. Abbott's theory is refuted.
As persons couched for cataract are always deprived of most of
the means by which Mr. Abbott thinks we naturally perceive dis-
tance and relief, (see Sight and Touch, p. 140,) the fact that they
appear to be devoid of these perceptions immediately after the
operation, cannot be regarded as a refutation of his theory. I
hardly think, however, that the evidence of the fact is fairly stated
in his tenth chapter. He argues, for instance, (page 147,) that
Cheselden's patient must have seen relief in solid objects before he
noticed it in paintings; for “if pictures had never (before this
time) conveyed the idea of solid figures, it is clear that solid figures
tº a º
ſº
; : * >
tº
86 SUPPLEMENTARY NOTES.
were not seen as plain pictures.” But Cheselden's patient had re-
cognized his father's likeness in a locket long before, and these
pictures were probably landscapes, the various objects in which it
took him some time to familiarize himself with. Again, in describ-
ing Franz's case, (page 158,) he lays much stress on the fact, that
the patient noticed at first sight that the sphere and the cube pre-
sented to him “were not drawings,” while he notices but very
slightly his repeated statements as to the flatness of all the objects
which he saw; and it should not be forgotten that he failed to
distinguish the sphere and cube from circular and square discs.
This patient, as I have already observed, could see, previously
to couching, a small patch of light through a lateral cleft in
the capsule; and, therefore, when the light was suffered to fall
on the whole retina instead of on this patch, it is not surprising
that he decribed his sensation as “an extensive field of light,” or
that objects appeared larger than before. Mr. Abbott again calls
attention to certain patients placing their hands close to the eye
and then moving them out in straight lines toward the objects;
but he does not observe that others continued groping about the
eye for something at a distance from it, or that Mr. Wardrop's
patient often placed her hand beyond the object and then moved it
inwards toward the eye. Franz's patient also continued groping at
the surface of a vessel of water for a ball sunk several inches be-
neath it. With these qualifications, I think, the cases reported by
Mr. Abbott will be found in perfect agreement with my views.
There can be do doubt that the patients afterwards came to see
distance and (in some degree at least) relief; and the fact that
their visual perceptions improved in this respect proves that asso-
ciation had something to do with the process; whether, as I think,
by connecting different visual impressions with each other, or, as
the Berkeleians maintain, by connecting visual with tactual and
muscular sensations.” The incident of the pictures would alone
* Having touched again on the common theory, I may here make a remark
which I omitted to make in its proper place. Even supposing that our pre-
sent visual perceptions arise from an association between the original sensa-
tions of sight and those of touch and the muscles, the current doctrine seems
needlessly complicated. A double association is postulated by Messrs. Mill
and Bain; first, between the visual sensations and the muscular sensations
which accompany the motions of the eye; and, secondly, between these mus-
º tº Q
º
º
*s :- tº C G - e.
G
• * *
ON MR. ABBOTT's THEORY. 87
suffice to establish this. But if distance from the eye and relief
are acquired perceptions in these instances, the natural presump-
tion is that they are so in all.
I should have wished to include some remarks on Mr. Levy's
book on Blindness and the Blind, in the present volume, but the
work did not appear until it was almost through the press. I find
in it a strong confirmation of my view, that names may be used by
different persons with the same denotation, but with different con-
notations. It should also be noticed that ordinary blindness is not
total insensibility to light; and blind men, by close attention to
their faint visual perceptions, may turn them to much better account
than we should do. This is, perhaps, the explanation of “facial
perception,” which at first sight seems rather startling. At all
events it gives no countenance to the modern theory, which lays
stress on the muscular sensibility rather than simple touch.
cular sensations and those arising from the movements of the arm or the leg.
Would not the theory be simplified by leaving out the intermediate link, as
indeed was done by Berkeley P What is there to prevent our visual sensa-
tions becoming directly associated with our muscular efforts in moving our
arms or legs? The true answer is, perhaps, as follows:—sight appeared to
afford more information about objects than the theory would account for, and
then a happy thought occurred to the theorists. When the eye is at rest, it
was said, it does not see space; and its motions can only alter this perception
by introducing associations with muscular sensations. The latter statement,
however, which is rather assumed than asserted, is quite untrue.
i
Printed by PoETEous & GIBBs, 18 Wicklow Street, Dublin.
CORRIGENDA.
ºmºmº
Preface, page ii., line 7 from end—for “vulgar,” read “the vulgar.”
Text, page 19, line 2, for “forcible,” read “a forcible.”
Page 50, line 9 from end—for “were,” read “was.”
Page 56, line 18—for “enable,” read “enables.”
Page 57, line 7—insert “See Hamilton's Reid, page 156.”
Page 68, line 5 from end of text—for “sensationsan d,” read “sensations.
and.”
Page 70, line 22—for “in,” read “into.”
Page 71, line 3 of note—for “cffort,” read “effort.”
Page 79, line 4–for “quoting,” read “quoting from.”
Page 79, line 6 from end—for “sveral,” read “several.”
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