Philosophy of Science, 77 ( January 2010) pp. 59–72. 0031-8248/2010/7701-0006$10.00
Copyright 2010 by the Philosophy of Science Association. All rights reserved.

59

Levels of Explanation Reconceived*

Angela Potochnik†‡

A common argument against explanatory reductionism is that higher-level explanations
are sometimes or always preferable because they are more general than reductive
explanations. Here I challenge two basic assumptions that are needed for that argument
to succeed. It cannot be assumed that higher-level explanations are more general than
their lower-level alternatives or that higher-level explanations are general in the right
way to be explanatory. I suggest a novel form of pluralism regarding levels of expla-
nation, according to which explanations at different levels are preferable in different
circumstances because they offer different types of generality, which are appropriate
in different circumstances of explanation.

1. Introduction. According to the classic reductionist conception of sci-
ence, the existence of different fields of science may now be a practical
necessity, but an ultimate aim of science is to show how all events follow
from microphysical laws and events (Oppenheim and Putnam 1958). On
this view, the explanation of an event is always improved by giving in-
formation about the lower-level determiners of the event—ideally, the
microphysical determiners. A common response to this reductionism
about scientific explanation is to defend the value of nonreductive expla-
nations on account of their generality (Fodor 1974; Putnam 1975; Gar-
finkel 1981; Kitcher 1984; Sober 1984, 1999; Jackson and Pettit 1992).
The basic argument is that, whereas a microphysical explanation applies
only to systems fitting that microphysical description, a higher-level ex-
planation has a much broader range of applicability. The reason is that
higher-level descriptions can be realized by a variety of microphysical
systems. This argument has been used to defend the value of a variety of

*Received July 2009; revised September 2009.

†To contact the author, please write to: Department of Philosophy, Oklahoma State
University, 246 Murray Hall, Stillwater, OK 74078; e-mail: angela.potochnik@okstate
.edu.

‡This article has benefited from comments by Michael Friedman, Peter Godfrey-Smith,
Elliott Sober, Michael Strevens, and Michael Weisberg, as well as two anonymous referees
for Philosophy of Science.



60 ANGELA POTOCHNIK

high-level explanations over their lower-level competitors. The idea is that
higher-level explanations are sometimes or always preferable because they
apply in a broader range of circumstances.1

Consider Putnam’s (1975) familiar example of the peg and the board.
There are two holes in a board: each has a diameter of 1 inch, but one
is circular and the other is square. The peg is a cube with sides slightly
shorter than 1 inch. If we are to explain why the peg passes through the
square hole but not the round hole, we do so by citing the dimensions
of the peg and the two holes, along with geometrical information about
the relationships among these dimensions. This higher-level explanation
is much more general—applies in a much broader range of circum-
stances—than a lower-level explanation that specifies the exact atomic
structure of the peg and the board and computes all possible trajectories
of these atoms in order to deduce that the peg never passes through the
round hole but does pass through the square hole.

This style of defense for higher-level explanations depends on the idea
that generality is of explanatory worth, but why general explanations
should be taken to be valuable often remains unarticulated.2 Examining
the motivation for this idea is the first step to assessing this popular line
of argument against explanatory reductionism. Garfinkel (1981) suggests
one possible motivation. He argues that higher-level explanations are pref-
erable because they focus on the causes that critically affect the outcome
to be explained, whereas the alternative lower-level explanations cite
overly specific causal details. Perhaps, then, general explanations are val-
uable because they identify the critical causal factors and exclude causal
information that is in some sense unimportant.

This idea can be put into broader terms that are compatible with a
range of views regarding the nature of explanation. The sense of generality
at issue is the breadth of an explanation’s applicability to different systems
or, equivalently, in different circumstances. An explanation’s generality is
inversely related to the amount of information it provides about the system
in question. Including more information more finely specifies the process,
which reduces the explanation’s breadth of applicability, whereas remov-
ing information eliminates specifications of the process, which extends the

1. Jackson and Pettit (1992) and Sober (1999) do not argue that general explanations
are always best; they are pluralists regarding levels of explanation. They claim that higher-
level explanations are valuable because they are more general, but in their view whether
generality or specificity is of explanatory value varies. I discuss this view in Section 4.

2. On the unification account of explanation (Friedman 1974; Kitcher 1981), generality
is constitutive of explanation. Generality is also an important consideration for Strevens’s
causal account of explanation (2009). However, the antireductionist arguments considered
here do not rely on a specific theory of explanation. I similarly avoid committing to a
particular account of explanation in my analysis.



LEVELS OF EXPLANATION RECONCEIVED 61

explanation’s breadth of applicability.3 A general explanation thus in-
cludes fewer details about the target system, thereby focusing attention
on the limited information it does contain. This information is more
perspicuous than it would be if lots of other features of the system were
also specified. Moreover, the explanation advertises the unimportance of
other features of the target system by neglecting them. For both of these
reasons, if the information featured in an explanation regards the ex-
planatorily important factors, then the explanation benefits from its gen-
erality. Accounts vary on what factors are explanatorily important. For
Garfinkel (1981) they are causes critical to the outcome to be explained.
Others who defend the value of general explanations think that the ex-
planatorily important factors are the difference makers (Strevens 2009)
or the factors that identify the system as fitting into a unified pattern
(Friedman 1974; Kitcher 1981).

This is one argument for the idea that explanations benefit from being
general. A general explanation focuses attention on the information that
is properly explanatory and advertises the unimportance of other features
of the system by neglecting them. I assume that those who defend the
value of higher-level explanations on the grounds of their generality would
be amenable to a justification along these lines. The examples they offer
suggest so. The square peg fails to travel through the round hole because
of a geometric relationship regardless of the particular atomic structure
of the peg and board (Putnam 1975), Gresham’s law governs economic
exchanges regardless of whether the exchanges involve wampum or dollars
(Fodor 1974), and modeling the evolution of altruism in terms of fitness
provides “a useful generality’’ lacking from physical descriptions of var-
ious instances of altruism and selfishness (Sober 1999).

Two claims are needed in order to use this argument for the explanatory
worth of generality to defend higher-level explanations. First, it must be
the case that higher-level explanations are more general than lower-level
explanations. Otherwise the explanatory value of generality would not
translate into the value of higher-level explanations. Fodor (1974) and
others appeal to the idea of multiple realizability to motivate this claim.
Second, it must be the case that the information included in a higher-
level explanation is important to the outcome to be explained, whereas
the neglected information is unimportant to the outcome. According to
the argument sketched above, the value of general explanations is that

3. This assumes that other features of the explanations in question are held constant.
The conception of generality as breadth of applicability may not include all senses in
which the generality of explanations varies, but this is the sense of generality that is at
work in the defenses of higher-level explanations under consideration. All claims about
generality in this article regard this particular sense of the word.



62 ANGELA POTOCHNIK

they focus attention on the important information. If neglected lower-
level information is explanatorily important, then the generality of a
higher-level explanation does not improve the explanation. Both of these
claims turn out to be problematic. The problems stem from the fact that
the relationship between higher- and lower-level explanations often cannot
be accurately described in terms of supervenience.

2. Supervenience and Levels of Explanation. The successful defense of
nonreductive explanations on the grounds of their generality relies on the
idea that higher-level explanations are more general than their lower-level
alternatives. This idea is widely accepted (Fodor 1974; Putnam 1975;
Garfinkel 1981; Jackson and Pettit 1992; Sober 1999; Strevens 2009) and
is motivated by the belief that higher-level properties are multiply realized
by lower-level properties—that is, that many lower-level properties indi-
vidually suffice as supervenience bases for a given higher-level property.
If a lower-level explanation cites properties that are only one way among
many to realize the properties cited in a higher-level explanation, then the
higher-level explanation is more general than the lower-level explanation.
That is, it applies to a broader range of systems. This is the situation in
Putnam’s peg and board example. The lower-level explanation cites the
atomic structure of the peg and board to account for why the square peg
cannot travel through the round hole, but this exact atomic structure is
only one way among many for the geometric relationship between the
peg and the board to be realized. A higher-level explanation that cites
only the geometric relationship is more general than the alternative lower-
level explanation.

Yet not all higher-level explanations are more general than their lower-
level alternatives. Let us grant that all higher-level properties supervene
on lower-level properties and that multiple realization is common. Even
so, this relationship among higher- and lower-level properties need not
result in the predicted relationship between higher- and lower-level ex-
planations. Establishing that implication requires an additional premise:
that lower-level explanations are formulated in terms of the supervenience
bases of the competing higher-level explanations. I see no reason to think
that this premise is always or even often true.

Aside from being multiply realized, higher-level properties are often
complexly realized. That is, higher-level properties are realized by a com-
plicated combination of lower-level properties, even in individual (token)
occurrences. Fodor (1974) points out that the physical description of an
economic exchange will sometimes involve dollar bills, sometimes strings
of wampum, sometimes signing one’s name to a check; this is multiple
realization. But consider the physical description of an individual event
that has the property of being an economic exchange—say, you putting



LEVELS OF EXPLANATION RECONCEIVED 63

a dollar bill into a vending machine yesterday and getting a candy bar
in return. What physical property qualifies this event as an economic
exchange? It is not enough that you gave up a dollar bill and received a
candy bar: a marooned explorer with a penchant for dramatics might
throw her cash to the wind just as her sole companion hands her the last
candy bar. The physical properties of your vending machine encounter
that jointly qualify it as an economic exchange must include (at least) the
relinquishing of the bill, the acquisition of the candy bar, the circulation
of relevantly similar bills in relevantly similar circumstances such that the
bill qualifies as valuable, the vending machine specifications such that the
encounter counts as its ordinary functioning, specifications of an economic
entity on behalf of which the machine acts, and your specifications such
that candy is of potential value to you and agreements are things you
can enter into.

Fields of science that are equipped to provide lower-level explanations
formulate those explanations for their own purposes, as part of investi-
gations into phenomena of interest in those fields. Consequently, lower-
level explanations feature properties that are relevant to those investi-
gations, not the complex, highly particular conjunctions of properties that
serve as supervenience bases. This is so even when the target of the ex-
planation is a higher-level event. Consider the relationship between an
organism’s genotype (genetic composition) and phenotype (observable
characteristics). Philosophers tend to think of genetic properties as oc-
cupying a lower level of organization than phenotypic properties (e.g.,
Dupré 1993; Rosenberg 1994), and evolutionary change is modeled by
focusing on one or the other. Yet phenotypes do not supervene on genes
or genotypes but on a complex combination of properties including many
other properties of the organism, properties of the environment, and some-
times even properties of other organisms. The lower-level properties under
investigation—genes—are not proper candidates for the supervenience
bases of phenotypes. As a result, explanations featuring genes or geno-
types are not related by supervenience to explanations featuring higher-
level, phenotypic properties, even when these properties are used to explain
the same evolutionary outcomes.

This example is not atypical. Because different fields of science for-
mulate explanations for different purposes, lower-level explanations and
higher-level explanations of the same phenomena are often not related
by supervenience. This is so even for some examples used to defend higher-
level explanations. Above I suggested that this is true for Fodor’s example
of monetary exchanges. Garfinkel (1981) provides a biology example that
also illustrates my point: competing explanations for the death of a rabbit.
Garfinkel considers a lower-level explanation for the death that cites the
presence of the unfortunate rabbit in the capture space of a particular



64 ANGELA POTOCHNIK

fox, and he considers a higher-level explanation that cites the high con-
centration of foxes in the area. Yet the property of high fox concentration
clearly does not supervene on the property of the rabbit being located in
the capture space of one fox but on a combination of the spatial location
of all foxes in the area. The only relationship apparent between these
higher-level and lower-level properties is that the high concentration of
foxes makes it more likely that the rabbit is in the capture space of some
fox or other. This bears no resemblance to supervenience.

Explanations that are not related by supervenience may still qualify as
lower and higher level. The explanations in Garfinkel’s rabbit example
are formulated at lower and higher levels in the sense that one deals with
an individual and the other with a population of individuals. In general,
a lower-level explanation cites properties of objects that stand in a part-
whole relationship to objects referenced in the competing higher-level
explanation. Genetic and phenotypic explanations are also related in this
way: genes are parts of organisms. Often, the properties cited in a lower-
level explanation are part of the complex group of properties on which
the properties cited in a higher-level explanation supervene, and occa-
sionally, lower-level properties are proper supervenience bases.

This lower/higher distinction may be less crucial when explanations are
not related by supervenience, but it remains significant that different fields
formulate competing explanations for a single event. Evolutionary ex-
planations provide a ready example of this. Population genetics and evo-
lutionary ecology provide competing explanations for why many traits
evolve. Genetic explanations show how the distribution of genotypes re-
lated to the trait changed generation by generation, whereas phenotypic
explanations show how the environment selectively advantaged the trait
in question. These explanations need not make competing claims about
the world: the genetic explanation is silent regarding the environmental
influences on selection, and the phenotypic explanation is silent regarding
the genetic causes of the trait (Potochnik, forthcoming). The explanations
do compete, though, in a way important to the debate about levels of
explanation. The two explanations cite different properties, at different
levels of organization, to explain the same event.

When higher- and lower-level explanations are not related by super-
venience, nothing ensures that higher-level explanations are more general
than their lower-level competitors. If a lower-level explanation does not
cite the supervenience bases of properties cited in a higher-level expla-
nation, then the former may not have a more limited range of applicability
than the latter. Without supervenience and multiple realization, competing
lower- and higher-level explanations may simply have different ranges of
applicability. Such explanations are general in different ways; they offer
different types of generality. By this I do not mean that they satisfy dif-



LEVELS OF EXPLANATION RECONCEIVED 65

ferent definitions of generality but that each applies to some range of
systems that the other does not.

So it is for genetic and phenotypic explanations of evolutionary phe-
nomena (Potochnik 2007). A phenotypic explanation of a trait is appli-
cable whenever the represented fitness relationship obtains, even if the
genetic details vary. For example, a single phenotypic model explains the
trait of when in the life cycle reproduction begins for a vast array of
species, even though the genes that influence that trait differ (Bull, Pfennig,
and Wang 2004). Genetic explanations are not general in this way. In
contrast, a genetic explanation of a trait is applicable whenever the rep-
resented combination of selection and transmission obtains, even if the
environmental sources of fitness are different. For example, a single genetic
model explains how a selectively advantaged trait influenced by two genes
evolves, regardless of the environmental sources of fitness (e.g., Curnow
and Ayres 2007). Phenotypic explanations represent the sources of fitness,
so they are not general in this way.

3. Contextual Explanation. In Section 1 I distinguished two claims that
must be true in order to use the explanatory worth of generality to defend
nonreductive explanations. I dealt with the first in Section 2. The second
claim is that higher-level explanations provide information that is im-
portant to the explanation and omit only unimportant information. Gen-
eral explanations are valuable if they focus attention on explanatorily
important information. If an explanation purchases greater generality by
omitting information that is important to the explanation, then the in-
creased generality impoverishes rather than improves the explanation.
Suppose that a higher-level explanation is more general than its lower-
level competitor (though I argued in Section 2 that this is uncommon).
This greater generality results in a better explanation only if it is the right
type of generality; the information omitted must be unimportant to the
explanation.

That more generality is not always better is demonstrated by taking
this idea to the extreme. Rigorously applying the precept of maximizing
generality would result in explanations that cite every possible circum-
stance that would lead to the event to be explained. So, for instance, the
most general explanation of a room getting dark is that either the light
was turned off, or the lightbulb burned out, or it is twilight, or someone
drew the curtains, or a storm is gathering outside, or the like.

This vast disjunction is maximally general, but it is not very infor-
mative.4 Expansive disjunctions like this surely cannot be the preferable

4. This is a version of the disjunction problem, which traces back to discussions of
Hempel’s theory of explanation; see Cover and Curd 1998, 786.



66 ANGELA POTOCHNIK

form of explanation, so it seems that explanations should not simply be
maximally general. Other arguments to this effect have been made for
different accounts of explanation. For instance, Strevens (2009) claims
that a causal account of explanation must specify which aspects of a causal
process belong in an event’s explanation and which aspects should be
omitted for the sake of generality. Kitcher (1981) claims that the generality
of an explanatory pattern should be limited by considerations of the
pattern’s stringency—the commonalities among its instances. Explana-
tions should not simply be maximally general. If generality is of explan-
atory worth, the trick is to find a more nuanced way to express this.

Consider evolutionary explanations again. A vast number of factors
influence individual evolutionary outcomes. For instance, Homo sapiens
have evolved large brains relative to our hominid kin. This is the result
of (at least) genetic mutations, developmental processes, environmental
conditions, other evolutionary events, and phylogenetic history. An ex-
planation that neglects some of these many influences focuses attention
on the influences that are cited and demonstrates that small variations in
other factors would not have interfered with the evolution of large brains.
For these reasons, a general explanation is preferable. Yet some ways of
increasing the generality of the explanation would be detrimental; for
example, an explanation that describes how large brains develop but does
not provide information about genetic factors and other evolutionary
influences would be ill suited as an evolutionary explanation.

Generality may be of explanatory worth, but explanations can be too
general or general in the wrong way. An exhaustive disjunction of all
possible causes of the room getting dark is too general to be a good
explanation. Citing only information about the development of humans’
large brains is general in the wrong way to be a good evolutionary ex-
planation. The degree and type of generality that benefits an explanation
must be specified. In the discussion that follows, I assume a broadly causal
view of explanation. An alternative formulation of my point is consistent
with a unification approach to explanation, but providing both formu-
lations here would be overly cumbersome. On the assumption that ex-
planations cite causal information, generality gained by leaving out un-
important causal information is good, whereas generality gained by
leaving out important causal information is bad. The key question is thus,
What causal information is important to an explanation?

What information about a causal process is important to explaining the
resulting event depends on which of the many causal relationships involved
are explanatory. This in turn depends on the role the explanation is intended
to play. Each causal factor leading to an event is potentially relevant to
explaining that event, or alternatively, each difference maker is of potential
relevance (Woodward 2003; Strevens 2009). Explanatory relevance follows



LEVELS OF EXPLANATION RECONCEIVED 67

from causal or difference-making relevance. Yet every event results from
innumerable causal factors; causal processes extend indefinitely back in time
and often involve several interacting influences at any given point in time.
The potential explanatory relevance of many causes, coupled with the ex-
planatory value of generality, makes it so that different causal relationships
are explanatorily important in different circumstances. Which causal re-
lationship is explanatory, and consequently what causal information is ex-
planatorily important, depends on the context of the call for explanation,
that is, on the research program to which the explanation contributes.5

Consider the different contexts in which an explanation may be sought
for why humans have large brains relative to other hominids. In a research
program in evolutionary ecology, the causal relationship between envi-
ronmental influences and the resultant selective advantage for large-
brained humans is explanatorily important. In a developmental biology
research program focused on the human nervous system, the causal re-
lationship of note is instead how certain developmental pathways in hu-
mans lead to the fabrication of big brains. Both of these causal relation-
ships—and many, many more besides—influence the event to be explained.
Which causal relationship is explanatorily important depends on the role
that the explanation is intended to play in scientific investigation.

Those who want to resist this move to contextual explanation might
argue that different contexts lead to different explananda. For the above
example, one might argue that in the first context one explains the evo-
lution of humans’ large brains, whereas in the second context one explains
the development of humans’ large brains. But individuating explananda
in this manner is simply a way of covertly specifying which causal rela-
tionship is of interest. The explanandum is that humans have large brains
or, if you like, that humans evolved to develop large brains. Depending
on research interests—that is, on context—this explanandum might be
expressed alternately as humans evolved (to develop) large brains or hu-
mans (evolved to) develop large brains. Both regard a single evolutionary/
developmental event, resulting from a single complex causal process. The
alternate expressions of the explanandum result from singling out a distal
part of this causal process and from singling out a proximate part of this
causal process, respectively (Mayr 1961).6 This is simply the context of

5. For the sake of brevity, I do not develop an account of this context dependence here.
See Potochnik, forthcoming, for discussion of what governs this pragmatic feature of
explanation.

6. Developmental systems theorists maintain that the evolutionary/developmental process
often cannot be prized apart in this way, that is, that it is not possible to adequately
answer evolutionary questions while bracketing development. If this is right, then in
contexts favoring evolutionary information, the explanandum would be the evolution of
the developmental process leading to large brains.



68 ANGELA POTOCHNIK

the explanation directing attention toward one part of the causal process,
which leads to bracketing other parts of the process.

Another way to resist the move to contextual explanation is to claim
that the role of context is captured by specifying the contrast class of the
explanandum. Contrastive explanation is the idea that an explanation is
in part determined by what one considers to be the alternatives to the
event to be explained (Dretske 1972; van Fraassen 1980; Garfinkel 1981).
For instance, one might wonder why humans have large brains rather
than small brains or why humans have large brains rather than chimps
having large brains. Although specifying different contrast classes may
lead to different explanations, this does not capture the full relevance of
context. Consider explanations for why humans have large brains rather
than smaller brains. Research interests in development will still result in
an explanation that focuses on developmental pathways (why it is that a
given genotype leads to large brains rather than small brains), whereas
research interests in evolution result in an explanation that focuses on
natural selection (why it is that large brains were selected for rather than
small brains). Specifying the contrast class does not fully account for the
relevance of context. On this, I agree with van Fraassen (1980).7

The idea that the explanatory importance of causal information is con-
text dependent undermines the claim that higher-level explanations always
focus on important information and neglect only unimportant informa-
tion. Which causal relationships are explanatorily important and which
should be neglected for the sake of generality depends on the research
program in which the explanation is developed. Even if a higher-level
explanation is more general than its lower-level competitor, there are likely
to be contexts in which the causal relationships that are focal in the lower-
level explanation are explanatorily important. When this is so, the gen-
erality of the higher-level explanation results in the omission of infor-
mation that is important to the explanation, thereby diminishing the value
of the explanation. The force of this point only increases when one takes
into account that higher- and lower-level explanations often are not related
by supervenience. Oftentimes, lower-level explanations and higher-level
explanations simply differ in which causal relationships they represent
and which they neglect and so are general in different ways. Whether the
higher-level explanation or the lower-level explanation offers the right

7. Much of what I say here about context dependence fits with van Fraassen’s views on
explanation. According to van Fraassen, why-questions do not even arise without specific
interests—“legal, medical, economic; or just an interest in optics or thermodynamics
rather than chemistry’’—and without abstracting, “[thinking] of Caesar’s death qua mul-
tiple stabbing, or qua assassination’’ (1980, 130).



LEVELS OF EXPLANATION RECONCEIVED 69

type of generality—that is, captures the important causal information and
neglects the unimportant information—depends on the circumstances.

4. New Reasons for Explanatory Pluralism. I have argued that, despite
common assumptions to the contrary, explanations formulated at different
levels are often general in different ways, and those different forms of
generality are valuable in different contexts of explanation. These points
undermine the traditional argument against explanatory reductionism and
suggest an alternate conception of the relationship among levels of ex-
planation. The result is a new form of pluralism regarding levels of ex-
planation, one that is sensitive to the complexity of real-world causal
processes and to the ways in which science grapples with this complexity.

The antireductionist argument considered here is right about at least
one thing: generality is always valuable to explanation. The generality of
an explanation is what enables it to apply to a range of systems, thereby
grouping together systems that bring about similar outcomes via similar
means. An explanation highlights the distinctive features of the range of
systems to which it applies and demonstrates the causal regularities that
are responsible for the outcome to be explained.8 The way in which an
explanation is general—what information it includes and what it ne-
glects—determines the range of systems to which the explanation applies.
Relatively general explanations apply to a broad range of systems and,
consequently, pick out a broad causal pattern. More specific explanations
pick out a more limited causal pattern, but the explanation still applies
to a range of systems that behave similarly to one another. The explanation
is still general in a way that makes perspicuous a causal regularity and
that indicates the range of circumstances in which that regularity holds.
Explanations that offer different types of generality, that apply to different
ranges of systems, offer information about different causal regularities
that are involved in bringing about the event in question.

The type of generality that is appropriate depends on the explanation’s
context. Of course, which causal relationships have the potential to be
explanatory in the first place is constrained by the nature of the causal
process that leads to the event to be explained. A causal relationship
cannot be explanatory if it is not involved in bringing about the event in
question. But which among the potentially explanatory causal relation-
ships belongs in a particular explanation is decided by the context in
which the explanation is formulated. This context is determined by the

8. This suggests how the present ideas are compatible with both causal and unification
approaches to explanation, though the idea that citing causal regularities is explanatory
is probably most closely associated with the difference-making approach to explanation
(Woodward 2003; Strevens 2009).



70 ANGELA POTOCHNIK

role to be played by the explanation, that is, the focus of the research
program in which the explanation is formulated. Because explanations
formulated on different levels are often not related via supervenience, they
are often general in different ways. That is, higher- and lower-level ex-
planations generalize to different ranges of systems, thereby highlighting
different causal regularities. Whether the higher-level or the lower-level
explanation is better depends on which of these causal regularities is
relevant to the guiding research interests.

Consider once more the distinct genetic and phenotypic explanations
for evolved traits. A genetic explanation provides insight into how the
dynamics of genetic transmission increased the frequency of the trait in
question. A phenotypic explanation provides insight into how the envi-
ronment selectively advantaged the trait in question and caused it to
predominate. These genetic and phenotypic explanations compete in the
sense that they are distinct explanations offered for a single event. More-
over, they cite properties that are reasonably described as lower and higher
level, respectively. Yet, as demonstrated in Section 2, they apply to dif-
ferent ranges of systems; in other words, they offer different types of
generality. By doing so, they highlight different causal regularities gov-
erning the processes by which traits evolve. Each type of explanation
captures one aspect of why, for example, humans have large brains. Which
explanation is better depends on the research interests that guide the
investigation. Accordingly, genetic explanations are typically developed
in population genetics, while phenotypic explanations are typically de-
veloped in evolutionary ecology.

Jackson and Pettit (1992) and Sober (1999) develop versions of the
classic antireductionist response that I criticize, but they are also pluralists
about levels of explanation. They grant that higher-level explanations are
more general than lower-level explanations, for they accept that higher-
and lower-level explanations cite properties related by supervenience and
that multiple realization is common. They are pluralists because they think
that explanations sometimes should be general (higher level) and other
times specific (lower level). This misidentifies the value of multiple levels
of explanation. Maximal generality of the right type always improves
explanations. Pluralism about explanation results from variance only in
what type of generality is called for, that is, what range of systems the
explanation should apply to. This is determined by contextual factors—
in particular, by what causal relationship is focal given the research pro-
gram at hand. I have shown that higher-level explanations need not be
more general than lower-level explanations and that the preferable level
of explanation depends on the type of generality that best captures the
causal relationship of explanatory importance in the context at hand.

This conception of the explanatory role of generality and its context



LEVELS OF EXPLANATION RECONCEIVED 71

dependence grounds a rich pluralism of approaches to explanation that
reflects the varied projects of science. Which of the many causal rela-
tionships involved in bringing about an event belongs in an explanation
of that event depends on the research interests that occasion the call for
explanation and determines the type of generality the explanation should
have. The rarity of supervenience relationships among competing expla-
nations undermines explanatory reductionism, as well as the common
antireductionist rejoinder. Indeed, the framework of “levels’’ of expla-
nation emerges as somewhat forced. It is more natural to distinguish
among alternate approaches to explanation that simply vary as to which
entities and properties are cited and thus which causal relationships are
focal.

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