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Citation for published version

Williamson, Jon  (2018) Justifying the Principle of Indifference.   European Journal for the Philosophy
of Science .    ISSN 1879-4920.

DOI

https://doi.org/10.1007/s13194-018-0201-0

Link to record in KAR

http://kar.kent.ac.uk/66127/

Document Version

Publisher pdf



Euro Jnl Phil Sci

https://doi.org/10.1007/s13194-018-0201-0

ORIGINAL PAPER IN FORMAL EPISTEMOLOGY

Justifying the principle of indifference

Jon Williamson1

Received: 21 November 2016 / Accepted: 13 February 2018

© The Author(s) 2018. This article is an open access publication

Abstract This paper presents a new argument for the Principle of Indifference. This

argument can be thought of in two ways: as a pragmatic argument, justifying the

principle as needing to hold if one is to minimise worst-case expected loss, or as

an epistemic argument, justifying the principle as needing to hold in order to min-

imise worst-case expected inaccuracy. The question arises as to which interpretation

is preferable. I show that the epistemic argument contradicts Evidentialism and sug-

gest that the relative plausibility of Evidentialism provides grounds to prefer the

pragmatic interpretation. If this is right, it extends to a general preference for prag-

matic arguments for the Principle of Indifference, and also to a general preference

for pragmatic arguments for other norms of Bayesian epistemology.

Keywords Principle of indifference · Bayesianism · Epistemic consequentialism ·

Accuracy

Many Bayesians are committed to some version or other of the Principle of Indif-

ference, which holds that in certain situations one’s degrees of belief should be

equivocal. Section 1 introduces three such versions in order of increasing strength. In

Section 2, I develop a consequentialist argument for the strongest version. This can

be thought of as motivating the principle in terms of its pragmatic consequences: if

one is to minimise worst-case expected loss, then one should satisfy the Principle of

Indifference. As I explain in Section 3, an analogous argument can be constructed

� Jon Williamson

j.williamson@kent.ac.uk

http://blogs.kent.ac.uk/jonw/

1 Philosophy Department, SECL, University of Kent, Kent, CT2 7NF, UK

http://crossmark.crossref.org/dialog/?doi=10.1007/s13194-018-0201-0&domain=pdf
http://orcid.org/0000-0003-0514-4209
mailto:j.williamson@kent.ac.uk
http://blogs.kent.ac.uk/jonw/


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to motivate the strongest version of the principle in terms of its epistemic conse-

quences: if one is to minimise worst-case expected epistemic inaccuracy, then one

should satisfy the Principle of Indifference. In Section 4, we shall see that this sort of

epistemic consequentialist argument conflicts with Evidentialism, which holds that

one’s beliefs are epistemically rational if and only if they are compatible with one’s

evidence. In Section 5 I argue that this is a worry not only for the epistemic justifi-

cation presented in Section 3, but also for any epistemic justification of the Principle

of Indifference: one should not be able to provide an argument for a Principle of

Indifference purely in terms of its epistemic consequences, because the Principle

of Indifference goes well beyond the evidence, and epistemic considerations should

at most motivate conforming to the evidence. I argue that this concern also calls

into question epistemic arguments for other Bayesian norms, such as Probabilism

(the view that the strengths of one’s beliefs should be probabilities). For this rea-

son, Bayesians are on safer ground motivating norms in terms of their pragmatic

consequences, rather than their epistemic consequences.

1 Three versions of the principle of indifference

Whilst radical subjectivist Bayesians would want to maintain that there are very few

constraints on rational belief and would reject the Principle of Indifference (e.g., de

Finetti 1937), many Bayesians are committed to some version or other of the Prin-

ciple of Indifference. Objective Bayesians often explicitly endorse norms on belief

which imply the Principle of Indifference, such as the Maximum Entropy Princi-

ple (Jaynes 1957; Williamson 2010). Moreover, as Hawthorne et al. (2017) argue,

Bayesians who endorse any principle which requires calibration of degrees of belief

to some probabilities of which one has evidence—e.g., the Principal Principle, the

Reflection Principle, or any testimony principle which posits deference to expert

authorities—are also committed to a version of the Principle of Indifference.

The Principle of Indifference has been formulated in many ways. In this paper

we shall consider three versions, which will be introduced in this section in order of

increasing strength.

We shall assume the following context in this section. First, we shall focus on a

particular agent and suppose that there is a most fine-grained set � of mutually exclu-

sive and exhaustive propositions that this agent can entertain or express: these are the

agent’s basic possibilities. Any other expressible proposition can be thought of as a

subset of �, the subset of possibilities in which that proposition is true. For exam-

ple, a simple artificial agent may be speculating about the results of an experiment

with three possible outcomes, ω1, ω2, ω3 respectively, in which case we might have

� = {ω1, ω2, ω3}; if ω1 is a positive outcome then the proposition that outcome is not

positive can be thought of as the subset {ω2, ω3} ⊆ �. Expressible propositions can

thus be represented by members of the power set P� of �. The Principle of Indiffer-

ence is well known to lead to complications or inconsistencies on infinite domains,

and its application there is very controversial (see, e.g., Keynes 1921, Chapter 4). For

this reason, we restrict our attention here to the case in which � is finite. Having said

that, it is worth pointing out that the principles of indifference which we discuss here



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can be extended in a consistent way to the case in which an agent’s language can be

modelled as a first-order predicate language and the (infinitely many) basic possi-

bilities represent truth assignments to the atomic propositions of the language—see

Williamson (2017). To keep things simple, we shall suppose that the agent in question

cannot express higher-order probability propositions—propositions about chances or

about degrees of belief—if she could, we would have to consider a variety of fur-

ther norms on degree of belief and specify how they interact with the Principle of

Indifference.

Suppose that available evidence E constrains the agent’s belief function P , which

represents her degrees of belief in the various propositions that she can express, to

lie in some convex set E of probability functions.1 For instance, evidence E might

consist of a set of expressible propositions; in this case, E constrains belief func-

tion P to lie in the convex subset E of probability functions which give probability

1 to each proposition in E. To return to our simple example, if E contains only

the proposition that the experiment did not yield a positive outcome, {ω2, ω3}, then

E = {P : P (ω2) + P (ω3) = 1} and E is convex. In general, however, we shall not

assume that the available evidence E consists entirely of propositions that the agent

can express, i.e., propositions in the domain P� of the belief function P . For exam-

ple, evidence E might alternatively consist of evidence of chances, which, we have

assumed, is not expressible in the sense outlined above. If E determines just that the

chance function P ∗ lies in convex E, a Bayesian who endorses some form of calibra-

tion to chances will want to say that the agent’s belief function P should also lie in E.

Thus if E says that P ∗({ω2, ω3}) = 0.9, so P
∗ ∈ E = {P : P (ω2) + P (ω3) = 0.9},

then arguably also P ∈ E, which is a convex set of probability functions. If E is

inconsistent, we shall take E = P, the whole space of probability functions on P�.

Let us now turn to the first of the three versions of the Principle of Indifference

that we shall consider in this paper. This first version says that if there is no evidence

at all then one should believe each basic possibility to the same extent:

PI1: If E = ∅ then P (ω) = 1
|�|

, for each ω ∈ �.

We shall call the probability function that gives the same probability to each basic

possibility the equivocator function and denote it by P=:

P=(ω)
df
=

1

|�|
, for each ω ∈ �.

PI1 is visualised in Fig. 1. Here there are three basic possibilities, � = {ω1, ω2, ω3}.

The triangle and its interior represent the set of probability functions: each vertex

represents the function that gives probability 1 to the corresponding basic possibility

and probability 0 to each of the other two; an edge represents the set of probability

functions that give probability 0 to the basic possibility at the opposite vertex; all

other probability functions are in the interior. If there is no evidence, E = ∅, then E

1A set of probability functions is convex if, for any two probability functions P and Q that are in the set,

any convex combination R of P and Q, defined by R(ω) = λP (ω) + (1 − λ)Q(ω) for some λ ∈ [0, 1]

and each ω ∈ �, is also in the set. Non-convex sets introduce complexities which would obscure the main

points of this paper, and as we shall see later, they do not affect the key results.



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Fig. 1 Visualisation of PI1

is the entire set of probability functions on P�, and PI1 says that P should be set to

the equivocator function, the mid-point of the triangle, which gives probability 1
3

to

each basic possibility.

The second version of the Principle of Indifference that we shall consider says that

if the evidence treats each basic possibility symmetrically, then one should believe

each such possibility to the same extent:

PI2: If E is invariant under permutations of the ω ∈ �, then P = P=.
2

This implies PI1, since if E is empty then it is invariant under permutations of the

basic possibilities. PI2 is depicted in Fig. 2. Here E is a strict subset of the set of

probability functions, invariant under 120◦ rotations.

The third version of the Principle of Indifference says that if it is compatible with

the evidence to believe each basic possibility to the same extent, then one should do

so:

PI3: If P= ∈ E then P = P=.

2E is invariant under permutations of the ω ∈ � if for any permutation π on �, permuting each ω to

π(ω) in each statement in E yields the same set E of probability functions. This corresponds to rotational

symmetry of E in the simplex P: symmetry under rotation by 120◦ in Fig. 2.



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Fig. 2 Visualisation of PI2

This principle, depicted in Fig. 3 is stronger still, implying PI2 and thereby also PI1.3

2 Pragmatic justification: controlling loss

Having introduced some principles of indifference, in this section I shall develop a

consequentialist argument for PI3 which appeals to some technical results of Landes

and Williamson (2013). In this section, PI3 will be explicitly motivated in terms of

its pragmatic consequences: it turns out that satisfying PI3 is advantageous in that it

minimises worst-case expected loss. Later, in Section 3, we shall see that an argument

with the same formal structure can be recast in epistemic consequentialist terms.

The argument presented here does more than justify the Principle of Indifference;

it also justifies other Bayesian norms. In particular, it justifies Probabilism—the

claim that P should be a probability function—and a calibration norm, which says

that P should be calibrated to chances. Thus in this section, we shall not presume

Probabilism, nor shall we assume that P should be calibrated to chances, as we did

in Section 1. Instead, we shall derive these norms. Let E be the set of evidentially-

compatible chance functions: evidence determines that the chance function P ∗ lies

3To see this, suppose that E is invariant under permutations of �. Because E is non-empty, convex and

invariant under rotational symmetry, P= ∈ E. Hence by PI3, P = P=, as required.

Note that PI3 only implies PI2 under the convexity assumption. Without convexity, PI2 is implausible,

because in that case PI2 may require that P �∈ E, i.e., that P should lie outside the set E of probability

functions that are compatible with the evidence.



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Fig. 3 Visualisation of PI3

within set E ⊆ P, and E is the smallest such set (there is no strict subset E′ ⊂ E such

that the evidence determines that P ∗ ∈ E′).4 I shall argue that the agent’s degrees of

belief should be representable by a function P within the set E. P is thus a probabil-

ity function and is also calibrated to chances insofar as there is evidence of chances.

In addition, we shall also see that P should satisfy PI3.

In this section we assume, purely for ease of exposition, that E is closed (i.e.,

contains its limit points) as well as convex. As before, we presuppose a finite set � of

basic possibilities, e.g., � = {ω1, ω2, ω3}, and we construe expressible propositions

as subsets of �, e.g., F = {ω2, ω3}. A partition π of propositions is a set of mutually

exclusive and exhaustive subsets of �. For example, {{ω1}, {ω2, ω3}} is a partition of

propositions. � will denote the set of all partitions of propositions.

We shall suppose that an agent’s degrees of belief can be represented by a function

bel : P� −→ R≥0 which attaches a non-negative real number to each express-

ible proposition. For example, one such belief function might set bel({ω1}) =

6, bel({ω2, ω3}) = 9, . . .. The set of belief functions is a much wider class of

4We thus presume that the chance function P ∗ is a probability function. The definition above presupposes

that the chance function is defined on the same domain P� as the agent’s belief function P . However,

this presupposition is not essential. Suppose instead that the chance function is defined on some other

domain, which may include certain propositions not expressible in P� and which may not include certain

propositions expressible in P�. In that case, we take the set evidentially-compatible chance functions to

be the set E of probability functions on domain P� that are consistent with evidence of chances in E: any

Q ∈ E is consistent with evidence of chances in E and any probability function Q defined on P� that

is consistent with evidence of chances in E is in the set E. As noted in the previous section, we do not

assume that the evidence E consists of propositions expressible in P�.



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functions than the set of probability functions. I shall argue that, in order to minimise

worst-case expected loss, the agent’s belief function should be a probability function,

in E, which satisfies the Principle of Indifference PI3.

Normalisation First, as a technical convenience, we shall normalise the belief func-

tions. Roughly speaking, we divide all degrees of belief by the maximum amount

of belief distributed amongst a partition of propositions. More precisely, for M =

maxπ ∈�
∑

F ∈π bel(F ) we normalise belief function bel by considering instead belief

function B : P� −→ [0, 1] defined by:

B(F ) =
bel(F )

M
for allF ⊆ �.

For example, if bel({ω1}) = 6, bel({ω2, ω3}) = 9, bel({ω2}) = 12, bel({ω3}) =

12, . . . and the partition of propositions {{ω1}, {ω2}, {ω3}} is given maximum belief,

M = 30, then bel would be normalised to give B({ω1}) = 0.2, B({ω2, ω3}) =

0.3, B({ω2}) = 0.4, B({ω3}) = 0.4, . . .. The symbol B will denote the set of all

normalised belief functions. For each such function B,
∑

F ∈π

B(F ) ≤ 1 for all π ∈ �, and
∑

F ∈π

B(F ) = 1 for some π ∈ �,

in virtue of the procedure used to normalise B. Note that the set P of all probability

functions P : P� −→ [0, 1] is a subset of B, since,
∑

F ∈π

P (F ) = 1 for all π ∈ �.

Thus if B({ω1}) = 0.2, B({ω2, ω3}) = 0.3 then B is not a probability function. The

task is to show that whilst in general a belief function B may lie outside the space

of probability functions (Fig. 4), in order to minimise worst-case expected loss the

agent’s belief function should lie inside E ⊆ P and should be the equivocator function

P=, if the equivocator function is in E. This will establish PI3.

Loss Next, we need to be more specific about the notion of loss under consideration.

Suppose the agent is not aware of the choices she will need to make, and is thus not

aware of the actual losses (or gains) which will be incurred by her degrees of belief.

What should she expect of her losses? Let L(F, B) denote the loss (aka disutility) one

should anticipate will be incurred by adopting belief function B when F turns out to

be true.5 We shall interpret this as the loss specific to F , i.e., the loss in isolation from

5The quantity L(F, B) is not interpreted as a formal expectation, for two reasons. First, since one of our

goals is to motivate Probabilism, we are not in a position to presuppose that any such belief distribution

is a probability function, which it would need to be in order to define a formal expectation. To get round

this problem, one might suggest that one could generalise the notion of formal expectation to weight the

loss by normalised degree of belief, rather than probability. However, a second problem would then arise,

as follows. The expectation would need to be an average, taken over all possible decision scenarios, of the

loss incurred in each scenario, weighted by the degree to which one believes that scenario will obtain. It

is unrealistic to think that one could enumerate all possible decision scenarios (i.e., all possible losses as

well as all possible sets of options) and specify a degree of belief in each scenario obtaining. Certainly,

we shall not assume here that the domain of the agent’s belief function is large enough to include every

possible decision scenario.



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Fig. 4 Belief function B outside the space P of probability functions

losses incurred by B on other propositions which may be implied by F or which may

imply F . We shall suppose:

L1: One should not anticipate any loss when one fully believes a proposition that

turns out to be true: L(F, B) = 0 if B(F ) = 1. (Fully believing the truth leads

to the best outcome.)

L2: One should anticipate that the loss L(F, B) will strictly increase as B(F )

decreases from 1 towards 0. (The less credence one gives to the truth, the worse

the outcome.)

L3: L(F, B) should depend only on B(F ), not on B(F ′) for F ′ �= F . (This repre-

sents the interpretation of L(F, B) as the loss incurred on F in isolation from

that incurred on other propositions.)

L4: Losses should be presumed to be additive when the space � of basic possibil-

ities is generated by independent subspaces: whenever ω ∈ � takes the form

ω1 ∧ ω2 where ω1 ∈ �1 and ω2 ∈ �2, B(�) = 1, and �1 and �2 are indepen-

dent in the sense that B(F1 × F2) = B(F1)B(F2) for all F1 × F2 = {ω1 ∧ ω2 :

ω1 ∈ F1, ω2 ∈ F2}, then L(F1 × F2, B) = L1(F1, B⇂�1 ) + L2(F2, B⇂�2 ),

where L1 and L2 are the loss functions on �1 and �2 respectively.

It turns out that these conditions force the loss function to be logarithmic:

L(F, B) = −k log B(F ),

for some positive constant k (Landes and Williamson 2013, Theorem 1).



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Expected loss Next, let us consider expected loss. If the chance function is P ∗, what

is the expected loss incurred by B over all the expressible propositions? A function

which measures expected loss is called a scoring rule.6 In fact, there are a range

of plausible scoring rules. For example, both the following measures seem equally

reasonable:

S�(P
∗
, B)

df
=

∑

π ∈�

∑

F ∈π

P
∗
(F )L(F, B),

SP�(P
∗
, B)

df
=

∑

F ⊆�

P
∗
(F )L(F, B).

Since there is no single scoring rule that stands out as being most appropriate, we

shall consider a whole class of scoring rules, indexed by a function g : � −→ R≥0
which attaches a non-negative weight to each partition:

Sg(P
∗
, B)

df
=

∑

π ∈�

g(π)
∑

F ∈π

P
∗
(F )L(F, B).

S� corresponds to the case in which each partition receives weight 1; SP� corre-

sponds to the case in which each partition of size 2 receives weight 1 and all other

partitions get weight 0.7

We shall impose two conditions on g. First, g is inclusive: every proposition F is

in some partition that is given positive weight (otherwise, that proposition will not

contribute to the score). Second, g is unbiased: it is invariant under permutations of

the basic possibilities ω ∈ � (otherwise, some possibilities are singled out a priori as

more important than others). The weighting functions for S� and SP� satisfy these

two conditions.

Given the anticipated loss function set out above, we have that

Sg(P
∗
, B) = −k

∑

π ∈�

g(π)
∑

F ∈π

P
∗
(F ) log B(F ).

6Here we take the expectation with respect to chance, P ∗, rather than normalised degree of belief, B.

Partly, this is because chance is assumed to be probabilistic but B is not. This is in line with the standard

view of scoring rules in the statistical literature (e.g., Grünwald and Dawid 2004), though some works in

the philosophical literature do weight by non-probabilistic belief (e.g., Leitgeb and Pettigrew 2010a). More

fundamentally, we focus on the concept of objective expectation rather than subjective expectation on the

grounds that: (i) objectively expected loss gives a better estimate of actual loss than subjectively expected

loss where the two disagree; and (ii) evidence tells us something about the chances (namely that P ∗ ∈ E)

so objectively expected loss is sufficiently accessible for our purposes. One might counter that—just as

objectively expected loss is preferable to subjectively expected loss—it would equally be better to focus

on the actual loss function rather than anticipated loss L(F, B) = −k log B(F ). However, the actual loss

function is not normally known, even to within some well circumscribed subset of possible loss functions.

The fact is that whilst evidence usually tells us quite a lot about chances, it rarely tells us much about the

losses we will be incurred in all future decision scenarios.
7Each F ⊆ � is in precisely one two-membered partition, namely {F, F̄ }, so

SP�(P
∗
, B) =

∑

F ⊆�

P
∗
(F )L(F, B) =

∑

π ={F,F̄ }

1
∑

G∈π

P
∗
(G)L(G, B).



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Worst-case expected loss Now, the precise chance function P ∗ will usually not be

known, so the expected loss is not fully accessible to the agent. The evidence only

determines that P ∗ lies in the set E of evidentially-compatible chance functions. The

evidence does, however, permit us to calculate the worst-case expected loss,

sup
P ∗∈E

Sg(P
∗
, B).

We can thus ask, which belief function B would incur minimum worst-case expected

loss? I.e., which B achieves

inf
B∈B

sup
P ∗∈E

Sg(P
∗
, B)?

It turns out (see Landes and Williamson 2013, Theorem 2) that the belief function

B ∈ B that incurs minimum worst-case expected loss is the probability function in E

which has maximum generalised entropy:

Hg(P )
df
= −

∑

π ∈�

g(π)
∑

F ∈π

P (F ) log P (F ).

This is depicted in Fig. 5. Since B ∈ E ⊆ P, this establishes that the optimal belief

function B is a probability function (i.e., Probabilism) and that the optimal belief

function B is in the set E of evidentially-compatible chance functions (Calibration).

Moreover, the equivocator function is the probability function in P that has maxi-

mum generalised entropy (Landes and Williamson 2013, Corollary 6). Hence, if the

Fig. 5 The optimum belief function B is the calibrated probability function with maximum generalised

entropy



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equivocator function is in E then it is bound to be the belief function that minimises

worst-case expected loss. This establishes that if P= ∈ E then the optimal belief func-

tion B is the equivocator function P=. This is PI3, the strongest of the three principles

of indifference. Note that PI3 still holds even if we drop the assumptions of closure

and convexity of the set E of evidentially-compatible chance functions (Landes and

Williamson 2013, Theorem 3).

3 Epistemic justification: reducing inaccuracy

The above argument for the Principle of Indifference sought to justify it in terms

of its pragmatic consequences: PI3 needs to hold if one is to minimise worst-case

expected loss. Interestingly, exactly the same formal argument can be cast in terms

of epistemic consequences. The only change involves interpreting the function L as a

measure of epistemic inaccuracy, instead of a loss function. Thus our four desiderata

become:

E1: L(F, B) = 0 if B(F ) = 1. (Fully believing the truth yields no inaccuracy.)

E2: Inaccuracy L(F, B) strictly increases as B(F ) decreases from 1 towards 0.

(The less credence one gives to the truth, the greater the inaccuracy.)

E3: Inaccuracy L(F, B) depends only on B(F ), not on B(F ′) for F ′ �= F . (This

represents the interpretation of L(F, B) as the inaccuracy of one’s degree of

belief in F , in isolation from inaccuracy on other propositions.)

E4: Inaccuracy is additive when the space � of basic possibilities is generated

by independent subspaces: whenever ω ∈ � takes the form ω1 ∧ ω2 where

ω1 ∈ �1 and ω2 ∈ �2, B(�) = 1, and �1 and �2 are independent in the

sense that P ∗(F1 × F2) = P
∗(F1)P

∗(F2) for all F1 × F2 = {ω1 ∧ ω2 : ω1 ∈

F1, ω2 ∈ F2}, then L(F1 × F2, B) = L1(F1, B⇂�1 ) + L2(F2, B⇂�2 ), where L1
and L2 are the inaccuracy functions on �1 and �2 respectively.

The only formal difference here is that E4 appeals to objective independence,

i.e., independence with respect to the chance function P ∗, rather than subjective

independence with respect to the belief function B.

Everything else follows through as before, but under an epistemic interpretation

rather than a pragmatic interpretation. The four conditions set out above force the

inaccuracy measure to be logarithmic:

L(F, B) = −k log B(F ).

A scoring rule now measures expected inaccuracy, rather than expected loss. As

before, we consider consider a whole class of scoring rules generated by the inclusive

and unbiased weighting functions. For any such weighting function, the (normalised)

belief function which minimises worst-case expected inaccuracy is a probability

function, in the set E of functions that are calibrated to chances, that has maxi-

mum generalised entropy. In particular, this optimal belief function satisfies PI3. The

Principle of Indifference can thus be motivated in terms of epistemic rationality: sup-

posing that epistemic rationality requires minimising worst-case expected inaccuracy,

degrees of belief must then satisfy principle of indifference PI3.



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We thus appear to be spoilt for choice when justifying the Principle of Indiffer-

ence. The formal argument for the Principle of Indifference can be given either a

pragmatic reading, which appeals to the notion of anticipated loss, or an epistemic

reading, which appeals to the notion of inaccuracy. The question arises, should one

prefer an epistemic justification over a pragmatic justification or vice versa?

Rather than evaluate the details of the particular argument presented above, I will

suggest a general answer to this question, in the hope that a general answer will

remain pertinent should new pragmatic or epistemic arguments for the Principle of

Indifference be put forward in the future. The answer that I will present is that the

pragmatic line of argument should be preferred, because of an inconsistency between

the epistemic line of argument and Evidentialism.

4 Evidentialism

In this section we shall explore the Evidentialism thesis and we shall see that this

thesis is inconsistent with the epistemic line of argument presented above.

Evidentialism Consider the following principle: one’s beliefs are rational if and only

if they are compatible with one’s evidence. There are compelling reasons to think that

the ‘if’ is too strong here: rationality seems to demand more than compatibility with

evidence. For example, suppose that a patient has evidence that the chance that she

will survive her cancer (s) given a certain genetic profile (g) is 0.8, i.e., P ∗(s|g) =

0.8, and that the chance she has genetic profile g is 0.7, i.e., P ∗(g) = 0.7. Regarding

survival, these two facts imply that P ∗(s) ∈ [0.56, 0.86].8 Assuming that one should

calibrate degrees of belief to chances, insofar as one has evidence of them, the patient

should believe s to some degree within the same interval, P (s) ∈ [0.56, 0.86]. Now,

it is known that a high degree of belief in survival after cancer influences the chance

of survival itself (e.g., Soler-Vila et al. 2005). It is thus in the patient’s interest that she

adopts a degree of belief in s that is at the higher end of the range. So there is a sense

in which the patient’s degree of belief would be irrational were it not sufficiently

close to 0.86: rationality requires more than mere compatibility with the evidence,

because a degree of belief of 0.56 is compatible with the evidence yet apparently

irrational. This sense of rationality is pragmatic however—it is motivated by the need

to survive, rather than the quest for truth. There is nothing to say that a higher degree

of belief is epistemically more appropriate than a lower degree of belief. After all,

just as higher degrees of belief lead to higher chances of survival, so do lower degrees

of belief lead to lower chances of survival—we may suppose here that all degrees of

belief within the interval [0.56, 0.86] are equally well calibrated to the chances.9

8This follows because P ∗(s) = P ∗(s|g)P ∗(g) + P ∗(s|¬g)P ∗(¬g) = 0.8 × 0.7 + P ∗(s|¬g) × 0.3 and

P ∗(s|¬g) ∈ [0, 1].
9Other pragmatic considerations may apply to the doctor who is treating the patient. If the arguments of

Section 2 are sound then one might argue that the doctor ought to believe s to some degree sufficiently

close to 0.71, the maximum entropy value. Again, this is for pragmatic reasons—other values incur an

avoidable increase in worst-case expected loss.



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This sort of example, which shows that there can be pragmatic grounds for going

beyond the evidence, suggests a reformulation of the above principle:

Evidentialism. One’s beliefs are epistemically rational if and only if they are

compatible with one’s evidence.

The motivation behind this principle is that the evidence provides all we have to go

on in our quest for truth. Advocates of Evidentialism include Conee and Feldman

(2004) and McCain (2014).

Having stated the Evidentialism principle, let us consider some points of clarifi-

cation.

When formulating evidentialism, ‘epistemically justified’ is sometimes used

instead of ‘epistemically rational’. Thus Conee and Feldman (2004, p. 83) provide

the following formulation of evidentialism: ‘Doxastic attitude D towards proposi-

tion p is epistemically justified for S at t if and only if having D towards p fits the

evidence S has at t .’ The two terms are often used interchangeably, although ‘epis-

temically rational’ could be interpreted as a slightly more permissive classification

than ‘epistemically justified’: that beliefs are justified suggests that there exists some

justification that rules them in as appropriate beliefs, whereas beliefs might be classi-

fied as rational just in case there are no grounds for ruling them out as irrational. Here

we shall stick with ‘epistemically rational’, which is more familiar in the Bayesian

context. I use ‘one’s beliefs’ to denote a belief state—perhaps a set of propositions in

the case of qualitative beliefs or a belief function in the case of Bayesian degrees of

belief.

DeRose (2000, §1) notes that there is a sense in which one ought not believe a

proposition unless one believes it on the basis of one’s evidence—i.e., one ought

not believe it for mistaken reasons, even if that belief is, in fact, compatible with

one’s evidence. As DeRose (2000) suggests, this consideration points to a distinction

between different kinds of ‘oughts’: a strong notion which presumes correct ‘basing’

and the weaker notion of the Evidentialism thesis as stated above. The distinction

can be put thus: one rationally believes if and only if one’s beliefs are rational (i.e.,

one’s belief state is compatible with one’s evidence) and that belief state is properly

believed on the basis of one’s evidence. The stronger sense of rationality is clearly a

different sense than that employed in the standard Bayesian notion of rational degree

of belief. The Bayesian notion is oblivious to the actual genesis of beliefs, caring

only whether the values in question are appropriate in the circumstances.

There is a further ambiguity that needs to be addressed. Baehr (2009) suggests

that, whilst there is a sense of epistemic rationality in which Evidentialism is true,

there is another sense in which the epistemic rationality of one’s beliefs depends on

whether the gathering of evidence was defective. According to this stronger ‘ought’,

one ought to gather enough evidence and gather it correctly, as well as ensure that

beliefs are appropriate given that evidence. This is a rather complex ‘ought’: the

individual who deliberately or negligently sees no evil and hears no evil may be both

epistemically defective (as is someone who sees and hears evil but fails to believe it)

and morally defective (as is someone who sees, hears and believes evil but fails to

speak out against it). Again, there is no need for the Bayesian to deny this stronger

‘ought’—it suffices to observe that the Bayesian is primarily interested in the weaker



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sense of epistemic rationality. As in the case of believing on the basis of the evidence,

it is plausible that an adequate account of the stronger ‘ought’ will need to invoke an

adequate account of the weaker notion as a component, so Evidentialism will be at

least part of the story.

It is important to note that Evidentialism as stated above is simply a biconditional

claim, a characterisation of rational belief rather than an analysis of it. It is there-

fore important to distinguish Evidentialism from an evidentialist epistemology, i.e.,

a detailed epistemological theory under which Evidentialism turns out to be true.

Such an epistemological theory would have as a minimum to give detailed accounts

of: rationality; the distinction between epistemic and pragmatic rationality; doxastic

deliberation; compatibility; the nature of evidence; and what it is to possess evi-

dence. Work towards an evidentialist epistemology from an internalist point of view

can be found in Conee and Feldman (2004), Dougherty (2011) and McCain (2014),

for example, and Williamson (2000) develops an evidentialist epistemology from the

externalist perspective (see Williamson 2000, §9.8). Clearly, one does not need to

provide a detailed evidentialist epistemology in order to advocate Evidentialism.

Bayesian Evidentialism Whilst advocates of Evidentialism are not bound to provide

a detailed theory of each of the terms that occur in the Evidentialism claim, it is

incumbent upon proponents of the claim to clarify when it applies. In particular, it is

important to be clear about what a belief state is and when a belief state is compatible

with evidence. Given our concern with norms of Bayesian rational degree of belief

in this paper, we shall explicate Evidentialism by appealing to the concepts set out

in Sections 1 and 2. Thus a belief state is construed as a (normalised) belief function

B and a belief function B is compatible with evidence just in case B ∈ E, the set of

evidentially-compatible chance functions.

The motivation behind this explication of compatibility with the evidence appeals

to the idea that degrees of belief should be calibrated to chances (a principle com-

mon to both the pragmatic approach and the epistemic approach under consideration

here). Recall that E is defined in Section 2 as the smallest set of probability functions

that contains the chance function P ∗, as far as can be determined by the evidence E.

If beliefs should be calibrated to chances then a belief function is compatible with

evidence just when it is calibrated to a chance function that is compatible with evi-

dence. This yields the claim that B is compatible with the evidence if and only if

B ∈ E. This claim can be motivated more precisely as follows.

First we shall see that if B �∈ E then B is incompatible with evidence. Suppose first

that E is a set of expressible propositions—propositions that are not statements about

chances. Then E is the set of all probability functions that give maximal probability

to propositions in E, E = {P ∈ P : P (θ ) = 1 for all θ ∈ E}. Now if B �∈ E

then there is some θ ∈ E such that B(θ) < 1. Such a belief function would be

problematic. It would be Moore-paradoxical to recognise that θ is evidence yet not

to fully believe θ . In general, one ought to calibrate one’s beliefs to truths, insofar as

one has evidence of them, yet this belief function is not calibrated. Hence, this belief

function B is incompatible with evidence. This argument can be extended to the case

in which E provides evidence of non-trivial chances. If B �∈ E then there is some

θ such that B(θ) �= P ∗(θ) for any P ∗ ∈ E. This is also problematic: one ought to



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calibrate one’s degrees of belief to chances, insofar as one has evidence of them, yet

this belief function is not calibrated. Hence, this belief function is incompatible with

the evidence.

On the other hand, if B ∈ E then B is compatible with the evidence, for the

following reason. That B ∈ E means that as far as can be determined from E, B may

well be the true chance function. Suppose new evidence is subsequently obtained that

determines that B is indeed the true chance function P ∗. This new evidence is clearly

compatible with the old evidence E. Call the new evidence base E′ and the new set

of evidentially-compatible chance functions E′. Now, B is compatible with E′: E′,

we may assume, is consistent, so there must be some belief function compatible with

E′; however, as we have just seen, any belief function outside E′ is incompatible with

E′; hence, B, the only function in E′, is compatible with E′. Since B is compatible

with E′ as a whole, it must be compatible with each item of evidence in E′. Hence it

is compatible with each item of evidence in E ⊆ E′, and therefore with E as a whole.

This provides some motivation for the view that belief function B is compatible

with evidence just when B ∈ E. Below, we will revisit the question whether this

characterisation is appropriate. In the meantime, we are now in a position to state a

Bayesian explication of Evidentialism:

BE: Belief function B is epistemically rational if and only if B ∈ E.

We shall call this specialisation of Evidentialism to degree of belief Bayesian

Evidentialism.

Note that BE circumvents one immediate concern with Evidentialism in the

Bayesian framework. This is the concern that Probabilism appears to conflict with

Evidentialism. For example, according to Probabilism, one ought to fully believe all

logical truths, even those for which one has no evidence that they are logical truths.

Thus one ought to fully believe that the millionth digit of π is 5, even if one has no

evidence that this is so. Probabilism, then, seems to be a constraint on rational degree

of belief that operates independently of the evidence, contrary to Evidentialism. In

response to this concern, it suffices to point out that Probabilism is treated as an ide-

alisation by the Bayesian: Probabilism is usually advocated on the grounds that it is a

simple and powerful approximation to a more nuanced, correct norm. In some cases,

such as the application of Bayesianism to mathematics (Corfield 2001), one may need

to invoke a more nuanced norm, but Probabilism suffices for most practical appli-

cations of Bayesianism. BE circumvents these issues by building Probabilism into

compatibility with the evidence: since E is construed as the set of chance functions

compatible with the evidence, it is a subset of P, the set of all probability functions.

Thus there is no conflict between Probabilism and BE. This means that BE should

also be treated as an idealisation—an approximation to a more nuanced claim that

would incorporate the more nuanced, correct norm which Probabilism approximates.

Another point of clarification. In the cancer example at the start of this section,

we supposed that all degrees of belief within the interval [0.56, 0.86] were equally

well calibrated to the chances. Consider a modification to the example where this is

not the case: suppose that individuals with degree of belief P (s) ≥ 0.86 have chance

0.86 of survival, those with degree of belief P (s) ≤ 0.56 have chance 0.56 of survival

and those with degree of belief strictly between 0.56 and 0.86 have some chance of



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survival other than that degree of belief, and that these facts are part of the patient’s

evidence. In this case, only the endpoints of the interval, 0.86 and 0.56, are possible

values of degrees of belief that are calibrated to the chances. This modified example

shows that, in general, some chance functions that are compatible with evidence may

not also be interpretable as belief functions compatible with evidence. The set E was

construed in Section 2 as the set of chance functions that are compatible with the

evidence. In its role in BE, E also needs to be construed as a set of belief functions

compatible with the evidence. Thus we must take E to be the set of those chance

functions compatible with evidence to which belief functions can be calibrated. This

qualification avoids a worry about Evidentialism of Reisner (2015).

Note finally that the evidence may be inconsistent, or it may be the case that there

is no chance function compatible with the evidence which can also be interpreted as

a belief function. Which belief functions are compatible with the evidence in such

situations? Whilst this question needs answering, it involves some subtleties that are

somewhat tangential to the concerns of this paper. For the purposes of this paper, we

may simply stipulate that E = P, the set of all probability functions, in both these

cases. See e.g., Williamson (2010, §3.3.1) for a fuller discussion.

Evidentialism and the Principle of Indifference Having explicated Evidentialism

in terms of BE, we can now move on to its connection to the Principle of Indifference.

The important point for our purposes is that BE is inconsistent with the claim that

if a belief function is epistemically rational then it satisfies Principle of Indifference

PI3. According to BE, any belief function that is compatible with evidence is epis-

temically rational; there is no further requirement that one’s degrees of belief should

equivocate between the basic possibilities. For example, if there is no evidence at

all, then any belief function satisfies the evidence, in particular, a belief function B0
that gives ω, one of the basic possibilities, degree of belief 0. BE would deem such

a belief function to be epistemically rational. On the other hand, if the claim that

epistemic rationality requires PI3 is true then this belief function B0 is not epistemi-

cally rational: one would need to give ω degree of belief 1/|�| rather than 0. Hence,

the claim that epistemic rationality requires PI3 contradicts BE. This claim was the

conclusion of the epistemic justification of the Principle of Indifference presented in

Section 3. Consequently, the epistemic justification is in tension with Evidentialism.

On the other hand, BE is not inconsistent with the claim that if a belief function

is rational simpliciter then it satisfies the Principle of Indifference PI3. BE is a claim

about epistemic rationality, whilst this claim invokes rationality simpliciter. This

claim is the upshot of the pragmatic justification of the Principle of Indifference of

Section 2. Therefore, the pragmatic justification is not in tension with Evidentialism.

The advocate of the pragmatic justification can say one of two things about the rela-

tion between BE and the claim that rationality requires the Principle of Indifference:

either she can point out that there is more to rationality than epistemic rationality, as

demonstrated by the cancer example at the start of this section, or she can deny that

there is such a thing as ‘epistemic rationality’, perhaps motivated by scepticism about

the tenability of a sharp distinction between pragmatic and epistemic rationality or

justification. Either way, the proponent of the pragmatic justification of the Principle



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of Indifference can deny that indifference is a requirement of epistemic rationality.

Hence, there is no tension with Evidentialism, under this view.10

Before exploring some of the consequences of the conflict between Evidential-

ism and the epistemic justification of the Principle of Indifference, let us consider a

possible response to the claim that there is such a conflict. One might suggest that

‘compatible with the evidence’ should be reinterpreted as follows. Instead of saying

that B is compatible with the evidence just when B ∈ E, one might say that B is

compatible with the evidence just when B = P
†
E

, the probability function in E with

maximum generalised entropy. With this stricter interpretation of ‘compatible with

the evidence’, the conflict between Evidentialism and the epistemic justification of

Section 3 dissolves, because the epistemic justification implies compatibility with the

evidence in the new sense.

The problem with this strategy is that the suggested reinterpretation is not a viable

interpretation of ‘compatible with the evidence’. This should already be clear, given

the motivation for BE provided above. To highlight how wrong the reinterpretation

is, suppose that E is correct but is incomplete, i.e., every proposition in E is true

but E does not determine the truth of every proposition. Let ω∗ be the truth function

that represents the true state of the world. Since E is correct, ω∗ ∈ E. Since E is

incomplete, there is some proposition θ such that ω∗(θ) = 1 (i.e., θ is true) but

P
†
E
(θ) < 1.11 Under the proposed reinterpretation, P

†
E

is the only belief function

compatible with the evidence. Therefore the belief function that coincides with the

truth function ω∗ is deemed to be incompatible with the evidence. This is perverse:

despite the fact that the truth is consistent with the evidence, believing the truth is

deemed incompatible with the evidence. The original interpretation of ‘compatible

with the evidence’ does not suffer from this problem.

Thus, the proposed reinterpretation is not viable—it strays too far from our usual

understanding of ‘compatible’. The conflict between Evidentialism and the epistemic

justification of the Principle of Indifference stands.

This epistemic justification is, however, consistent with a significant weakening

of the Evidentialism thesis:

Supervenience. One’s beliefs are epistemically rational if and only if they super-

vene upon one’s evidence.

This Supervenience thesis is consistent with the view that compatibility with the

evidence is neither sufficient nor necessary for epistemic rationality. Particular imple-

mentations of the Supervenience thesis might reinstate necessity, however. One

might, for instance, hold that degrees of belief should be compatible with evidence,

i.e., B ∈ E, and, in addition, B should have maximum generalised entropy. In any

case, according to Supervenience, the evidence is only a part of the story.

10On the second view—denying epistemic rationality—the proponent of a pragmatic justification of the

Principle of Indifference will deny the force of Evidentialism as well as the force of the epistemic

justification.
11The probability function with maximum generalised entropy will give a proposition that is not con-

strained to have probability 1 a probability lower than 1—see Landes and Williamson (2013, Proposition

18).



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In the literature, it is not always clear whether an author is endorsing the Eviden-

tialism thesis or the Supervenience thesis. Thus it is not always clear whether the

inconsistency identified above arises. For example, Mittag (2015, §1) cites Bertrand

Russell as an early evidentialist. Russell did indeed say some things which accord

with this. For example,

the reason for believing no matter what must be found, after sufficient analysis,

in data, and in data alone. Russell (1948, p. 401.)

Despite this, Russell (1948, p. 404) explicitly advocates PI1 and PI2. Since the Prin-

ciple of Indifference apparently goes well beyond the data, the inconsistency between

the epistemic justification of the Principle of Indifference and Evidentialism might

seem to pose a problem for Russell. However, it is not entirely clear that Russell was

a genuine evidentialist, i.e., an advocate of Evidentialism rather than Supervenience.

Without further textual evidence, we cannot conclude that Russell falls foul of this

inconsistency.

On the other hand, we have seen that it is indeed clear that a proponent of the epis-

temic justification of the Principle of Indifference cannot consistently also advocate

Evidentialism. We shall suggest next that this inconsistency reflects negatively on the

epistemic justification.

5 Consequences for consequentialism

Thus far we have seen that the Principle of Indifference can be given a pragmatic

justification in terms of avoiding avoidable loss; it can also be given an epistemic

justification in terms of reducing inaccuracy; however, this latter form of justification

conflicts with Evidentialism. We shall now explore some of the consequences of this

tension.

The pragmatic vs the epistemic interpretation Let us first turn to the main ques-

tion of the paper: should the new formal justification of the Principle of Indifference

be given a pragmatic reading in terms of loss or an epistemic reading in terms of

inaccuracy? I shall argue as follows. Evidentialism is prima facie plausible, and,

moreover, extant objections to Evidentialism miss the mark. Therefore, the con-

flict between the epistemic argument and Evidentialism should be taken to reflect

negatively on the epistemic argument. On the other hand, the pragmatic version

of the justification coheres well with Bayesianism, and as we have seen, is not in

tension with Evidentialism. On balance, then, Evidentialism favours the pragmatic

interpretation of Section 2 over the epistemic interpretation of Section 3.

First, Evidentialism is, at least prima facie, rather plausible. Beliefs can be useful

in various ways, as we saw in the cancer example. However, their epistemic value lies

in the extent to which they latch on to the truth. In that sense, the epistemic aim of

belief is truth. Now, evidence and inference appear to provide our only route to truth.

Unless one can infer something about a proposition from one’s evidence, there is



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no reason to suspect that the proposition is true.12 Beliefs are epistemically rational,

then, just when they are compatible with (what can be inferred from) evidence.

In order to provide a full defence of Evidentialism, one would need to develop

a detailed evidentialist epistemology which validates the principle. This will not be

attempted here, for two reasons. First, this is a major enterprise, and not one that can

be adequately carried out in the remaining pages of this paper. Second, any detailed

epistemological theory is likely to involve so many controversial components as to

render the whole less plausible than the Evidentialism principle itself. Hence, such a

theory will not be very confirmatory.

Instead, I shall aim to explain why key objections related to Evidentialism miss

the mark in the sense that they do not help the proponent of the epistemic justifica-

tion of the Principle of Indifference. Whilst this will not go far enough to establish

the truth of Evidentialism, when taken together with the prima facie plausibility of

Evidentialism it will arguably render Evidentialism in a strong position.

Objections to Evidentialism offer no succour to proponent of the epistemic justi-

fication for six main reasons. (i) Some are objections to evidentialist principles other

than the Evidentialism thesis as explicated by BE. For example, Sharadin (2016)

observes that non-evidential considerations can play a role as motivating reasons for

beliefs (a self-fulfilling prophesy, such as belief in survival after cancer, can be such

a consideration). However, as Sharadin acknowledges, this fact would not undermine

the normative version of Evidentialism set out above (Sharadin 2016, §3). (ii) Some

are objections to features of evidentialist epistemologies, rather than to the Eviden-

tialism thesis itself. For example, there are many objections to features and details

of Conee and Feldman’s epistemological theory, such as to its internalism (see, e.g.,

Dougherty 2011, Part V). There are also many objections to Timothy Williamson’s

externalist epistemology, such as to its identification of evidence and knowledge (see,

e.g., Williamson 2015). These features are not implied by Evidentialism, so prob-

lems with these features do not falsify Evidentialism. (iii) Some objections arise

from showing that Evidentialism turns out false under one or other non-evidentialist

epistemology (see, e.g., Stich 1990; Axtell 2011). As noted above, any detailed epis-

temological theory will be so controversial as to offer little scope for confirming

or undermining Evidentialism. (iv) Some are objections to arguments in favour of

Evidentialism, rather than to Evidentialism itself. For example, Steglich-Petersen

(2008), Yamada (2010), Sharadin (2016) and Rinard (2017, §8) provide objections

to an argument for Evidentialism put forward by Shah (2006). (v) Some are objec-

tions which, if successful, would not only undermine Evidentialism but would also

undermine Bayesianism, so cannot be used to favour the epistemic argument over

Evidentialism for the purposes of justifying the Principle of Indifference. For exam-

ple, as discussed in Section 4, DeRose (2000, §1) and Baehr (2009) are concerned

that Evidentialism handles too weak a sense of epistemic rationality. However, the

weak sense is precisely the sense of epistemic rationality that Bayesianism tack-

12Perhaps certain logical or mathematical truths can be inferred without evidence. But then they can triv-

ially also be inferred from evidence. Perhaps certain other propositions are self-evident. But then they too

can be inferred from evidence.



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les. (vi) Some objections, if successful, would not only undermine Evidentialism but

would also undermine the epistemic argument so do not help proponents of such a

justification of the Principle of Indifference. For example, Marušić (2012) argues that

decisions and promises require believing propositions which may be contrary to the

evidence. If so, these are beliefs which are also more inaccurate than those in accord

with the evidence. To take another example, Littlejohn (2012, §7.3) argues against

Evidentialism by claiming that belief is factive. As we shall see below (c.f. principle

I5), this claim is as unattractive to the proponent of an inaccuracy argument as it is to

the evidentialist.

We have seen that objections to Evidentialism fail to help the proponent of

the epistemic justification of the Principle of Indifference. Since Evidentialism is

prima facie plausible, the fact that Evidentialism is in tension with the epistemic

justification casts some doubt on that justification.

On the other hand, no such doubt is cast on the pragmatic version of the jus-

tification of PI3. This is because, as we saw above, there is no tension between

Evidentialism and the pragmatic justification. Furthermore, the pragmatic justifica-

tion is relatively unproblematic because it rests on the idea that one should avoid

avoidable loss, and this goal is central to Bayesianism. The main existing argument

for Probabilism—the Dutch book argument (de Finetti 1937)—is based on exactly

this premiss, since it shows that degrees of belief must be probabilities if one is

avoid avoidable sure loss in a particular betting set-up. Moreover, Bayesianism is

intended as a practical theory which can guide decision making, and the supposition

behind Bayesian decision theory is that one should avoid avoidable loss by max-

imising expected utility. The pragmatic reading of the justification of the Principle

of Indifference is clearly also of the form avoid avoidable loss: in this case, avoid

avoidable worst-case expected loss. Hence, the pragmatic reading coheres well with

Bayesianism.

In sum, then, the epistemic justification of Section 3 comes out worse than the

pragmatic justification of Section 2 from its clash with Evidentialism.

Other epistemic justifications of the Principle of Indifference It is worth not-

ing that the epistemic argument of Section 3 is not the only epistemic argument for

the Principle of Indifference. Pettigrew (2016b), for example, gives another argu-

ment in terms of epistemic inaccuracy. This argument is for PI1; recall that this says

that when there is no evidence one should believe each basic possibility to the same

extent. Suppose, then, that there is no evidence, E = ∅. Pettigrew supposes that a

measure I (ω, B) of inaccuracy measure should satisfy the following two require-

ments. First, there is no other belief function B that has as low inaccuracy as the

equivocator function for all basic possibilities: if B �= P= then there is some ω

such that I (ω, B) > I (ω, P=). Second, Pettigrew requires that I is invariant under

isomorphisms acting on the set of propositions. (As with the earlier requirement

that a weighting function be unbiased, this second requirement ensures that inaccu-

racy measures are invariant under permutations of the basic possibilities.) The first

requirement forces the equivocator function to be less inaccurate than B for some

basic possibility, and the second forces the inaccuracy of the equivocator function to

be the same for every basic possibility. Hence the equivocator function is the belief



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function with minimum worst-case inaccuracy, where the worst case is taken over all

ω. Thus, Pettigrew argues, PI1 must hold if one is to minimise worst-case inaccuracy.

It is important to observe that the grounds for preferring the pragmatic justifica-

tion of Section 2 over the epistemic version of Section 3 also apply to other epistemic

justifications of the Principle of Indifference, such as Pettigrew’s justification. This

is because it is the claim that epistemic rationality requires the Principle of Indiffer-

ence that is inconsistent with Evidentialism, and any epistemic justification of the

Principle of Indifference will yield that claim. Thus, if this inconsistency favours

the pragmatic justification of Section 2 over the epistemic justification of Section

3, it also favours the pragmatic justification of Section 2 over any other epistemic

justification of the Principle of Indifference.

A further worry is relevant here. Any other epistemic justification of the Prin-

ciple of Indifference that appeals to inaccuracy will need to provide an account of

inaccuracy that is somewhat different to that given in Section 3. Such an account of

inaccuracy will hinge on a complex package of claims which can be hard to justify.

This complexity, when contrasted with the simplicity and prima facie plausibility of

Evidentialism, may cast further doubt on the inaccuracy account.

To get a sense of this complexity, consider that any account of epistemic inaccu-

racy which underpins a purely epistemic justification of the Principle of Indifference

will depend upon the following claims I1–5, which I shall collectively call the

Inaccuracy Package:

I1: Some specific function f can measure the inaccuracy of a belief function.

In the case of our epistemic justification of Section 3, a logarithmic function was

singled out as most appropriate. Pettigrew (2016b) favours the quadratic Brier score,

though his justification of PI1 considered a class of inaccuracy measures. Different

classes of inaccuracy measures have appeared in the literature, often delineated by

technical fruitfulness rather than philosophical considerations—e.g., ‘strictly proper’

inaccuracy measures are particularly conducive to proving the required theorems

(Landes 2015). As yet, we are far from a consensus as to which functions are appro-

priate as inaccuracy measures. Worse, it is controversial whether inaccuracy is the

sort of thing which can be measured by a single number, and which depends only

on the belief function in question and the true state of the world. It is still an open

question whether the quest for an inaccuracy measure will turn out to be as quixotic

as the closely related quest for a measure of verisimilitude. This stands in contrast

to loss (negative utility), which is so well entrenched within the Bayesian framework

that Bayesians find it uncontroversial that one can measure loss by a single number

that depends on the belief function and the state of the world.

I2: Some unique norm N(f ) governs inaccuracy.

In the epistemic justification of Section 3, the norm is to minimise worst-case

expected f . In Pettigrew’s justification, it is to minimise worst-case f . Another norm

often invoked by proponents of inaccuracy arguments is to avoid dominated f . Note

that these norms conflict; we saw that the first norm warrants PI3 but the second

norm does not. Apparently, the third norm, which can be used to motivate Probabil-

ism, cannot be used to justify even PI1 (Pettigrew 2016a, Chapter 12). Therefore, the



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proponent of inaccuracy arguments needs to provide grounds for singling out which

norm, or which combination of norms, should be applied. This has not been done as

yet.

I3: N(f ) is a purely epistemic standard.

Inaccuracy needs to be a purely epistemic standard if it is to provide a purely epis-

temic justification of the Principle of Indifference. Now, the word ‘inaccuracy’ has

epistemic connotations, but in the light of the epistemic justification of Section 3,

which characterises the inaccuracy measure in just the same way that the measure

of loss was characterised in Section 2, one might worry whether inaccuracy is just

loss in disguise. This casts some doubt on whether inaccuracy is a purely epis-

temic consideration. Is minimisation of inaccuracy an appropriate standard? Truth is

uncontroversially an epistemic standard, but a small improvement in the accuracy of

a belief is less obviously so—in many circumstances, a miss is as good as a mile,

and one might hold that truth is the only epistemic standard to which beliefs should

conform.

I4: N(f ) is necessary for rationality of belief.

Truth is an epistemic standard but it is not normally thought of as necessary for

rationality of belief. Even if minimisation of inaccuracy is an epistemic standard,

some further consideration—currently lacking—needs to be provided before we can

be convinced of its necessity for rationality. Furthermore this consideration needs to

be clearly epistemic. Suppose our beliefs fit our evidence; should we follow norm

N(f ) in addition? f doesn’t tell us any more about what is true than the evidence

does. Of course, we might worry that we’ll lose out in proportion to our inaccuracy—

but then avoiding inaccuracy is a pragmatic desideratum, rather than an epistemic one.

I5: N(f ) is sufficient for rationality of belief.

Now, N(f ) can only be sufficient for epistemic rationality if no other norm is nec-

essary. One concern for sufficiency is a slippery slope: if accuracy is necessary for

rationality, then why isn’t truth also necessary? The proponent of inaccuracy argu-

ments needs to say why such a move is not warranted. The proponent will want

to resist such a move, not only because requiring truth over and above controlling

inaccuracy contradicts the sufficiency of N(f ), but also because if rational belief

is factive then very implausible principles of indifference will follow. For example,

suppose that truth were also necessary for rationality of belief in the sense that a

degree of belief in basic possibility ω which surpasses some threshold τ , P (ω) > τ ,

is epistemically rational only when ω is true. Unless the evidence forces the truth of

some possibility ω, it is impossible to determine for sure whether ω is true, and hence

whether it is epistemically rational to set P (ω) > τ . Hence, the Bayesian can nor-

mally only be sure of following this truth norm by setting P (ω) ≤ τ for every ω, i.e.,

by being sufficiently indifferent between the basic possibilities. In cases where, for

some ω, evidence fails to force the truth of ω but implies that the chance of ω exceeds

the threshold, P ∗(ω) ≥ τ , this truth norm violates the requirement that P ∈ E. Advo-

cating degrees of belief which are incompatible with the evidence clearly goes much

further than most Bayesians would like.



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There is another, more well known, worry about I4–5, presented by Greaves (2013,

p. 918):

Emily is taking a walk through the Garden of Epistemic Imps. A child plays

on the grass in front of her. In a nearby summerhouse are n further children,

each of whom may or may not come out to play in a minute. They are able to

read Emily’s mind, and their algorithm for deciding whether to play outdoors

is as follows. If she forms degree of belief 0 that there is now a child before

her, they will come out to play. If she forms degree of belief 1 that there is a

child before her, they will roll a fair die, and come out to play iff the outcome

is an even number. More generally, the summerhouse children will play with

chance (1 − 1
2
q(C0)), where q(C0) is the degree of belief Emily adopts in the

proposition (C0) that there is now a child before her. Emily’s epistemic decision

is the choice of credences in the proposition C0 that there is now a child before

her, and, for each j = 1, . . . , n, the proposition Cj that the j th summerhouse

child will be outdoors in a few minutes’ time.

The problem is as follows. Assume Emily knows all the facts set out above. Emily’s

evidence determines that C0 is true. The Bayesian will want to say that she should

fully believe C0. Then, for j = 1, . . . , n, the chance of each Cj will be
1
2

and the

Bayesian will prescribe degree of belief 1
2

in each Cj . These latter degrees of belief

will be deemed inaccurate by some typical inaccuracy measures f , leading to low

overall accuracy. Much greater total accuracy would be achieved if Emily were to

fully disbelieve C0, contrary to her evidence, and fully believe C1, . . . , C10, which

would then all be true. Now, if following N(f ) were necessary and sufficient for

rationality of belief then Emily should adopt the latter beliefs. Such a norm, which

requires violating the evidence, would be very unpalatable to the Bayesian.13

Such examples strengthen the conflict between Evidentialism and inaccuracy

arguments. Evidentialism states that compatibility with the evidence is necessary and

sufficient for rationality of beliefs. Inaccuracy arguments can conflict both the neces-

sity and sufficiency of this claim: whilst the fact that inaccuracy arguments are used

to justify the Principle of Indifference contradicts sufficiency, the Epistemic Imps

example tells against necessity.

The Inaccuracy Package, then, is not only complex, it is also riddled with lacunae

and challenges. This is not to say that all these challenges are insuperable, rather that,

currently, the Inaccuracy Package is merely a promissory note. On the other hand, I

have argued that Evidentialism is prima facie plausible and objections to Evidential-

ism miss the mark. Consequently, Evidentialism is more credible than the Inaccuracy

Package, and, because of the inconsistency between inaccuracy arguments and Evi-

dentialism, a pragmatic justification of the Principle of Indifference will be on firmer

ground than any epistemic justification which appeals to inaccuracy.

Let us consider a response to this line of argument, to be found in some comments

of Pettigrew (2016b, §3.1). Pettigrew expresses the worry that evidentialists need to

13Easwaran and Fitelson (2012) present some other situations in which avoiding inaccuracy requires

violating the evidence.



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invoke multiple cognitive goals. For example, it is not enough to fit our beliefs to the

evidence we have—we ought to gather new evidence too. On the other hand, pro-

ponents of inaccuracy arguments need only invoke a single cognitive virtue, namely

accuracy. According to Pettigrew, the single goal of accuracy explains both the need

to fit evidence and the need to gather new evidence because they both help to reduce

inaccuracy. Thus the Inaccuracy Package should be preferred over Evidentialism on

the grounds that the former is more explanatory than the latter.

There are various compelling rejoinders open to the evidentialist, however. First,

avoiding inaccuracy fails to explain the need to fit evidence. As Pettigrew acknowl-

edges, examples like the Epistemic Imps example set out above show that one should

not always fit the evidence if one is to minimise inaccuracy. Second, avoiding inaccu-

racy also fails to explain the need to gather new evidence. If accuracy were the only

goal, one should not gather new evidence wherever that evidence is likely to lead to

less accurate degrees of belief—i.e., one should not gather evidence that is likely to

reveal that the chances are such as to lead to higher values of the chosen inaccuracy

measure f when degrees of belief are calibrated to those chances. Third, although

the proponent of inaccuracy arguments might claim that the inaccuracy approach is

more explanatory, that plays into the hands of the evidentialist, whose objection is

precisely that inaccuracy arguments explain too much: they explain pragmatic norms

such as the Principle of Indifference when they should only be explaining epistemic

norms. Fourth, it is far from clear that the evidentialist fails to explain both the need to

gather evidence and the need to fit beliefs to evidence. BE cashes out Evidentialism

in terms of calibration of degrees of belief to chances. Better calibration to chances

can explain the need to gather more evidence of chances as well as to fit degrees of

belief to current evidence of chances.

Consequently, Pettigrew’s response does not succeed. The inconsistency between

Evidentialism and inaccuracy arguments for the Principle of Indifference does indeed

favour the former over the latter. A Bayesian seeking a justification for the Principle

of Indifference should prefer a pragmatic justification over the epistemic justification

of Section 3.

Inaccuracy arguments in general. We have seen that inaccuracy arguments for the

Principle of Indifference are problematic, and that pragmatic arguments arguably fare

better in motivating the Principle of Indifference. But these worries extend beyond

the Principle of Indifference. This is because inaccuracy arguments for the Principle

of Indifference are of exactly the same kind as inaccuracy arguments for other norms

of Bayesianism—not only Probabilism but also norms, such as the Principal Princi-

ple, which require calibration of degrees of belief to chances (see, e.g., Joyce 1998;

Leitgeb and Pettigrew 2010a, b). If these arguments fail with respect to the Princi-

ple of Indifference, then that is a problem for the Inaccuracy Package in general. The

whole project of using inaccuracy arguments to provide an epistemic consequentialist

justification of Bayesianism is brought into question.

One possible response to this problem is to somehow differentiate inaccuracy

arguments for the Principle of Indifference from those for other Bayesian norms, in

order to prevent objections to the former from affecting the latter. Perhaps the most

promising point of differentiation is in the choice of norm—I2 of the Inaccuracy



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Package. If one restricts inaccuracy arguments by appealing solely to avoiding dom-

inance of inaccuracy, not worst-case inaccuracy nor worst-case expected inaccuracy,

one may hope to firewall Probabilism and the Principal Principle from the incon-

sistency with Evidentialism. As noted above, avoiding dominance of inaccuracy can

be used to justify Probabilism and the Principal Principle, but not the Principle of

Indifference. The proponent of inaccuracy might then accept that the Principle of

Indifference is a pragmatic norm, but maintain that Probabilism and the Principal

Principle are epistemic norms, to be justified in terms of avoiding inaccuracy. But

what are the grounds for dismissing the suggestion that one should minimise worst-

case inaccuracy or minimise worst-case expected inaccuracy? As yet, we are lacking

a principled answer to this question.

Even if a principled response can be given to this question, proponents of

inaccuracy arguments are left with a further question: why should some norms

of Bayesianism—e.g., Probabilism—be given a non-pragmatic justification and

others—such as the Principle of Indifference—a pragmatic justification? Having two

forms of justification seems otiose, especially when one can get away with a single,

unified, pragmatic justification of all the norms of Bayesianism, such as that provided

in Section 2.

Epistemic Consequentialism The conflict between Evidentialism and inaccuracy

arguments for Bayesianism renders the latter implausible, if the above reasoning

is sound. But inaccuracy arguments for Bayesianism are the best available epis-

temic consequentialist arguments for Bayesianism. So epistemic consequentialism

in general is on thin ice. If Bayesianism can’t be justified in terms of its epistemic

consequences, the pragmatic approach remains the most promising.

Of course epistemic consequentialism isn’t a one-horse race. Reliabilism is an

alternative option to inaccuracy arguments for Bayesian norms. In the context of

degrees of belief, the reliabilist approach has been much less thoroughly investi-

gated than the inaccuracy approach, but Dunn (2015) has suggested that the reliabilist

approach is preferable to the inaccuracy approach. Besides taking issue with the inac-

curacy approach, Dunn argues for an alternative that measures the reliability of a

belief-formation process by how well calibrated the resulting degrees of belief are to

chances. There are three main worries about this sort of approach, as we shall now

see.

First, a Bayesian calibration norm is one of the things that we would like to

justify—a justification which itself appeals to calibration is hardly likely to be very

convincing there.

Second, it appears that this reliability account might conflict with Probabilism,

which would be a serious concern for the Bayesian.14 Suppose that a visual process is

correct 95% of the time and an auditory process is correct 80% of the time. Someone

who sees no evil and hears no evil would be perfectly rational, under this sort of reli-

abilist approach, to believe that there is no evil to degree 0.95 and to degree 0.8—i.e.,

14I am very grateful to Jeff Dunn for pointing this out.



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to believe the same proposition to two different degrees, contradicting Probabilism.

One might try to save Probabilism by modifying the account to characterise ratio-

nality in terms of the reliability of the process determining the belief function as a

whole, rather than individual degrees of belief. In our example, if the prior probabil-

ity that there is no evil present is 0.5, then its posterior probability, given that evil is

neither seen nor heard, is about 0.99, so this latter (unique) degree of belief would be

rational.15 But then Probabilism follows too easily—it follows directly from the stip-

ulation that the one should focus on the reliability of the belief function as a whole,

together with the stipulation that rationality is assessed in terms of calibration to

chances and the assumption that chances are probabilistic. For such a move to be con-

vincing, the two stipulations would need to be given some independent motivation.

This is currently lacking.

The third worry about this reliabilist approach is that, although it is unclear

whether or not such a line of argument will extend to justifying the Principle of

Indifference, difficulties arise either way. On the one hand, so-called ‘concentration

theorems’ suggest that probability functions that are indifferent (i.e., have maximum

entropy) are overwhelmingly likely (see, e.g., Jaynes 2003, §11.4), so perhaps a case

can be made for an indifferent belief function being better calibrated to chances. But

then there would be a conflict between reliabilism, which would deem the Principle

of Indifference to be an epistemic norm, and Evidentialism, which would class it as

non-epistemic. This takes the epistemic consequentialist back to square one, a con-

flict with Evidentialism. On the other hand, if the Principle of Indifference does not

admit a reliabilist justification, the epistemic consequentialist is left with the prob-

lem of motivating a mixture of pragmatic and epistemic justifications for different

Bayesian norms, instead of simply adopting a unified pragmatic justification, such as

that of Section 2.

Given the current state of play, then, Bayesianism is best motivated pragmati-

cally. Epistemic consequentialism remains an interesting project, but there is a lot

more to do before either an approach based on the Inaccuracy Package or a reliabilist

approach can offer a viable alternative to pragmatic justifications of Bayesian norms

in terms of avoiding avoidable losses.

Acknowledgments I am grateful to the UK Arts and Humanities Research Council and to the Lever-

hulme Trust for supporting this research, to Teddy Groves for commenting on a previous version of this

paper, and to Jeff Dunn, Michael Wilde and the anonymous referees for helpful comments and suggestions.

15Let e, s and h denote respectively that there is no evil present, no evil has been seen, no evil has been

heard. Suppose that seeing and hearing evil are probabilistically independent conditional on the presence

or absence of evil. We have that P (e) = 0.5, P (s|e) = P (¬s|¬e) = 0.95, P (h|e) = P (¬h|¬e) = 0.8.

Hence,

P (e|sh) =
P (s|e)P (h|e)P (e)

P (s|e)P (h|e)P (e)+P (s|¬e)P (h|¬e)P (¬e)
=

0.95×0.8 × 0.5

0.95×0.8×0.5+0.05 × 0.2 × 0.5
≈ 0.987.



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Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0

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tribution, and reproduction in any medium, provided you give appropriate credit to the original author(s)

and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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	Justifying the principle of indifference
	Abstract
	Three versions of the principle of indifference
	Pragmatic justification: controlling loss
	Normalisation
	Loss
	Expected loss
	Worst-case expected loss



	Epistemic justification: reducing inaccuracy
	Evidentialism
	Evidentialism
	Bayesian Evidentialism
	Evidentialism and the Principle of Indifference



	Consequences for consequentialism
	The pragmatic vs the epistemic interpretation
	Other epistemic justifications of the Principle of Indifference
	Inaccuracy arguments in general.
	Epistemic Consequentialism



	Acknowledgments
	Open Access
	References