Coherence in Epistemology and Belief Revision*

SVEN OVE HANSSON

COHERENCE IN EPISTEMOLOGY AND BELIEFREVISION*

ABSTRACT. A general theory of coherence is proposed, in which systemicand relational coherence are shown to be interdefinable. When this theory isapplied to sets of sentences, it turns out that logical closure obscures thedistinctions that are needed for a meaningful analysis of coherence. It isconcluded that references to ‘‘all beliefs’’ in coherentist phrases such as ‘‘allbeliefs support each other’’ have to be modified so that merely derivedbeliefs are excluded. Therefore, in order to avoid absurd conclusions,coherentists have to accept a weak version of epistemic priority, that sortsout merely derived beliefs. Furthermore, it is shown that in belief revisiontheory, coherence cannot be adequately represented by logical closure, buthas to be represented separately.

1. INTRODUCTION

The notion of coherence has mostly been discussed in relationto coherentist epistemology, but its area of application ismuch wider. Coherence has a role also in some epistemologi-cal systems commonly classified as foundationalist, such asthat of Lewis (1946). It has important applications in otherbranches of philosophy, including moral philosophy. Coher-ence, or attempts to achieve it, is essential in John Rawls’sethical theory. Others have questioned the feasibility of afully coherent ethical system, and claimed that there is a basicheterogeneity in the realm of values, distinguishing it fromthe realm of facts (Daya, 1960). Coherence is also an impor-tant property of systems of goals and plans. Hence, Millgram(2000) claims that coherent plans make more sense thancoherent theory choice.

*Contribution to ‘‘Seven Bridges’’

Philosophical Studies (2006) 128:93–108 � Springer 2006DOI 10.1007/s11098-005-4058-7

The literature on coherence is large, but most of it is focusedon specific application areas, and too little emphasis has beenput on the more general structural issues. Furthermore,although semi-formal treatments are common, the advantagesof a fully formalized system have seldom been made use of. Inthis paper, I intend to show that philosophically importantinsights can be obtained in this area by applying formal meth-ods, including those developed in belief revision theory. Theneed for precise formalizations was also pointed out by Bender(1989) in his plea for a ‘‘move from mere ’theory-sketch’ toactual theory’’ in studies of coherence.

Section 2 introduces a general definition of coherence. Insection 3, this theory is applied to sets of sentences, i.e. setsthat have a logical structure. The major conclusions of thisinvestigation are summarized in section 4.

2. A GENERAL THEORY OF COHERENCE

2.1. Basics

I will assume that coherence is a property of sets, i.e. unor-dered classes of elements. In section 2, no structure is imposedon the elements of the sets in question. Intuitively speaking,coherence refers to some property that keeps the elements to-gether. In epistemology, coherence may be inferential, eviden-tial, explanatory, or probabilistic (Hansson and Olsson, 1999).In applications to sets of plans or goals, coherence may referto the extent to which the achievement of some goals or plansfacilitates the achievement of other goals or plans.

Coherence has often been treated as a categorical (all-or-nothing) notion, i.e. a set is assumed to be either (fully)coherent or not coherent at all. As was observed by Brendel(1999), a more general approach should include a gradationalnotion of coherence, one in which coherence comes in degrees.Given a gradational notion, it is a trivial matter to introduce acategorical notion by just inserting a limit on the scale ofdegrees of coherence, below which a set is counted as incoher-ent and above which it is counted as coherent. The gradational

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approach will be adopted here; for simplicity it will be assumedthat degrees of coherence can be expressed as real numbers.(An alternative approach would be to introduce a binary rela-tion ‘‘at least as coherent as’’. This approach is more generalsince the relation may be incomplete and intransitive. Thenumerical approach is chosen for reasons of simplicity.)

Bender (1989) introduced the terms ‘‘systemic’’ and ‘‘rela-tional’’ coherence. Systemic coherence is the coherence of abelief system as a whole, whereas relational coherence holdsbetween a part of such a system and the rest of the sys-tem. Both these notions have a tradition in epistemology.Quine’s and Bonjour’s coherentism refers to systemic coher-ence whereas the coherence discussed by Lehrer comes clo-ser to the relational variant. The distinction can easily betransferred to other subject-matter than knowledge, and itcan therefore be included in a general theory of coherence.

Bender (1989) asked the important question whether sys-temic coherence holds in a set just in case there is relationalcoherence between each of its elements and the remainingmembers. We can generalize this question by asking: Can sys-temic coherence be defined in terms of relational coherence?The obvious converse question (already treated in Olsson1999) is: can relational coherence be defined in terms of sys-temic coherence?

A further issue of interdefinability arises if we relate thesystemic coherence of a set to that of its proper subsets:Given that we know the systemic coherence of each of theproper subsets of a set A (its ‘‘subsystemic coherence’’), canwe then infer the systemic coherence of A itself?

To treat these issues, we need to introduce two measures,one for systemic and one for relational coherence. For each setA, let s(A) be its systemic coherence, and for each pair (A,B)of two sets, let r(A,B) be the relational coherence of A with B.

Clearly, the addition of an element to a set can eitherincrease or decrease its coherence. Therefore, systemiccoherence will not be assumed to be monotonic or antimono-tonic, i.e. from A � B neither s(A) ‡ s(B) nor s(A) £ s(B)can be concluded.1 Relational coherence is assumed to be

COHERENCE IN EPISTEMOLOGY AND BELIEF REVISION 95

order-independent, i.e. r(A,B)=r(B,A) for all A and B. Thisgeneralizes Thagard’s (1991) proposal, in a categorical con-text, that coherence is symmetric in the sense that A cohereswith B if and only if B coheres with A.

2.2. Systemic and Subsystemic Coherence

There are good reasons why the systemic coherence of a setcannot in general be derived from the systemic coherence ofits proper subsets. Consider the three elements:

(a) Roger is married.(b) Roger has been ordained.(c) Roger is a catholic.

Given reasonable background beliefs, the set {a,b,c} ismuch less coherent than any of its proper subsets. It is alsopossible to construct sets of four etc. elements in which thecoherence of the whole set depends on features that are notpresent in any of its proper subsets. However, for larger num-bers of elements these examples will have to be more con-trived, and the argument loses much of its practical relevance.

To make this precise, we can use an additive model inwhich for each set A, its systemic coherence s(A) is the sumof the (positive or negative) contributions to its coherenceobtained from each of its proper subsets. We can introduce afunction b that represents the uniquely contributed coherence.For each set A, let b(A) be the contribution to its coherencethat is not obtained from any of the proper subsets ofA. Hence, in general:

ð1Þ sðBÞ ¼ RfbðXÞjX � Bg

Note that b only distributes s(B) among the subsets of B; theintroduction of b does not add any new assumption or prop-erty to the function s that represents systemic coherence.Indeed, s and b are interdefinable in finite domains; if wehave the value of s(A) for every set A, then we can obtainb(A) for every set A by recursion on the number of elements,since for every A, b(A)=s(A))

P{b(X) | X�A}.

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We can assume that s(Ø)=0. Then b(Ø)=0. It is also rea-sonable, at least for some purposes, to assume that b({x})=0for all singletons {x}.2 It then follows that b({x,y})=s({x,y})for all sets {x,y} with exactly two elements.

In some applications it may be reasonable to assume thatb(A)=0 for all sets A above a certain size limit. For a simpleexample, let this size limit be 3, so that the degree of systemiccoherence of any set is equal to the sum of the contributionsof its subsets with at most three elements. Assuming forsimplicity that singletons do not contribute to coherence, sothat b({x,y})=s({x,y}) for all x and y, this means for a set{a,b,c,d} of four elements that:

sðfa;b; c;dgÞ ¼bðfa;b; cgÞþ bðfa;b;dgÞþ bðfa; c;dgÞþ bðfb; c;dgÞþ bðfa;bgÞþ bðfa; cgÞþ bðfa;dgÞþ bðfb; cgÞþ bðfb;dgÞþ bðfc;dgÞ¼sðfa;b; cgÞþ sðfa;b;dgÞþ sðfa; c;dgÞþ sðfb; c;dgÞ� sðfa;bgÞ� sðfa; cgÞ� sðfa;dgÞ� sðfb; cgÞ� sðfb;dgÞ� sðfc;dgÞ:

Similarly, for a set {a,b,c,d,e} with five elements we obtain

sðfa;b;c;d;egÞ¼ bðfa;b;cgÞþbðfa;b;dgÞþbðfa;b;egÞþbðfa;c;dgÞþbðfa;c;egÞþbðfa;d;egÞþbðfb;c;dgÞþbðfb;c;egÞþbðfb;d;egÞþbðfc;d;egÞþbðfa;bgÞþbðfa;cgÞþbðfa;dgÞþbðfa;egÞþbðfb;cgÞþbðfb;dgÞþbðfb;egÞþbðfc;dgÞþbðfc;egÞþbðfd;egÞ¼ sðfa;b;cgÞþ sðfa;b;dgÞþ sðfa;b;egÞþ sðfa;c;dgÞþ sðfa;c;egÞþ sðfa;d;egÞþ sðfb;c;dgÞþ sðfb;c;egÞþ sðfb;d;egÞþ sðfc;d;egÞ�2sðfa;bgÞ�2sðfa;cgÞ�2sðfa;dgÞ�2sðfa;egÞ�2sðfb;cgÞ�2sðfb;dgÞ�2sðfb;egÞ�2sðfc;dgÞ�2sðfc;egÞ�2sfd;egÞ:

In the literature on epistemic coherence, several authors haveemphasized that a coherent set of beliefs should not contain


‘‘isolated’’ subsystems that are unconnected with the rest ofthe set. (For references, see Hansson and Olsson (1999). Seealso Spohn (1999).) In the present gradational approach, twomutually exclusive sets A and B can be defined as discon-nected if b(X)=0 for all X such that A\X „ Ø „ B\X. Suchdisconnectedness leads to a drastic reduction in the numberof possible contributions to systemic coherence. If A and Bhave four elements each, then the condition that b(X)=0 ifA\X „ Ø „ B\X reduces the maximal number of subsets Xof A[B with b(X) „ 0 from 247 to 22. This example confirmsthe high relevance of disconnected subsets for overall sys-temic coherence.

2.3. Systemic and Relational Coherence

A definition of relational coherence in terms of systemiccoherence was proposed by Olsson (1999) in a categoricalframework. According to that proposal, A is coherent withB if and only if the combination of A and B (for sets: A [B) is coherent. This proposal can be transferred to our gra-dational framework through the formula r(A,B)=s(A [ B).However, cases can easily be found in which this proposal isimplausible. Consider the following four statements aboutbat species:

(a) The Noctilio leporinus has longer legs, toes and clawsthan other bats.

(b) The Noctilio leporinus flies low over coastal water, catch-ing surface crustacea with its legs.

(c) The incisors of the Desmodus rotundus are razor-edged,whereas its cheek teeth are degenerate.

(d) The Desmodus rotundus (‘‘true vampire’’) lives entirelyon fresh blood that it obtains by making incisions in theskin of living animals.

Clearly, {a,c} coheres more with {b,d} than {a,b} cohereswith {c,d}.

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A more promising alternative is to define the relationalcoherence of two mutually exclusive sets A and B as theincrease in total coherence obtained by joining them, thus:

ð2Þ rðA;BÞ ¼ sðA [ BÞ � sðAÞ � sðBÞ ðfor A \ B ¼ [ÞTo see why the formula is only applicable when A and B aremutually exclusive, consider the extreme opposite case whenA=B. The formula would then reduce to r(A,A)=)s(A),making relational self-coherence inversely correlated to sys-temic coherence, which is absurd.

With this definition, systemic coherence of finite sets turnsout to be derivable from relational coherence by recursion onthe number of elements, using the following two formulasthat both follow from (2):

ð3Þ sðB [ fagÞ ¼ rðfag;BÞ þ sðfagÞ þ sðBÞðfor a j2 BÞ

ð4Þ rðfag; fbgÞ ¼ sðfa; bgÞ3

Combining (1) and (2) we obtain a definition of r in terms ofb, according to which the coherence between two mutuallyexclusive sets is equal to the sum of the coherence contrib-uted by each subset of their union that connects them:

ð5ÞrðA;BÞ ¼ RfbðXÞjX 2 }ðA [ BÞ n ð}ðAÞ [ }ðBÞÞg

whenever A\B ¼ [

Hence, with reasonable definitions, systemic, contributed, andrelational coherence are all interdefinable. This result is prob-lematic for philosophical views that favour either systemic orrelational coherence to the exclusion of the other.4

3. COHERENCE OF SETS OF SENTENCES

3.1. Basics

In this section, we are going to add the further condition thatthe sets under study consist of sentences. This assumptionincreases the epistemological relevance of our deliberations,since beliefs can be represented by sentences. Admittedly,


actual beliefs do not necessarily have the structure of sen-tences in a language. However, although sentences do notcapture all aspects of beliefs, they provide the best availablegeneral-purpose representation of beliefs. (Hansson, 2003)

An immediate consequence of sentential representation isthat logical relationships enter the scene. This they do inessentially two ways. First, coherence can be based at least inpart on logical implication. The two sets {p,q} and {p ” q,p�q} are logically equivalent, and it would be strange to denythat they have a high degree of relational coherence. On theother hand, it would be too demanding to require that episte-mic coherence always be based on logical inference. Weakerrelationships such as ‘‘increases the probability of’’ or ‘‘makesit more justified to believe that’’ can provide the basis forcoherence. However, these relations typically have logicalimplication as a limiting case. Therefore, we can assume thatthe addition to a set of sentences of some sentence that itimplies should not decrease coherence, or in other words:

ð6Þ If B ‘ a; then sðB [ fagÞ � sðBÞor more generally:

ð7Þ If A � CnðBÞ then sðB [ AÞ � sðBÞðlogical additionÞThe other way in which logical relationships have impact oncoherence is the negative way: with inconsistency comes inco-herence. As was noted by Olsson (1999) in a categoricalframework, that coherent sets be consistent ‘‘is the only thingthat researchers tend to agree upon when it comes to thequalitative nature of coherence’’. One way to express thisintuition in a gradational account is the following:

ð8Þ If A is consistent and B inconsistent, then sðAÞ> sðBÞ:In what follows, we will be concerned with sets of sentencesthat represent belief states. The basic assumption is that foreach belief state K there is a belief-representing set K of sen-tences, such that the degree of coherence of K is adequatelyrepresented by the degree of coherence of K, as measuredwith a systemic coherence measure s.

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As was noted in section 2.1, systemic coherence is notmonotonic. It follows that there should be cases in which theremoval of a sentence from a belief-representing set increasesthe degree of systemic coherence:

ð9ÞThere is some belief-representing set K and some

set A such that KnA is a belief-representing set

and that s(KnA)>s(K). (non-monotonicity).

3.2. Logical Closure

It is generally assumed in belief revision theory that all belief-representing sets are logically closed, i.e.:

ð10ÞIf K is a belief-representing set, then K ¼ CnðKÞðlogical closure)

Similarly, we should expect the resulting new set after aremoval to be logically closed. However, this gives rise to thefollowing formal result:

Observation 1. Let Ø „ A ˝ K=Cn(K) and K\A=Cn(K\A). Then Cn(A)=K.

Proof. Excluding trivial limiting cases, let a 2 A and b 2 K\A.It follows from the logical closure of K\A and a j2 K\A thata ” b j2 K\A, hence a ” b 2 A, hence b 2 Cn(A). Since thisholds for all b 2 K\A we have K\A ˝ Cn(A), hence Cn(A)=K.

This means that if the both the original set of beliefs andthe set resulting from the removal of A are logically closed,then the removed unit A logically implies the original set.This is implausible, and gives us reason to give up therequirement of logical closure. However, before doing that weshould consider alternatives. Perhaps, if we retain logical clo-sure for the original set of beliefs, but give it up for the setremaining after the removal, then the removed unit (A inthe above observation) can have a more plausible structurethan in Observation 1. This amounts to adding a structuralrequirement on A to the non-monotonicity postulate. There


are two obvious ways to do this. We can require A to be asingleton, and thus let the postulate refer to the removal of asingle belief, or we can require that A be logically closed,thus letting the units of coherence have the same structure assets of belief-representing sentences. This amounts to the fol-lowing two versions of the postulate:


a such that sðK n fagÞ>sðKÞ:ðnon-monotonicity 1Þ:


A such that A ¼ CnðAÞ � K and sðKnAÞ > sðKÞ:ðnon-monotonicity 2Þ:

However, neither of these two variants of the postulate iscompatible with the other postulates given above:

Observation 2. The following conditions are incompatible:

If A ˝ Cn(B) then s(B[A) ‡ s(B) (logical addition)If K is a belief-representing set, then K=Cn(K) (logicalclosure)There is some belief-representing set K and some a suchthat s(K\{a})>s(K). (non-monotonicity 1).

Proof. Excluding a trivial case, we have a 2 K. It followsfrom logical closure that there is some b 2 K\{a} and thata�b, a��b 2 K\{a}. Hence Knfag ‘ a, so that K ˝Cn(K\{a}). Logical addition then yields s(K) ‡ s(K\{a}), con-trary to non-monotonicity 1.

Observation 3. The following conditions are incompatible:

If A ˝ Cn(B) then s(B[A) ‡ s(B) (logical addition)If K is a belief-representing set, then K=Cn(K) (logicalclosure)There is some belief-representing set K and some A such thatA=Cn(A) � K and s(K\A)>s(K). (non-monotonicity 2).

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Proof. Since A � K, there is some b 2 K\A. Let a 2 A. Thendue to the logical closure of A, a ” b 2 K\A, hence a 2Cn(K\A). Thus A ˝ Cn(K\A). Logical addition yields thats((K\A) [ A) ‡ s(K\A), i.e. s(K) ‡ s(K\A), contrary to non-monotonicity 2.

The above three observations give us reason to give up theidea that belief-representing sets of sentences should be logi-cally closed. It does not seem possible to apply a measure ofsystemic coherence to logically closed sets of sentences in away that reflects the effects of logical relations on coherence.(A similar conclusion was reached for a categorical notion ofcoherence in Hansson and Olsson (1999).) The reason for thiscan be seen from the simple example of a person believing intwo logically independent sentences p and q and their logicalconsequences. If we move from using {p,q} to Cn({p,q}) asa belief-representing set, then we add a large number of‘‘connecting sentences’’ such as p&q, p ” q, p � q, r fi p&q,�r fi p&q, etc. These ‘‘logical additions’’ to the belief-representing set tend to add so much coherence that distinc-tions are lost.

It is important to note that this argument is not directedagainst the view that the logical consequences of beliefs arethemselves beliefs. Someone who believes in p and q is alsocommitted to believe in p&q, p ” q, p � q, etc. Therefore, log-ical closure of the total set of beliefs held by an agent is stilla useful idealization (but a total set of beliefs should be dis-tinguished from a belief-representing set of sentences in thesense introduced in section 3.1.). What is at stake is insteadthe assumption that all beliefs are on an equal footing withrespect to contributions to the coherence of the state ofbeliefs. If I believe that r and that q, I also believe that r&q,but it does not follow that the latter belief has in all respectsthe same standing as r and q. In particular, the fact that itimplies r and q does not necessarily mean that it contributesto their justification or to the coherence of the belief state.For a concrete example, since I believe that Paris is the siteof the French foreign ministry (p) I also believe that either


Paris or Quito is the site of the French foreign ministry(pVq), but this latter belief does not have an independentstanding; it stands or fall with the former. Such merelyderived beliefs do not seem to contribute to the coherence ofthe belief state.

3.3. Consequences for Epistemic Coherentism and for BeliefRevision

These formal results do not invalidate the basic coherentistideas, but they nevertheless have consequences for coheren-tism. The view that ‘‘all beliefs’’ are capable of contributingto the justification (etc.) of other beliefs has to be modified sothat ‘‘mere logical consequences’’ are excluded. Coherentistshave to recognize that there are two categories of beliefs,such that the beliefs in one of these categories (the merelyderived beliefs) depend entirely on those in the second cate-gory. However, this second category of beliefs does not corre-spond to the basic beliefs referred to in foundationalistepistemology. Our formal analysis give us no reason to main-tain that they have to be based on experience. Indeed, as faras these results are concerned they need not have any otherjustification than the coherence that holds among them.

In belief revision theory it has mostly been taken for gran-ted that models in which the belief state is represented by alogically closed set of sentences (such as the AGM model, seeAlchourron et al. (1985)) correspond to coherentist epistemol-ogy, whereas models employing ‘‘belief bases’’ that are notlogically closed represent foundationalism. (Gardenfors, 1990;see further references in Hansson and Olsson (1999).) How-ever, as should be clear from the above, the logical relation-ships among the elements of a logically closed set do notadequately represent epistemic coherence. A representation ofcoherence is something that has to be added.

An explicit representation of coherence was introduced intoa belief revision framework in by Olsson (1997, 1998). In(Hansson, 2000a) such a representation was employed in astudy of contractions, i.e. changes in which beliefs are

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removed but no new beliefs are added. Coherence was treatedas categorical; hence it was assumed that belief statesare either coherent or non-coherent, with no intermediatedegrees. Clearly, if the current belief state is coherent it doesnot follow that all subsets of the current set of beliefs repre-sent the beliefs held in some coherent belief state that can bereached through contraction.5 For instance, I presently be-lieve that there is a radio transmitter operating on the surfaceof Mars (r). I also believe that a spaceship from Earth hasbrought such a device to Mars (s). There are various ways inwhich my belief state may be contracted, i.e. changed so thatmy set of beliefs is reduced to a subset of the original set ofbeliefs. It holds for all coherent belief states that I can arriveat through contraction that if r is retained, then so is s.(There may be ways in which I can be brought to coherentlybelieve in r but not in s, but these involve the acquisition ofsome new belief such as that extraterrestrial beings have sentinstruments to Mars.)

Based on this reasoning, we can postulate that there is a setC consisting of those subsets of the current set of beliefs, K,that represent a coherent belief state. K is assumed to be anelement of C. Note that the elements of C are logically closed;as was observed above the idealization that the total set ofbeliefs is logically closed does not have to be given up. Thecentral result obtained in (Hansson, 2000a) was based on thefurther assumptions that C is finite and all its elements finite-based, and that it forms a semi-lattice for K (i.e Cn(Ø) 2 C,K 2 C, and if X, Y 2 C then Cn(X[Y) 2 C). Under these con-ditions, a contraction operator on K such that K ‚ a 2 C forall a can be obtained as the closure of a partial meetcontraction on a finite belief base for B, i.e. for all a, K ‚a=Cn(B)a) where B is a set such that Cn(B)=K and ) isan operator of partial meet contraction for B. This is ‘‘coheren-tist contraction’’ in the sense that the outcome of contracting aset K of beliefs by a non-tautologous sentence a is always acoherent subset of K that does not imply a. The fact that co-herentist contraction brings us to an operation that can be con-structed with a belief base shows again that the conventional


association between belief bases and foundationalist epistemol-ogy is misleading.

With this approach, the dynamic behaviour of the beliefstate determines which beliefs are basic. This solves one ofthe major problems with belief bases, namely that it is diffi-cult to determine which are the basic sentences. It should beemphasized, however, that the approach to coherence in(Hansson, 2000a) is too simplistic; a gradational representa-tion of coherence should be useful here as well.

4. CONCLUSION

The formalization of philosophical concepts can provide uswith new insights that have implications for informal philoso-phy. (Hansson, 2000b) The results reported here have twomajor such implications. (1) Systemic and relational accountsof coherence have been treated in informal epistemology ascompetitors. We have shown that systemic and relationalcoherence are interdefinable. Therefore, they should be trea-ted as alternative formulations of essentially the same con-cept. (2) Epistemic coherentism has been interpreted asmeaning that ‘‘all beliefs support each other’’. We have seenthat this interpretation is untenable. In order to avoid absurdconclusions, coherentists have to accept a weak version ofepistemic priority, that sorts out merely derived beliefs.

NOTES

1 However, a plausible stability property should be noted that is some-what related to monotonicity: If it holds for a set B that s(A)<s(B) for allA � B, then B is stable against coherence-driven contraction.2 Cf. Rescher’s (1973, p. 32) view that coherence is a property of setswith at least two elements.3 This holds under the assumption that singletons make zero contribu-tion to overall systemic coherence. If singletons make non-zero contribu-tion, then recursion is still possible provided that r({a},{a})=s({a}).4 See Olsson (1999) for a critical discussion of Lehrer’s renunciation ofsystemic coherence.5 Note again the distinction between (1) the set of all beliefs held in a

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certain belief state, and (2) a set representing that belief state, and suchthat the interrelations among its elements correspond to the relations ofcoherence in that belief state. In section 3.2 we saw that a set satisfying(2) cannot be logically closed. In the rest of this section we will be con-cerned with logically closed sets that satisfy (1).

Acknowledgements

I would like to thank Erik J Olsson for valuable comments on an earlierversion of this paper.

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Department of Philosophy and the History of TechnologyRoyal Institute of Technology (KTH)Teknikringen 78B,100 44 StockholmSwedenE-mail: [email protected]

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Coherence in Epistemology and Belief Revision*