PhilosStud(2017)174:735–758 DOI10.1007/s11098-016-0705-4 The a priority of abduction Stephen Biggs1 • Jessica Wilson2 Publishedonline:18June2016 (cid:2)SpringerScience+BusinessMediaDordrecht2016 Abstract Here we challenge the orthodoxy according to which abduction is an a posteriori mode of inference. We start by providing a case study illustrating how abduction can justify a philosophical claim not justifiable by empirical evidence alone. While many grant abduction’s epistemic value, nearly all assume that abductive justification is a posteriori, on grounds that our belief in abduction’s epistemicvaluedependsonempiricalevidenceabouthowtheworldcontingentlyis (e.g., parsimonious, or such that more parsimonious theories better track truth). Contra this assumption, we argue, first, that our belief in abduction’s epistemic value is notandcouldnotbe justified aposteriori,and second, thatattentiontothe rolesexperienceplaysinabductivejustificationsupportstakingabductiontobeana priorimodeofinference.Weclosebyhighlightinghowourstrategyforestablishing theapriorityofabductionpositivelycontrastswithstrategiesinBonjour(Indefense of pure reason. Cambridge University Press, New York, 1998), Swinburne (Epis- temic justification. Claredon Press, Oxford, 2001), and Peacocke (The realm of reason.OxfordUniversityPress,Oxford,2004)aimingtoestablishtheapriorityof certain ampliative modes of inference or abductive principles. Keywords Abduction (cid:2) Inference to the best explanation (cid:2) Epistemology of modality, identity, and essentiality (cid:2) Philosophical method (cid:2) A priority (cid:2) Parsimony & StephenBiggs [email protected] JessicaWilson [email protected] 1 DepartmentofPhilosophyandReligiousStudies,IowaStateUniversity,413CattHall,Ames, IA50010,USA 2 DepartmentofPhilosophy,UniversityofToronto,170St.GeorgeSt.,Toronto,ONM5R2M8, Canada 123 736 S.Biggs,J.Wilson 1 Introduction Abduction, or inference to the best explanation, is an inferential procedure that proceedsbyassessingtheextenttowhicheachofarangeofcandidatetheoriessatisfies certain abductiveprinciples(‘theoreticalvirtues’),suchasprinciplesofontological parsimony, ideological simplicity, elegance, plausibility, compatibility with other beliefs,andsoon.Touseabductionwhendecidingamongcompetingtheoriesisto infer to the truth of (or justified belief in) the theory that best explains some target explanandum, where the underlying abductive principles and their weightings determinehowtheoriesaretoberanked.AsHarman(1965)describestheprocedure: In making this inference one infers, from the fact that a certain hypothesis would explain the evidence, to the truth of that hypothesis. In general, there will be several hypotheses which might explain the evidence, so one must be abletorejectallsuchalternativehypothesesbeforeoneiswarrantedinmaking theinference.Thusoneinfers,fromthepremisethatagivenhypothesiswould provide a ‘‘better’’ explanation for the evidence than any other hypothesis, to theconclusionthatthegivenhypothesisistrue.(…)Suchajudgmentwillbe based on considerations such as which hypothesis is simpler, which is more plausible, which explains more, which is less ad hoc, and so forth. (89) Itisnearlyuniversallypresumedthatabductionisanaposteriorimodeofinference. Herewechallengethisorthodoxy.Westartbyconsideringhowphilosophicalclaims pertaining to necessity, essentiality, and identity might best be justified, given that suchclaimsappeartogobeyondtheempiricalevidence;weregisterconcernsabout whetherexisting(intuition-orconceiving-based)modesofapriorideliberationcan justify such claims, and present a case study supporting the increasingly common suppositionthatabductioncandoso(Sect. 2).Thoughphilosophersoftengrantthat abduction can justify philosophical claims, they nearly universally assume that abduction is an a posteriori mode of inference, on grounds thatourjustification for thinking that abduction has epistemic value (roughly: results in justified beliefs; a morespecificdefinitionfollows)dependson(whatwemightcall)‘indirect’empirical evidence,pertainingtohowtheworldcontingentlyis(e.g.,parsimonious,orsuchthat moreparsimonioustheoriesaremorelikelytobetrue).Contrathislineofthought,we argue that no empirical evidence—direct or indirect, actual or hypothetical—is or could be relevant to assessing the epistemic value of abductive principles (e.g., parsimony),ormoregenerally,ofabduction,concludingthatourjustificationforthe beliefthatabductionhasepistemicvalueisnotaposteriori(Sect. 3).Wethenargue thatourjustificationforthisbeliefisapriori,byidentifyingtherolesthatexperience can play in forming a justified belief, and considering the bearing of such roles on whether a given belief is justified a priori or a posteriori (Sect. 4). We close by highlightinghowourstrategyforestablishingtheapriorityofabductioncontrastswith strategies in Bonjour (1998), Swinburne (2001), and Peacocke (2004) that aim to establish the a priority of certain ampliative modes of inference or abductive principles,andshowingthatouraccountisnotsubjecttocertainconcernswiththese accounts(Sect. 5). 123 Theapriorityofabduction 737 Fourpreliminaryclarifications.First,weuse‘abduction’asalabelfor‘inference tothebestexplanation’forthesakeofsimplicity,nottoimplythatinferencetothe best explanation just is Peirce’s abduction.1 Second,wetakeamodeofinferenceMtohaveepistemicvalueifnecessarily,for any subject s, conclusion C, and premise(s) P: if s justifiably believes P and uses MtoinferCfromP,then(absentdefeaters)sjustifiablybelievesC.Nothinginwhat follows depends specifically on epistemic values’ involving justified belief; those taking knowledge, entitlement, warrant, or some other epistemic feature to enter more basically into what Beebe (2009) calls the ‘‘epistemic goodness of [a given mode of] inference’’ (619) can substitute accordingly. Third,inwhatfollowswesetasidegeneral skepticismaboutabduction.Weassume thatweareactuallyjustifiedinbelievingthatabductionhasepistemicvalue,andaim heretoestablish(amongotherresults)thatourjustificationforbelievingthisisapriori.2 Weseethisasdialecticallyreasonable,giventhepopularityofandseemingneedfor abductionbothinthesciencesandinphilosophy(forwhichweprovidefurthersupport inSect.2),asperLipton’sclaimthat‘‘InferencetotheBestExplanationisapopular account’’ofinference(1991/2004,1),Ladyman’sclaimthat‘‘naturalistsmustagreethat inferencetothebestexplanationisindispensableinscience’’(2007,184),andHawley’s claim that ‘‘rejection of inference to the best explanation … is incompatible with standardversionsofscientificrealism’’(2006,454).3 Fourth, we do not take any stand here on exactly how abduction works, beyond thesortofroughsketchthatHarmanprovides,andthepresuppositionthatprinciples suchasontologicalparsimonyandcompatibilitywithexistingbeliefsareamongthe abductiveprinciples.Ourcaseforabduction’sbeinganapriorimodeofinferenceis compatiblewithawiderangeofcompetingviewsaboutwhichprinciplesareamong the abductive principles, how abductive principles are to be weighted, how abduction relates to other forms of inference, and so on.4 2 How are philosophical claims justified? Philosophers often attempt to discern whether certain claims are necessary or contingent, whether certain properties of an object are essential or accidental to that object,whethercertainspatiotemporallycoincidententitiesareidenticalordistinct,and soon.Onthefaceofit,empiricalevidencealonedoesnotprovideajustificatorybasisfor 1 For reasons to distinguish Peircean abduction and inference to the best explanation see, e.g., Minnameier(2004),Campos(2011),Plutynski(2011),andMackonis(2013). 2 Forargumentsthatabductionhasepistemicvaluesee,e.g.,Lipton(1991/2004)andWeintraub(2013); forargumentsthatabductionlacksepistemicvaluesee,e.g.,vanFraassen(1980,2002). 3 Skepticsmaynonethelessbeinterestedinourreasonsforthinkingthatthebeliefthatabductionhas epistemicvaluecannotbejustifiedaposteriori. 4 Forconsiderationoftheprinciplesatplayinabductionsee,e.g.,Thagard(1978),Lipton(1991/2004), Beebe(2009)(esp.609–611),andMackonis(2013).Forattemptstoformalizesomeabductiveprinciples see,e.g.,McGrew(2003)andShupbachandSprenger(2011).Forcompetingviewsonhowabduction relatestootherformsofinferencesee,e.g.,Harman(1970),vanFraassen(2002),Weisberg(2009),and Henderson(2014). 123 738 S.Biggs,J.Wilson suchclaims,sinceitdoesnotdistinguishbetweenthefeaturesatissue:experiencecan tellusthatSocratesismortal,butnotwhetherthisisnecessaryorcontingent;itcantellus thatRuthisaphilosopher,butnotwhetherthisisessentiallyoraccidentallyso;itcantell us that water and H O are spatiotemporally coincident, but not whether these are 2 identicalordistinct.Astrictempiricistmightconcludethatphilosophersarewrongto seekanswerstoquestionsaboutnecessity,essence,identity,andthelike,butwefollow thosewhothinkthatwecanprovideepistemicallyprincipledanswerstosuchquestions, evenintheabsenceofdecisiveempiricalevidence. 2.1 Justifying philosophical claims: intuition- and conceiving-based approaches How can philosophers fill the gap between empirical evidence and claims about necessity, essentiality, and identity (to focus on three especially important philosophical notions)? So far as a priori modes of deliberation are concerned, twooptionsarestandardlyonoffer.Onthefirst,itissupposedthatanaprioriform of evidence fills the gap, along the lines of Bealer’s (2000, 2002) accounts of intuitionasanon-inferential,aprioriformofrational‘seeming’,whichencodesour insight into the concepts entering into the claim at issue, and which constitutes evidence for claims about necessity, essentiality, and identity. On the second, it is supposed that an a priori mode of inference fills the gap, as with Chalmers and Jackson’s (2001) and Chalmers’s (2002) account of conceiving as an a priori form of rational inference, which takes empirical evidence as input and which—upon consideration of how we (or more specifically: idealized versions of ourselves) would be inclined to apply our concepts in hypothetical scenarios—delivers indefeasible claims about necessity, essentiality, or identity as output. We find these currently available accounts of a priori deliberation into philosophical questions unsatisfying, for two reasons in particular. First, a priori conclusions based in either intuition or conceiving require that our conceptual competence be such as to eventuate in determinate results in any scenario relevant to assessing the philosophical claim at issue: we must be able to consider whether thereareanyscenarioswhereourunderstandingof‘Socratesismortal’comesapart fromourconceptoftruth,whereourconceptsof‘Ruth’and‘philosophizing’come apart,whereourconceptsof‘water’and‘H O’comeapart.Butashasbeenargued 2 by Wilson (1982, 2006), Block and Stalnaker (1999), Byrne and Pryor (2006), Melnyk (2008), and ourselves (Biggs and Wilson 2016, in progress), our concepts are typically to some extent indeterminate, thus preventing their determinate applicationintherequisitefullrangeofscenariosusingtheresourcesofintuitionor conceiving, understood as involving some sort of rational insight into pre-existing conceptual connections, alone.5 As such, we are inclined to agree with Williamson (2007) that, at the end of the day, ‘‘few philosophical questions are conceptual questions in any distinctive sense’’ (3). 5 Nor,paceChalmers,cantherequisiteampliative(read:abductive)resourcesbebuiltintotheconcepts atissue,withoutmultiplyingconceptsandintroducingotherproblems(seeBiggsandWilson2016,in progress). 123 Theapriorityofabduction 739 Second,resultsbasedinintuitionandconceiving toooftenleavephilosophersat an impasse, reflecting that individuals registering different opinions about what is intuitively true or conceivable have no clear means of resolving their disputes, and hence no clear means of resolving the answers to the original questions about necessity,essentiality,andidentity.AsBlockandStalnaker(1999)observe,‘‘claims about conceivability seem at least as fragile and fallible as intuitions about what is metaphysically possible’’ (6). Moreover, and more problematically, there is a kind of ‘black box’ aspect to these forms of a priori deliberation, which renders differences in claims about what is intuitive or conceivable largely brute, and as such not amenable to reasoned reconciliation. Weare inclined toseetheseconcernsasrepresentinginsuperabledifficulties for intuition- and conceiving-based approaches to the justification of philosophical claims. 2.2 Justifying philosophical claims: abduction Luckily, there is a better available approach to the justification of philosophical claims—namely, abduction. It’s increasingly common for metaphysicians to characterize philosophical investigations as proceeding by way of this mode of inference.6Forexample,Hawley(2006)arguesthatabductioncanjustifybeliefsin metaphysics no less than in science, Rayo (2013) argues that abduction can justify beliefsaboutidentity(‘‘justis’’statements),Biggs(2011)arguesthatabductioncan justify beliefs about necessity and possibility, and Sider (2009) is explicit that metaphysical investigations generally appeal to abductive principles: Competing positions are treated as tentative hypotheses about the world, and are assessed by a loose battery of criteria for theory choice. Match with ordinary usage and belief sometimes plays a role in this assessment, but typicallynotadominantone.Theoreticalinsight,considerationsofsimplicity, integrationwith otherdomains(forinstancescience,logic, andphilosophyof language), and so on, play important roles. (385) By way of illustration, consider the oft-registered claim that water is identical with H O. What justifies, or could justify, this claim? To start, as above, the 2 justification is partly dependent on empirical experience—in particular, on water and H O’s being actually spatiotemporally coincident.7 On the other hand, the 2 6 Thisisnottosuggestthatmetaphysicianshaveonlyrecentlycharacterizedphilosophyasproceedingby wayofabduction.Forexample,asFisher(2015)notes,DonaldWilliams’smethodfordoingmetaphysics included‘‘theweighingupofcompetinghypothesesagainstascientificpictureoftheworld,common sense,andatheory’sexplanatorypowerandsimplicity’’,suchthatWilliams‘‘canbeseenasanearly defenderof‘inferencetothebestexplanation’’’asamethodfordoingmetaphysics(6). 7 The claim that water and H O are actually coextensive (and identical) has been widely (albeit not 2 universally) accepted since Putnam (1962, 1975) and Kripke (1980). As has been observed (in, e.g., Weisberg 2005), however, chemists take H O to be a genus that includes a series of distinct isomers 2 amongitsspecies,someofwhich,theysay(atleastinordinarycontexts),arenotwater.Supposing,then, thatwedefertochemists’usageof‘water’tofixthecontentofourownuses,waterisnotcoextensive withHO.GrantingWeisberg’spoint,itremainsthatchemists(hencewe)mightmaintainthatwateris 2 123 740 S.Biggs,J.Wilson justification for the identity claim must go beyond this experience, for spatiotem- poral coincidence is compatible with different theories about the relation between water and H O, including, to start: 2 Identity: Water is identical with H O 2 and Distinctness: Water is distinct from H O. 2 Distinctness,inturn,iscompatiblewithvariousspecificaccountsoftherelationat issue,intermsofconstitution,composition,functionalrealization,thedeterminate- determinable relation, strong (physically unacceptable) emergence, and so on. Given these options, why endorse Identity rather than some version of Distinctness? Going by what philosophers actually do, it will be natural to start by appealing to the abduction-relevant fact that Identity is the most ontologically parsimonious explanation of the spatiotemporal coincidence, in that it posits a single type, or kind, rather than two or more, as on any version of Distinctness. That’s not the end of the story, since acceptance of Identity on grounds of its providing the most parsimonious explanation of the spatiotemporal coincidence requires that other things be equal—on Identity doing as well as or better than Distinctnessalongotherabductivedimensions,includingconsistency,elegance,and compatibility with existing beliefs. And though Identity appears to do better than Distinctness with respect to some such dimensions (e.g., elegance), that it does so for others (e.g., compatibility with other beliefs) is less clear. For example, if we have reason to think that water is multiply realizable, or that the term ‘water’, in entering into distinctive special science laws, is naturally seen as picking out a distinctive higher-level property, then a proponent of Identity needs to say more. And, indeed, reductionists about special science kinds do address these sorts of concerns in the course of defending their view.8 Indeed, notwithstanding common acceptance of Identity, this debate isn’t over. Howeverthedebateeventuallyplaysout,thefirstpointwewanttomakeisthatitis very natural to see philosophical investigations into these options as appealing to abduction and associated abductive principles, as Hawley, Rayo, Sider, and others suggest.9 The second point is that, notwithstanding the provisional status of even Footnote7continued identicalwithaparticularisomer(orsetofisomers)ratherthanmerelyspatiotemporallycoincidentwith thatisomer(orsetofisomers),andruntheargumenttofollowaccordingly.Forpurposesofillustration westickwiththeusualidentityclaim. 8 Forexample,areductionistmightaimtoaccommodatemultiplerealizabilitybytakingtheidentityto involveadisjunctivelower-leveltype(alaAntony1999)orbyembracingspecies-leveltype-identities(a laKim1972);andareductionistmightrejectseemingreferencetohigher-leveltypesaspresupposingan incorrect’PictureTheory’ofmeaning(alaHeil2003). 9 Ladyman(2007)worriesthatabductioncannotjustifybeliefsinmetaphysicalclaimsaboutrealentities (e.g.,water)becauseittakesintuitionsasitsexplananda,andthoseintuitionsmaynotreflectthenatureof thoseentities.Buttypicallytheexplanandaatissuewhenusingabductiontojustifyphilosophicalclaims will also consist in empirical evidence—e.g., evidence that water and HO are spatiotemporally 2 coincident. 123 Theapriorityofabduction 741 commonlyendorsedclaimssuchasIdentity,thereisnoclearreasontointerpretthis status as indicating that abductive assessments of philosophical claims are doomed to be inconclusive, as opposed to simply reflecting that we are currently some distancefrom the end ofinquiry (see Biggs and Wilson 2016, inprogress). No one ever said that justifying philosophical claims was going to be easy—we just don’t want it to be insuperably difficult, as it arguably is (due to widespread conceptual indeterminacy, and to brute differences in what is considered intuitive or conceivable) on intuition- or conceiving-based approaches. And though we cannot fully defend these claimshere,a case can be made(see Biggs andWilson 2016, in progress)thatabduction,inbeinganampliativemodeofinferenceandininvolving explicit criteria as opposed to brute intuition- or conceiving-based seemings, has resources both for overcoming conceptual indeterminacy (if it turns out that conceptual connections are ever relevant to abductively justifying philosophical claims) and for allowing disputants to engage in substantive debate. In what follows, then, we assume, following Hawley, Rayo, Sider and many others, that philosophical claims are justified by appeal to abduction, and that as such, we are actually justified in believing that abduction has epistemic value. 3 Where it is argued that belief in the epistemic value of abduction is not justified a posteriori Given that we are justified in believing that abduction has epistemic value, is this justification a priori or a posteriori? The nearly universal answer is that this justification is a posteriori: as Douven (2011) says in discussing the status of abductionashavingepistemicvalue,‘‘alldefensesthathavebeengivensofarareof anempiricalnatureinthattheyappealtodatathatsupposedlysupporttheclaimthat (in some form) abduction is a reliable rule of inference’’.10 The line of thought underlying this answer seems to be that whether abduction has epistemic value depends on contingent, empirical fact(s) registering that the world is as the abductive principles suggest it to be—e.g., ontologically parsimonious, or such that (other things being equal) ontologically parsimonious theoriesarelikelytobetrue—andcorrespondingly,thatwhetherwearejustifiedin believingthatabductionhasepistemicvaluedependsonwhetherwearejustifiedin believing that the contingent, empirical facts are as abduction suggests them to be. Hence Sober (1988) says that ‘‘Appeal to simplicity is a surrogate for stating an empirical background theory’’ (64); Bonjour (1998) wonders, ‘‘Why, after all, should it be thought … that the world is somehow more likely to be simple than complex?’’ (91); and Beebe (200?) says, more generally: [P]ractically everyone who works on abductive inference believes that such inferencesarejustifiedempiricallyandthatthetheoretical virtuesarebroadly empirical and contingent marks of truth. (625) 10 Dependingonwhatcountsasa‘defense’,Douvenmaybeoverstatingthecase;seeSect.5. 123 742 S.Biggs,J.Wilson We now argue that, notwithstanding this line of thought, the justification for the belief that abduction has epistemic value is not and indeed could not be a posteriori.11 We illustrate our argumentative strategy by attention to the following principle of parsimony: Parsimony:Forany theoriesT andT*, andentityE:if T andT* differ inthat T*includesEsasfundamentalbutTdoesnot,thenceterisparibusweshould believe T.12 Ourstrategygeneralizes, mutatismutandis,tootherabductiveprinciples,andmore generally to abduction itself, understood as constituted by application of these principles.13 And for short, we sometimes refer to our target (that is, the question whether our belief in the epistemic value of abduction/abductive principles is justified a priori or a posteriori) in simpler terms as concerning whether the epistemic value of abduction/abductive principles is a priori or a posteriori. We start with two clarificatory points, which will structure what follows. First, note that two different sorts of claim might be seen as entering into the line of thought according to which the epistemic value of abduction (and associated principles) is a posteriori. Applied to Parsimony: one claim is that the epistemic valueofParsimonydependsonwhethertheworldis‘‘simpleratherthancomplex’’; theotherclaimisthattheepistemicvalueofParsimonydependsonwhether,atthe world in question, theories satisfying Parsimony are more likely to be true. We address each claim in turn. Second, as Bonjour (1998) notes, ‘‘the precise character of the distinction between a priori and a posteriori justification remains more than a little obscure’’ (6).Acommonfirstpasshasitthatabeliefinatruepropositionisjustifiedapriori if it is justified independently of any appeal to experience, and is justified a posteriori if it is justified by at least some appeal to experience. But what sort of experience is at issue? As is familiar, certain kinds of experience—e.g., that required to understand the proposition at issue—play a role even in a priori deliberation, requiring second-pass refinement of the distinction (an issue to which we will return in Sect. 4). We can also ask, however, what sort of experience is at issueinaposteriorijustification.Thisistypicallyqualifiedasbeing‘empirical’oras involving empirical evidence, where the notion of the empirical is tied to in- principle observability or other broadly sensory or causal notions. Hence Bonjour suggests that 11 Swinburne(2001)andPeacocke(2004)alsotakebroadlyabductiveapplicationsof(somethingakin to)theprincipleofsimplicitytobeapriorijustified.Asprefigured,inSect.5wehighlightthemainpoints of contrast of our account with these other approaches, as well as with Bonjour’s (1998) account of inductionasapriorijustified. 12 Asabove,thereareseveralconceptionsofparsimonyonoffer;wefocusonParsimonyforsimplicity (nopunintended).Fordiscussionofvariousconceptionsofparsimonyandrelatedprinciples,see,e.g., Baker(2003,2004/2010)andSober(2003). 13 Sincesomeprinciplesmaybeintension,abductionasageneralmodeofinferencealsopresumably encodeshowthesearetoberankedandweighted;wereturntothisissueattheendofthissection. 123 Theapriorityofabduction 743 the relevant notion of experience should be understood to include any sort of processthatisperceptualinthebroadsenseof(a)beingacausallyconditioned response to particular, contingent features of the world and (b) yielding doxasticstatesthathaveastheircontentputativeinformationconcerningsuch particular, contingent features of the actual world as contrasted with other worlds. (8) Wewillhavesomethingalongtheselinesinmindwhenassessingwhetheritmakes sense to suppose that the epistemic value of Parsimony is justified a posteriori, but downtheline(bywayofanobjectiontoourconclusionthatthisepistemicvalueis not justified a posteriori) will consider a more ‘metaphysical’ conception of the experience or empirical evidence at issue. Tostart,then.Accordingtothefirstclaim,theepistemicvalueofParsimonyisa posteriori,independingonwhethertheactualworldisparsimonious.Now,ifwhat it is for a world to be simple or complex, parsimonious or not, depends on how many fundamental entities there are at a world, then this is presumably something for which we could have empirical evidence. But any such evidence would be irrelevant to the epistemic value of Parsimony. How many fundamental kinds (for example)doesaworldhavetocontaininordertobeunparsimonious?Two?Forty- two? A billion? These questions are silly, precisely because Parsimony is a principle or a norm that guides theory choice, not a descriptive fact that holds, or doesn’thold,ataworld.14Henceevenifwecouldmakesomesenseofitsbeingtrue that‘Aworldwithabillionfundamentalkindsisunparsimonious’,thataworldhad a billion fundamental kinds would be irrelevant to the epistemic value of Parsimony. It would still remain, in such a world, that in theorizing one should not posit more fundamental kinds (e.g., one billion and one, in the world at issue) than are needed. Accordingtothesecondclaim(whichsomemayhavehadinmindinendorsingthe first),theepistemicvalueofParsimonyisaposteriori,independingonwhether,inthe actualworld,theoriessatisfyingParsimonyaremorelikelytobetrue.Iftherewere somewaytoempiricallydetermine,inaworld,whethertheoriessatisfyingParsimony weremorelikelytobetrue,thenthiswould,perhaps,berelevanttotheepistemicvalue ofParsimony;inparticular,suchempiricalevidencewould,perhaps,berelevantto whetherbeliefinthisepistemicvaluewasjustifiedaposteriori.15Butinanycasewe couldnevergainempiricalevidence,however‘indirect’,tothiseffect.Parsimony,like all abductive principles, requires that ‘other things be equal’. Consequently, no empirical evidence could, even in principle, distinguish between a world in which theoriessatisfyingParsimonyweremorelikelytobetrue,andonewherethiswasnot thecase.Ifthereweresuchempiricalevidence—if,forexample,experimentswereto indicate that the world contained more fundamental kinds than our best theory 14 NorisParsimonyapieceof‘‘information’’thatmightbeentailedbybasefactsataworld,asChalmers and Jackson (2001) suggest in responding to Block and Stalnaker’s (1999) objection that appeal to simplicityconsiderationsisrequiredinordertoovercomeconceptualunderdetermination. 15 We say ’perhaps’ since one might deny that justification (or whatever epistemic good is seen as enteringintothecharacterizationoftheepistemicvalueofagivenmodeofinference)hingesontruth,or likelytruth,oranyothermetaphysicalnotions. 123 744 S.Biggs,J.Wilson implies—then the ceteris paribus condition in Parsimony would not be met: one theory would be explanatorily better than the other (vis-a`-vis another abductive principle).Wecouldneverbeinempiricalpositiontoknow,then,thattheactualworld isnotcooperatingwithParsimony. The previous considerations suggest that our justification for believing in Parsimony’sepistemicvaluedoesnotandindeedcannotrelyonempiricalevidence, whetherthisevidence is supposed to indicate that ourworldis parsimoniousrather thanunparsimonious(whateverexactlythiscomesto),orisrathersupposedtoindicate that our world is one where more parsimonious theories are more likely to be true. Eitherway,thereisnowaytogainsuchevidence:inthefirstcase,becausefactsabout thenumberoffundamentalentitiesorkindsataworldareirrelevanttotheepistemic valueofParsimony;inthesecondcase,becauseanyempiricalevidencethatmightbe broughttobeartoshowthatmoreparsimonioustheoriesarelesslikelytobetruewould violatetheceterisparibusconditioninParsimony,andsoagainbeirrelevanttothe epistemicvalueofthisprinciple.Weconclude,then,that (C1) The belief that Parsimony has epistemic value is not actually justified a posteriori. Thesameconsiderationssupporting(C1)alsoservetoshowthatinanyworldwhere we are justified in believing that Parsimony has epistemic value, that belief would not be justified a posteriori. In any such world, it would remain that there could be no (hypothetical) empirical evidence that would be relevant to assessing the epistemicvalueofParsimony,asappliedinsuchaworld.Indeed,giventhatweare actuallyjustifiedinbelievingthatParsimonyhasepistemicvalue,wecanprovidea furtherargumentforthisclaim,accordingtowhichinreasoningabouthypothetical statesofaffairs,weshouldapplyouractualnorms.Onthisview,thereisnomorea world where one (epistemically) should implement Anti-Parsimony (or otherwise failtoimplementParsimony)thanthereisaworldwhereone(morally)shouldharm othersforfun:thedetailsoftheworldareirrelevanttowhatoneshoulddo.Wecan thus drop the reference to ‘actually’ in (C1), and more generally conclude that (C2)ThebeliefthatParsimonyhasepistemicvalueisnotjustifiedaposteriori. Considerations similar to those just canvassed for Parsimony hold, mutatis mutandis,forotherabductiveprinciples—whicharealsoprinciplesornormsrather thandescriptivefacts,which areournorms,andwhichalsoincludeceteris paribus clausesholdingotherexplanatoryconsiderationsconstant,renderingitthecasethat no empirical evidence, direct or indirect, actual or hypothetical, could disprove these principles. Since the application of these principles is constitutive of abduction, it follows, more generally, then, that (C3)Thebeliefthatabductionhasepistemicvalueisnotjustifiedaposteriori. InthenextSection,wewillarguethatthebeliefthatParsimonyhasepistemicvalue, and more generally the belief that abduction has epistemic value, is moreover justified a priori. Before moving on, however, in the following sub-Section we consider and respond to four objections to what we have claimed thus far. 123