Explanation Versus Prediction Which Carries More Evidential
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Explanation Versus Prediction: Which Carries More Evidential
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TheBookofEvidencePeterAchinstein
Printpublicationdate:2001PrintISBN-13:9780195143898PublishedtoOxfordScholarshipOnline:November2003DOI:10.1093/0195143892.001.0001
ExplanationVersusPrediction:WhichCarriesMoreEvidentialPeterAchinstein(ContributorWebpage)
DOI:10.1093/0195143892.003.0010
AbstractandKeywords
Accordingtoonestandardview,apredictionofanewfactalwayscountsasstrongerevidenceforahypothesisthananexplanationofaknownone.Accordingtoanotherview,itisthereverse.Bothviewsareshowntobemistaken.Whatisimportantforevidenceisnotwhetheritwaspredictedorexplained,but(inthesortofcasesusedbyeachside)whatselectionprocedureswereusedtoobtaintheevidence.StephenBrush'sdefenseofexplanationismandPatrickMaher'sdefenseofpredictivismarecriticallyexamined,asisClarkGlymour'sproblemofoldevidence.
Keywords:Brush,evidence,explanationism,Glymour,Maher,oldevidence,prediction,predictivism,scientificexplanation,selectionprocedures
1.TheHistoricalThesisofEvidenceAccordingtoastandardview,predictionsofnewphenomenaprovidestrongerevidenceforatheorythanexplanationsofoldones.Moreprecisely,atheorythatpredicts
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phenomenathatdidnotprompttheinitialformulationofthattheoryisbettersupportedbythosephenomenathanisatheorybyknownphenomenathatgeneratedthetheoryinthefirstplaceandthatthetheorywasusedtoexplain.Sosayvariousphilosophersofscience,includingWilliamWhewellinthenineteenthcenturyandKarlPopperinthetwentieth,tomentionjusttwo.1
StephenBrushtakesissuewiththisonhistoricalgrounds.2Hearguesthat,generallyspeaking,scientistsdonotregardnewphenomenapredictedbyatheory,evenonesofakindtotallydifferentfromthosethatpromptedthetheoryinthefirstplace,asprovidingbetterevidentialsupportforthattheorythanisprovidedbyalreadyknownphenomenaexplainedbythetheory.Bycontrast,Brushclaims,therearecases,includinggeneralrelativityandtheperiodiclawofelements,inwhichscientiststendtoconsiderknownphenomenaexplainedbyatheoryasconstitutingmuchstrongerevidencethannovelpredictions.3
(p.211) Boththepredictionistandtheexplanationistarecommittedtoaninterestinghistoricalthesisaboutputativeevidenceeandahypothesish,viz.
Historicalthesis:Whethereiftrueisevidencethath,orhowstrongthatevidenceis,dependsoncertainhistoricalfactsaboute,h,ortheirrelationship.4
Forexample,whethereiftrueisevidencethath,orhowstrongitis,dependsonwhetherewasknowntobetruebeforeorafterhwasformulated.Varioushistoricalpositionsarepossible,asAlanMusgravenotedyearsagoinaveryinterestingarticle.5Onasimplepredictionistview(whichMusgraveclassifiesaspurelytemporal),eisevidencethathonlyifewasnotknowntobetruewhenhwasfirstproposed.Onanotherview(whichMusgraveattributestoElieZaharandcallsheuristic),eisevidencethathonlyifwhenhwasfirstformulateditwasnotdevisedinordertoexplaine.Onyetathirdhistoricalview(whichMusgravehimselfaccepts),eisevidencethathonlyifecannotbeexplainedbyapredecessortheory,thatis,byacompetingtheorywhichwasdevisedbyscientistspriortotheformulationofh.
Thesearethreeexamplesofhistoricalviewsconcernedwiththetemporalorderinwhicheandhwereformulatedorknowntobetrue.Butotherhistoricalpositionsarepossible.Forexample,itmightbeheldthatwhethereiftrueisevidencethath,andhowstrongthatevidenceis,alwaysdependsonhistoricalfactsconcerninghowtheresultsreportedinewereobtained,forexample,whatsamplingmethodswereinfactusedbythosewhoreportedthate(moreonthisinsection2).
Thehistoricalthesisisnotthateorharethemselvespropositionsaboutparticularhistoricalevents.(Forexample,emightbethatlightcanbediffracted,andhmightbethatlightisawavemotion.)Ratherthethesisisthatevenifneitherenorhdescribesaparticulareventorsetofeventsthatoccurred,whethere,iftrue,isevidencethathdependsupontheoccurrenceofsomeparticulareventorsetofeventspertainingtowhenorhowe,h,ortherelationshipbetweenthemcametobeformulatedorbelieved,orhowtheresultsinewereobtained.
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Isthehistoricalthesistrueorfalse?Itisclearlytrueinthecaseofsubjectiveevidence.Whethereissomeperson'sevidencethathdependsoncertainhistoricalfactsaboute,h,andtheirrelationship,viz.whetherthatpersoninfacthasorhadcertainbeliefsaboute,h,andtheirrelationship.However,defendersofthehistoricalthesisarenotspeakingofsubjectiveevidence.Theyarenotspeakingofsomethingthatsomeonetakestobeevidencebutofsomethingthatis(objective)evidence.Incasesofthelattersort,Iproposetoarguethatthehistoricalthesisissometimestrue,andsometimesfalse,dependingonthetypeofevidenceinquestion.Iwillshowhowthiscomportswithmyowntheoryofpotentialandveridicalevidence.ThenIwillconsiderwhatimplications,ifany,thishasforthe(p.212) debatebetweenBrushandthepredictionistsandfortheproblemofoldevidenceraisedbyClarkGlymour.
Beforebeginning,however,letmementionacuriousbutinterestingfactaboutcertainwellknownphilosophicaltheoriesordefinitionsofobjectiveevidence(otherthanmine),includingCarnap'stheoryofconfirmation,6Hempel'ssatisfactiontheory,7Glymour'sbootstrapaccount,8andtheusualhypotheticodeductiveaccount.Thesetheoriesareincompatiblewiththehistoricalthesis.9Theyholdthatwhether,ortheextenttowhich,eisevidencethathisanapriorifactabouttherelationshipbetweeneandh.Itisinnowayaffectedbyempiricalissuessuchasthetimeatwhichhwasfirstproposed,orewasfirstknown,orbytheintentionswithwhichhwasformulated,orhowinformationreportedinewasobtained.Defendersoftheseviewsmustrejectboththepredictionistandtheexplanationistclaimsaboutevidence.Theymustsaythatwhether,ortheextenttowhich,esupportshhasnothingtodowithwhetherewasfirstformulatedasanovelpredictionfromhorwhetherewasknownbeforehandhwasconstructedtoexplainit.
Accordingly,wehavetwoextremeorabsolutistpositions.Thereistheposition,reflectedinthehistoricalthesis,thatevidenceisalwayshistorical(inthesenseindicated).Andthereisacontrastingposition,reflectedinaprioriviews,thatevidenceisneverhistorical.Doesthetruthlieateitherextreme?Orisitsomewhereinthemiddle?InwhatfollowsIwillpursuethesequestionswithrespecttomyconceptsofpotentialandveridicalevidence,theonesmostcrucialtoscientists(althoughwhatIwillsayisalsoapplicabletoESevidence).
2.SelectionProceduresSupposethataninvestigatordecidestotesttheefficacyofadrugDinrelievingsymptomsS.Thehypothesisunderconsiderationis
h:DrugDrelievessymptomsSinapproximately95%ofthecases.
AsIhaveemphasizedinchapter9,whether(andtheextenttowhich)sometestresultisevidencethatacertainhypothesisistruedependsontheselectionprocedureusedtoobtainthatresult.Herearetwoofthemanypossibleselectionproceduresfortestingh:
SP1:Chooseasampleof2000personsofdifferentages,bothsexes,whohavesymptomsSinvaryingdegrees;dividethemarbitrarilyintotwogroups;giveonegroupdrugDandtheotheraplacebo;determinehowmanyineachgrouphavetheirsymptomsrelieved.
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SP2:Chooseasampleof2000femalesaged5allofwhomhavesymptomsSinaverymildform;proceedasinSP1.
(p.213) Nowsupposethatourinvestigatorobtainsthefollowingresult:e:Inagroupof1000personswithsymptomsStakingdrugD,950personshadreliefofS;inacontrolgroupof1000SsufferersnottakingDbutaplacebononehadsymptomsSrelieved.
IfresultewasobtainedbyfollowingSP2,thene,althoughtrue,wouldnotbeparticularlygoodevidencethath,certainlynotasstrongasthatobtainedbyfollowingSP1.Thereason,ofcourse,isthatSP1,bycontrastwithSP2,givesasamplethatisvariedwithrespecttofactorsthatmaywellberelevant:age,sex,andseverityofsymptoms.(Hypothesishdoesnotrestrictitselfto5yearoldgirlswithmildsymptoms,butassertsacurerateforthegeneralpopulationofsuffererswithvaryingdegreesofthesymptomsinquestion.)
Thismeansthatiftheresultdescribedineisobtained,thenwhetherthatresultsodescribedconstitutesevidencethath,andhowstrongthatevidenceis,dependsonahistoricalfactaboute,viz.howinfactewasobtained.IferesultedfromfollowingSP1,theneisprettystrongevidencethath;ifewasobtainedbyfollowingSP2,theneisprettyweakevidencethath,ifitisevidenceatall.Justbylookingateandh,andevenbyascertainingthateistrue,weareunabletodeterminetowhatextent,ifany,e,iftrue,supportsh.Weneedtoinvokehistory.
Hereisathirdselectionprocedure:
SP3:Chooseasampleof2000personsallofwhomhaveSinvaryingdegrees;dividethemarbitrarilyintotwogroups;giveonegroupdrugsDandD(whereDrelievessymptomsSin95%ofthecasesandblockspossiblecurativeeffectsofDwhentakentogether);givetheothergroupaplacebo.
Ifthiswasusedtogeneratee,thenresultefailstoprovideanysupportforh.And,again,thatthisissocanbeascertainedonlybylearningahistoricalfactaboute,viz.thatwhate(truly)reportswasobtainedbyfollowingSP3.10
Sofarthenweseemtohavesupportforthehistoricalthesisaboutevidence.Canwegeneralizefromexampleslikethistoallcases?Canwesaythatforanye,andanyhypothesish,whether,ortowhatextent,e,iftrue,isevidencethathdependsuponhistoricalfactsabouthowewasobtained?
Consideranothertypeofcase,inwhichtheaimistotestthehypothesis
h:Johnwillwinthelottery.
Atthepresenttimesupposethatthiscanbedoneonlyindirectlybyobtaininginformationaboutwhoboughtticketsandhowmany.Twoinvestigatorsproceedtoobtaininformationofthissort,eachonefollowingadifferentselectionprocedure:(p.214)
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SP4:Determinewhoboughttickets,andhowmany,byaskinglotteryofficials.SP5:Determinethisbyconsultingthelocalnewspaper,whichpublishesthisinformationasaservicetoitsreaders.
Theseresultsareobtained:e1:Investigator1reportsthatinthelottery1000ticketsweresold,ofwhichJohnowns999,andnofurtherticketswillbesold.e2:Investigator2reportsthesamething.
Whethere1ore2orbothareevidencethathdependsonwhatselectionprocedurewasemployedbytheinvestigatorsinobtainingtheinformationintheirreports.Supposethatinvestigator1followedSP4,whileinvestigator2followedSP5.Andsupposethatnewspapers,bycontrasttolotteryofficials,areusuallyunreliableinsuchreports.Then,althoughe1,iftrue,isstrongevidencethath,e2isnot.Inanycase,todeterminewhether,ortheextenttowhich,e1ore2supportshweagainneedtodetermineahistoricalfactconcerninghowtheticketinformationreportedine1ande2wasobtained.
Now,however,letusdistilltheinformationreportedine1ande2intothefollowing:
e:Inthelottery1000ticketsweresold,ofwhichJohnowns999,andnofurtherticketswillbesold.
Inthiscase,whethere,iftrue,isevidencethath,andhowstrongitis,doesnotdependonhistoricalfactsconcerninghowinformationreportedinewasobtained,orconcerningwhenorhowe,h,ortheirrelationshipcametobeformulated,believed,orknown.Tobesure,whether,ortheextenttowhich,e,iftrue,supportshdoesdependuponotherhistoricalfacts,suchaswhetherthelotteryisfair,whethercertainconditionswillinterfere,andsoforth.Butthesearenothistoricalfactsofthekindrelevantforthehistoricalthesisofevidence.Inthiscase,unlikethedrugcase,inorderfore,iftrue,tobeevidencethath,itisirrelevanthoworwhentheinformationinewasobtained,orevenwhetheritwasobtained.Accordingly,wehaveacasethatviolatesthehistoricalthesisofevidence.
Sinceexamplessimilartoeachoftheabovecanbereadilyconstructed,wemayconcludethattherearecasesthatsatisfythehistoricalthesisofevidence,andothersthatfailtosatisfyit.Withrespecttoahypothesish,ifespeaksofobservationsortestsmadethatyieldcertainresults,thenwhethereiftrueisevidencethath,andifso,howstrongthatevidenceis,dependsonwhatselectionprocedurewasemployedinmakingtheseobservationsortests.Thatisahistoricalfactofthekindthatconformstothehistoricalthesisofevidence.However,therearemanycases,suchasthelastonenoted,inwhichedoesnotspeakofobservationsortests,butofcertainfactsthataredescribedindependentlyofobservationsortests.Suchcasesdonotconformtothehistoricalthesis.
Whatimplications,ifany,doesthisholdforwhetherpredictionsorexplanationsprovidestrongerevidence?
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(p.215) 3.PredictionsVersusExplanationsLetusreturntotheoriginalquestionproposedbyBrush.Donovelfactspredictedbyatheoryprovidestrongerevidenceforthattheorythanknownfactsexplainedbythetheory,asWhewellandPopperclaim?Oristhereversetrue?
Apreliminarypointisworthmaking.Asshouldbeobviousfromdiscussionsinearlierchapters,themerefactthatsometheoryorhypothesishwas(orindeedcanbe)usedsuccessfullytopredictsomenoveledoesnotsufficetomakeeevidencethath.Nordoesthefactthatsometheoryorhypothesishwas(orcanbe)usedtoexplainsomeknownfactesufficetomakeeevidencethath.Supposethereisalotterywhose1millionticketsgoonsaleonTuesday;onlyoneticketcanbepurchasedbyanygivenperson;andthedrawingwillbemadeonFriday.Myhypothesisisthatyouwillwinthelottery.SupposeIusethishypothesistopredictthatyouwillbuy1ticket.Thefactthatmypredictionturnedouttobecorrectisnotevidencethatyouwillwinthelottery.SupposethatonSundayitcomestobeknownthatyouareinagoodmood.AndsupposeIusethehypothesisthatyouwonthelotterytoexplainthisknownfact.Thiswouldnotsufficetomakethefactthatyouareinagoodmoodevidencethatyouwonthelottery.Itisreadilyshownthatontheaccountof(potentialandveridical)evidenceIpropose,neitherthefactthatewas(orcanbe)correctlypredictedfromahypothesish,northefactthathwas(orcanbe)usedtoexplainaknownfacte,sufficestomakeeevidencethath.
Accordingly,whatthepredictionistandexplanationistmaywanttosayisthis.Whethereisanovelfactcorrectlypredictedfromahypothesish,orwhethereisaknownfactthathwasinvokedtoexplain,isrelevantforthequestionofwhethereisevidencethath,andparticularlyforthequestionofhowstrongthatevidenceis.Onthepredictionistview,anovelfactcorrectlypredictedbyahypothesisprovidesstrongerevidencethandoesanalreadyknownfactexplainedbythehypothesis.Foranexplanationistitisthereverse.Whichviewiscorrect?
Myansweristhis:Neitherone.Sometimesapredictionprovidesbetterevidenceforahypothesis,sometimesanexplanationdoes,andsometimestheyareequallygood.Whichobtainshasnothingtodowiththefactthatitisapredictionofnovelfactsorthatitisanexplanationofknownones.
Toshowthis,Iwillbeginwithacasethatviolatesthehistoricalthesisofevidence.Hereitshouldbeeasytoshowthatwhethertheputativeevidenceisknownbeforeorafterthehypothesisisformulatedisirrelevantforwhetheritisevidencethathorhowstrongthatevidenceis.Let
h:Thiscoinisfair,thatis,iftossedinrandomwaysundernormalconditionsitwilllandheadsapproximatelyhalfthetimeinthelongrune:Thiscoinisphysicallysymmetrical,andinaseriesof1000randomtossesundernormalconditionsitlandedheadsapproximatelyhalfthetime.
Letussupposethateisempiricallycompletewithrespecttoh(thatis,whethereisevidencethath,andhowstrongthatevidenceis,doesnotdependonempirical(p.216)
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factsotherthane).11Inparticular,whethereisevidencethathdoesnotdependonwhen,how,orevenwhether,ecomestobeknown,oronwhetherewasknownfirstandhthenformulated,oronwhetherhwasconceivedfirstandethenstatedasapredictionfromit.Putativeevidenceesupportshypothesishanddoesso(equallywell)whetherornoteisknownbeforeorafterhwasinitiallyformulated,indeedwhetherornothwaseverformulatedoreiseverknowntobetrueoranyselectionprocedurewasusedtoobtaine.
Thesameholdsinacasethatisanalogousexceptthateisnotempiricallycompletewithrespecttoh.Let
e:Inthelottery1000ticketsweresold,ofwhichJohnowned999atthetimeofthedrawing.h:Johnwon.
Whethereisevidencethath,andifsohowstrongitis,dependsonthetruthofcertainfactsotherthane,suchaswhetherthelotterywasfair.Butitdoesnotdependonwhetherewasknownbeforeorafterhwasformulated,oronwhetherewaseverknown,oronwhatselectionprocedure,ifany,wasemployedtoobtaine.
Soletusfocusinsteadoncasesthatsatisfythehistoricalthesisofevidence.Wemightsupposethatatleastinsuchcasesexplanations(orpredictions)arealwaysbetterforevidence.Returnonceagaintoourdrughypothesis
h:DrugDrelievessymptomsSinapproximately95%ofthecases.
Considertwoevidenceclaims,thefirstapredictionaboutanunknownfutureevent,thesecondareportaboutsomethingalreadyknown:
e1:Inthenextclinicaltrialof1000patientswhosufferfromsymptomsSandwhotakeDapproximately950willgetsomerelief.e2:Inatrialthathasalreadytakenplaceinvolving1000patientswithSwhotookD(weknowthat)approximately950gotsomerelief.
Onthepredictionview,e1(iftrue)isstrongerevidenceforhthanise2.Ontheexplanationview,itisthereverse.Andtosharpenthecase,letussupposethate2,bycontrasttoe1,wasnotonlyknowntobetruepriortotheformulationofh,butthathwasformulatedwiththeintentionofexplaininge2.Whichviewiscorrect?Neitherone.
Letustakethepredictioncasee1first.Whethere1iftrueisevidencethath,andhowstrongitis,dependsontheselectionproceduretobeusedinthenextclinicaltrial.Supposethisselectionprocedurecallsforchoosingjust5yearoldgirlswithverymildsymptomswhoinadditiontoDarealsotakingdrugDwhichamelioratessymptomsSin95%ofthecasesandpotentiallyblocksDfromdoingso.Thene1wouldbeveryweakevidencethathistrue,ifitisevidenceatall.Thisisso,despitethefactthate1isacorrectpredictionfromh,onenotusedingeneratinghinthefirstplace.Bycontrast,supposethattheselectionprocedureusedinthepasttrialmentionedine2ismuch
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betterwithrespecttoh.Forexample,itcallsforchoosinghumansofbothsexes,ofdifferentages,with(p.217) symptomsofvaryingdegrees,whoarenotalsotakingdrugDoranyotherthatcanpreventDfromworking.Thene2wouldbeevidencethath,indeedmuchstrongerevidencethane1.Insuchacase,aknownfactexplainedbyhwouldprovidemoresupportforhthananewlypredictedfactwould.
Obviouslythesituationsherecanbereversed.Wemightsupposethattheselectionprocedureusedtogeneratethepredictionofe1istheonecitedinthepreviousparagraphasbeingusedtogeneratee2(andviceversa).Inthissituationanewlypredictedfactwouldprovidemoresupportforhthananalreadyexplainedone.
Inthesecaseswhethersomefactisevidence,orhowstrongthatevidenceis,hasnothingtodowithwhetheritisbeingexplainedorpredicted.Ithastodowiththeselectionprocedureusedtogeneratethatevidence.Inonesituationwhetheritinvolvessomethingthatisexplainedorpredictedwehaveaputativeevidencestatementgeneratedbyaselectionprocedurethatisagoodonerelativetoh;intheothercasewehaveaflawedselectionprocedure.Thisiswhatmattersforevidence,notwhethertheputativeevidenceisbeingexplainedorpredicted.
4.AResponse:Maher'sAccountAresponseofthepredictionistandexplanationistwillnowbeconsideredinthissectionandthetwothatfollow.Itinvolvesformulatingtheinformationconcerningwhethereisapredictionoranexplanationaspartoftheevidencestatementitselforatleastaspartofthebackgroundinformation.12
Intheexampleabove,let
e:Inaclinicaltrialof1000patientswhosufferfromsymptomsSandwhotakedrugD,approximately95%gotsomereliefb1:eisapredictionfromhb2:hwasdevisedtoexplaineh:DrugDrelievessymptomsSinapproximately95%ofthecases.
Accordingtothepredictionist,e&b1isstrongerevidencethaththanise&b2(oreisstrongerevidencethathgivenb1thanitisgivenb2).
Accordingtotheexplanationist,e&b2isstrongerevidencethaththanise&b1(oreisstrongerevidencethathgivenb2thanitisgivenb1).
PatrickMaheroffersapredictionistresponseofthissort,whichheformulatesasfollows.13Supposethatahypothesishisgeneratedonsomeoccasionby(p.218) amethodM.LetMhbethatMgeneratedahypothesisthatentailsh.LetObethat(asMaherputsit)ewasavailabletoMwhenitgeneratedh.Then,accordingtoMaher,thepredictionistthesisisthis:
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(PT)p(h/Mh&e&O)>p(h/Mh&e&O).14
Theprobabilityontheleftisconditionalontheassumptionthatewasnotusedtogenerateh.(Thisisthepredictioncase.)Theprobabilityontherightisconditionalontheassumptionthatewasusedtogenerateh(whichMahercallsaccommodation).Maherdoesnotclaimthat(PT)holdsuniversally.Buthedoesproveatheoremshowingthatitholdsundercertainconditions,which,heclaims,usuallyobtaininscience.15
Beforediscussinganyoftheseconditions,IwillgiveanexampleMaherhimselfoffers,andthenshowhowhewoulddealwiththedrugcaseabove.Maher'sexampleinvolvescointossinganditpurportstoshowthatpredictionprovidesstrongerevidencethanaccommodation.16Heimaginestwocases,asfollows:
(a)Accommodation.Afaircoinisrandomlytossed99timesbyanexperimenter,andtheoutcomeofeachtossisrecordedinasentencee.Thehypothesishisaconjunctionofewiththeproposition(h)thatthecoinwilllandheadsonthe100thtoss.Letbcontaintheinformationthattheresultsofthe99tosseswerefirstrecordedbytheexperimenter,followingwhichtheexperimenterformulatedtheconjunctivehypothesishtoaccommodatee.(Thisistheexperimenter'smethodMofgeneratinghypothesish.)AccordingtoMaher,
(1)p(h/e&b)=p(h/e&b)=.
Thatis,giventhatoneachofthefirst99tossesthecoinlandedinthemannerdescribedbyeandgiventheaccommodationdescribedinb,theprobabilitythatthecoinwilllandheadsonthe100thtossandthatinthefirst99tossesitlandedinthemannerdescribedbyeisjusttheprobabilitythatitwilllandheadsonthe100thtoss,giventhatitlandedthewayitdidonthefirst99tossesandgivenaccommodation.Assumingthecoinisfairandtossedrandomly,thelatterprobabilityshouldbe.
(b)Prediction.Propositionse,h,andhareasabove,butbischanged.Insteadoffirsttossingthecoin99timesandrecordingtheresults,theexperimenterpredictstheresultsofthefirst99tosses,viz.e,andalsopredictsthatthe100thtosswillyieldheads.(Callthisconjunctivefactb(e).)17Thenthecoinistossed99timesandtheresultseareexactlyaspredicted.Now,accordingtoMaher,
(2)p(h/e&b(e))=p(h/e&b(e))approximately1.
(p.219) LetCPibethattheexperimentercorrectlypredictstheresultoftheithtoss.Wherehisthatthe100thtosswillbeheads,letP(h)bethattheexperimenterpredictsthatthe100thtosswillbeheads.Thene&b(e)isequivalenttoCP1...CP99&P(h).Sofrom(2)wehave
(3)p(h/CP1...CP99&P(h))=approximately1.
Inthiscase,giventhattheexperimentersuccessfullypredictstheresultsofthefirst99tosses,viz.e,andpredictsthatthe100thtosswillbeheads,theprobabilitythathis
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prediction(h)ofheadsonthe100thtosswillbecorrectiscloseto1.Theonlydifferencebetween(1)and(3)isthatin(1)thedatarecordedineareaccommodatedbytheexperimenterusingtheconjunctivehypothesish(=h&e),whilein(3)thedatarecordedinearecorrectlypredictedbytheexperimenterinadvanceoftheexperiment.
Maherattributesthedifferenceinprobabilitiestothefactthatinthesuccessfulpredictioncase,butnottheaccommodationcase,thereisastrongreasontosupposethattheexperimenterhasareliablemethodofpredictingcointosses.Thisisthebasisforhisclaimthatpredictionsprovidestrongerevidencethanaccommodations.
5.MaherandtheDrugCaseReturningnowtomydrugcase,hereishowMaher'saccountwouldproceed.(InwhatfollowsIaltertheexampletoconformtothisaccount.)Supposethereare11clinicaltrials,eachtodeterminetheeffectivenessofdrugDinrelievingsymptomsS.Let
e1:Inthefirst10clinicaltrialsinvolving1000SsuffererswhotookdrugD,95%gotreliefin9outofthe10trials.Inoneofthetentrials,75%gotrelief.e2:Inthe11thtrialinvolving1000SsuffererswhotookdrugD,95%gotrelief.M:Iftherelativefrequencyof95%successintenclinicaltrialsinvolving1000Ssufferersis.9,inferthesame95%successrateinanothertrialinvolving1000SsuffererswhotookD.Me2:Mwasusedtogenerateahypothesisthatentailse2.Mh:Mwasusedtogenerateahypothesisthatentailsh.Oe(i):eiwasavailabletoMwhenMgeneratedei.Oh:eiwasavailabletoMwhenMgeneratedh.h:e1&e2.
Now,accordingtothepredictionistthesis,
(PT)p(h/Mh&e1&Oh)>p(h/Mh&e1&Oh)
Theprobabilityontheleftrepresentsthepredictioncase,theoneontherightrepresentsaccommodation.Thatis,ife1isaprediction,thenhhasahigherprobabilitythanife1isaccommodated(thatis,ife1wasavailabletoMwhenMgeneratedh).
(p.220) Inthepredictioncase,Maherclaims,thefactthate1,whichisentailedbyh,wascorrectlypredicted(indicatedbye1&Oh)booststhereliabilityofthemethodMusedtogenerateh.Thisinturnbooststheprobabilityofh.Bycontrast,intheaccommodationcase,thefactthate1wasaccommodated(indicatedbye1&Oh)doesnotboostthereliabilityofthemethodMusedtogenerateh,ordoesnotboostitasmuchasthepredictioncasedoes.Henceinthiscaseh'sprobabilityisnotashighasitisinthepredictioncase.
OneofthecentralassumptionsofMaher'sgeneraltheoremconcernsthelackofsuchaboostinthecaseofaccommodation.MaherspeaksofthereliabilityRofamethodforgeneratingahypothesis.Thisisthe(objective)probabilitythatthehypothesisgenerated
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bythismethodwillbecorrect.LettheexpectedvalueofthereliabilityofamethodMforgeneratingahypothesishbedenotedbyE(RM(h)).18ConsiderMabove,amethodthatwillgeneratee2.Mahermakesanassumptionforhistheoremwhichhasthefollowingconsequence[p.336,see(22)]:
(1)E(RM(e2)/Me2&e2&Oe2&e1)=E(RM(e2)/e1).
Heassumesthatifeissimplyaccommodated(butnotpredicted),thentheexpectedvalueofthereliabilityofmethodMwithrespecttoe2isnotincreased.
Nowinthedrugcaseabove,thisassumptiondoesnothold.Accordingtoe2,theresultinthe11thtrialsatisfiesM.ThisshouldboosttheexpectedvalueoftheprobabilitythatahypothesisgeneratedbyMiscorrect,whethertheresultsinthe11thtrialwereaccommodatedorpredicted.
Maher'sresponseistoagreethatinthissortofcasethepredictionistthesis(PT)doesnothold.19Insuchacase,hesays,itiscertainwhatthemethodMwouldpredict,thatis,
(2)p(Me2/e2&Oe2&e1=p(Me2/e2&Oe2&e1)=1.
Thisviolatesanassumptionforhistheoremfromwhich(1)abovefollows.Althoughheagreesthat(PT)doesnotholdwhen(2)istrue,heclaimsthatintheusualscientificcases(2)isfalse.Maherwrites:
Onlyinveryspecialcasescanwepredictwithcertaintywhathypothesesscientistswillgenerate.20
Inresponse,notefirstthat(2)isconcernednotwiththeprobabilitythatascientistwillgeneratethehypothesise2butwiththeprobabilitythatmethodMwillgeneratee2.Iagreethattheformerprobabilityisnot1,butitisthelatterprobabilitythatisofconcernin(2).SoisMaherclaimingthatintypicalcasesofprediction,whereascientistusesamethodforgeneratingapredictivehypothesish,theprobabilitythatthemethodgenerateshislessthan1?Theanswerseemstobeyes.TheonlyjustificationMaheroffersforthisansweristhattypicallyinpredictive(p.221) casesthemethodbeingusedisnotunderstoodwellenoughtoallowittobecertainthatthemethodgeneratesthepredictioninquestion.
Asmentionedinnote14,Maheremploysasubjectiveinterpretationofprobabilityinhispredictionistthesis(PT).SohispresentclaimwouldbethatatypicalscientistwhomakesapredictionusingamethodMdoesnotunderstandMwellenoughtobecompletelysurethatMyieldsthatprediction.21ThescopeandinterestofMaher'spredictionistthesis(PT),then,dependsonhowtypicalitisforscientistsusingamethodtopredictthetruthofahypothesishnottounderstandthemethodwellenoughtoknowforsurewhetherhisactuallypredictedbythatmethod.Tosaytheleast,thisisverydifferentfromtheusualpredictionistidea.Moreimportantly,itisonewhosewideapplicabilityseemsquestionable.Frequently,ascientistwillbeinasituationinwhichthemethodofpredictionconsistsinderivingthepredictiondeductivelyfromatheoryorhypothesis,
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wherethescientistknowswhetherornotthederivationiscorrect(withoutnecessarilyknowingwhetherwhatisderivedistrue).
6.Balmer'sFormulaItmaybeusefultociteasimplehistoricalexample,viz.Balmer'sformula,thatissimilarincertainrespectstooneemployedbyMaher.22Whenlightfromhydrogenisanalyzedusingaspectroscope,itisseentoconsistinseriesofsharplinesofdefinitewavelengths.In1885,JohannJakobBalmerintroducedageneralformulathatentailedthewavelengthsofthefourlinesknownbyhimatthattime.Theformulacanberepresentedasfollows:
whereisthewavelengthofagivenline,Risaconstant,andn=3,4,5,6forthefourlines.Balmerdoesnotclaimtobeexplainingwhythelinesoccurorhavethewavelengthstheydo,butsimplytoberepresent[ing]thewavelengthsofthedifferentlinesinasatisfactorymanner.23ThisseemstobeacasesatisfyingMaher'snotionofaccommodation.
(p.222) NowBalmerindicatesthatheusedhisformulatoobtainthewavelengthofafifthline(lettingn=7).Hesaysheknewnothingofsuchalinewhenheperformedthecalculation,andwaslaterinformedthatitexistsandsatisfiestheformula.Sothefifthlinewas,fromhisstandpoint,apredictionthatturnedouttobecorrect.Moreover,hereportsbeinginformedthatmanymorehydrogenlinesareknown,whichhavebeenmeasuredbyVogelandHugginsinthevioletandultravioletpartsofthehydrogenspectrumandthespectrumofthewhitestars(p.362).WhatimpressesBalmer,however,isnotthefactthathehasmadeasuccessfulprediction,butsimplythefactthatallofthelines,whetheraccommodatedorpredicted,satisfyhisformula.Hewrites:
Fromthesecomparisonsitappearsthattheformulaholdsalsoforthefifthhydrogenline....ItfurtherappearsthatVogel'shydrogenlinesandthecorrespondingHugginslinesofthewhitestarscanbepresentedbytheformulaverysatisfactorily.WemayalmostcertainlyassumethattheotherlinesofthewhitestarswhichHugginsfoundfurtheronintheultravioletpartofthespectrumwillbeexpressedbytheformula.(p.362)
AsfarasBalmerisconcerned,itisthefactthatthevariouslines,whetherfirstknownandlateraccommodatedorfirstpredictedandlaterknown,allsatisfyhisformulathatprovidesstrongevidenceforthelastclaimintheabovepassage.
LetB(i)meanthatlineisatisfiesBalmer'sformula.Balmer'sclaimisthat
(3)p(B(5)/B(1)...B(4))isveryhigh.
Thishasnothingtodowithaccommodationorprediction.However,letthemethodforgeneratinghypothesesoftheformB(i)beasfollows:
M:UseBalmer'sformulatoobtainB(i).
1/ = R( 1/ )n 14 n2
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TheprobabilityinwhichMaherisinterestedforaccommodationis(4)p(h/M&e&O),
thatforpredictionis(5)p(h/M&e&O),
wherehisB(5)&e,eisB(1)...B(4),andOsaysthatewasavailabletoMwhenitgeneratedh.
IfMaher'spredictionistthesis(PT)holdsforthiscase,thentheprobabilityin(5)shouldbegreaterthanthatin(4).But,Isubmit,theyareequal.Therelevantconsiderationisthatinbothcaseslines1through4satisfyBalmer'sformula,andthatthehypothesisofinterestconcerningline5wasgeneratedusingtheBalmerformula.ThetruthvalueofclaimsB(1)...B(4)isrelevant,butitisirrelevantwhetherornotthetruthvaluesoftheseclaimswereavailabletomethodMwhenitgeneratedB(5).
Tobesure,insuchacaseMaherwillsaythatBalmer'smethodMissuchthatwhetherB(i)isapredictionusingMisknownforcertain.24ButIsubmit,itisjust(p.223) thesekindsofpredictions,viz.deductionsfromhypothesesand/orapplicationsofquantitativeformulastonewcases,thatarefrequentlymeantbythosewhodefendapredictionistthesis.Moreover,Isubmit,theyarenotatypicalinscience.
7.BrushReduxBrushisclearlydenyingageneralpredictionistthesis.Bycontrast,hecitescasesinwhichscientiststhemselvesregardedknownevidenceexplainedbyatheoryasstrongersupportforthattheorythannewevidencethatwassuccessfullypredicted.Andheseemstoimplythatthiswasreasonable.Heoffersanexplanationforthisclaim,viz.thatwithexplanationsoftheknownphenomena,bycontrastwithsuccessfulpredictionsofthenewones,scientistshadtimetoconsideralternativetheoriesthatwouldgeneratethesephenomena.Now,evenifBrushdoesnotdoso,IwanttoextendthisideaandconsideramoregeneralexplanationistviewthatiscommittedtothefollowingthreethesesthatBrushinvokesforsomecases:
(1)Aselectionprocedurefortestingahypothesishisflawed,oratleastinferiortoanother,otherthingsbeingequal,ifitfailstocallforexplicitconsiderationofcompetitorstoh.(2)Thelongertimescientistshavetoconsiderwhetherthereareplausiblecompetitorstohthemorelikelytheyaretofindsomeiftheyexist.(3)Withputativeevidencealreadyknownbeforetheformulationofhscientistshave(had)moretimetoconsiderwhetherthereareplausiblecompetitorstohthanisthecasewithnovelpredictions.
Iwouldchallengeatleastthefirstandthirdtheses.Inthedrugexampleofsection2,selectionprocedureSP1forthedrughypothesisdoesnotcallforexplicitlyconsideringcompetitorstothathypothesis.Yetitdoesnotseemflawedonthataccount,orinferiortoonethatdoes.However,evensupposingitwereinferior,whetherornotaselection
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procedurecallsforaconsiderationofcompetitorsiscompletelyirrelevanttowhethertheputativeevidenceclaimisapredictionoraknownfactbeingexplained.Inthecaseofaprediction,nolessthanthatofanexplanation,theselectionproceduremaycallforaconsiderationofcompetitors.
Forexample,inourdrugcase,wherehisDrugDrelievessymptomsSinapproximately95%ofthecases,andeisthepredictionInthenextclinicaltrialof1000patientssufferingfromsymptomsSwhotakeD,approximately950will(p.224) getsomerelief,theselectionproceduretobeusedforthenextclinicaltrialmightincludetherule
Inconductingthisnexttrial,determinewhetherthepatientsarealsotakingsomeotherdrugwhichrelievesSinapproximately95%ofthecasesandwhichblocksanyeffectivenessDmighthave.
Suchaselectionprocedurecallsfortheexplicitconsiderationofacompetitortoexplaine,viz.thatitwillbesomeotherdrug,notD,thatwillrelievesymptomsSinthenexttrial.Thisissoeventhougheisaprediction.Moreover,torespondtothethirdthesisabouttimeforconsideringcompetitors,aninvestigatorplanningafuturetrialcanhaveasmuchtimeasshelikestodevelopaselectionprocedurecallingforaconsiderationofacompetinghypothesis.Moregenerally,indesigninganovelexperimenttotestsomehypothesish,asmuchtimemaybespentinprecludingcompetinghypothesesthatwillexplainthetestresultsasisspentinconsideringcompetinghypothesesforolddata.
8.ThomsonVersusHertz:TheWaveTheoryofLightLetmeinvoketwoscientificexamplesemployedearlier.ThefirstinvolvesthedisputebetweenHeinrichHertzandJ.J.Thomsonoverthenatureofcathoderays,discussedinchapter2above.Recallthatin1883Hertzobservedthatthecathoderaysinhisexperimentswerenotdeflectedbyanelectricfield.Hetookthistobestrongevidencethatcathoderaysarenotchargedparticlesbutsometypeofetherwaves.In1897ThomsonrepeatedHertz'sexperimentsbutwithamuchhigherevacuationofgasinthecathodetubethanHertzhadbeenabletoobtain.Thomsonbelievedthatwhencathoderayspassthroughagastheymakeitaconductor,whichscreensofftheelectricforcefromthechargedparticlescomprisingthecathoderays.Thisscreeningoffeffectwillbereducedifthegasinthetubeismorethoroughlyevacuated.InThomson's1897experimentselectricaldeflectionofthecathoderayswasdetected,whichThomsontooktobestrongevidencethatcathoderaysarechargedparticles.
IwanttoconsidertheevidentialreportofHertzin1883,notofThomsonin1897.Let
1. e=InHertz'scathoderayexperimentsof1883,noelectricaldeflectionofcathoderayswasdetected.
2. h=Cathoderaysarenotelectricallycharged.
Hertzconsideredetobestrong(veridical)evidencethath.In1897ThomsonclaimedthatHertz'sresultsasreportedinedidnotprovidestrongevidencethath,sinceHertz'sexperimentalsetupwasflawed:Hewasemployinginsufficientlyevacuated
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tubes.Tousemypreviousterminology,ThomsonwasclaimingthatHertz'sselectionprocedurefortestinghwasinadequate,andhencethateisnot(potentialorveridical)evidencethath.
HerewecanpickuponapointemphasizedbyBrush.Hertz,wemightsay,failedtouseaselectionprocedurecallingforconsideringacompetitortohtoexplainhisresults(viz.thatcathoderaysarechargedparticles,butthatthetubes(p.225) Hertzwasusingwerenotsufficientlyevacuatedtoallowanelectricalforcetoactontheseparticles).ButandthisisthepointIwanttoemphasizeindeterminingwhether,ortowhatextent,eis(potentialorveridical)evidencethath,itisirrelevantwhetherHertz'sewasanovelpredictionfromanalreadyformulatedhypothesishoranalreadyknownfacttobeexplainedbyh.Hertzwritesthatinperformingtherelevantexperimentshewastryingtoanswertwoquestions:
Firstly:Dothecathoderaysgiverisetoelectrostaticforcesintheirneighbourhood?Secondly:Intheircoursearetheyaffectedbyexternalelectrostaticforces?25
Inhispaperhedidnotpredictwhathisexperimentswouldshow.Norweretheresultsofhisexperimentstreatedbyhimasfactsknownbeforehehadformulatedhishypothesish.Onceheobtainedhisexperimentalresultshethenclaimedthattheysupportedhistheory:
Asfarastheaccuracyoftheexperimentallows,wecanconcludewithcertaintythatnoelectrostaticeffectduetothecathoderayscanbeperceived.(p.251)
Tobesure,wemightsaythatHertz'stheoryitselfpredictedsomesuchresults,evenifHertzhimselfdidnot(thatis,evenifHertzdidnothimselfdrawhisconclusionbeforegettinghisexperimentalresults).Butevenifwespeakthisway,Hertzdidnotclaimorimplythathisexperimentalresultsprovidebetter(orweaker)evidenceforhistheorybecausethetheorypredictedthembeforetheywereobtained.NordidThomsoninhiscriticismofHertzalludetooneortheotherpossibility.WhicheveritwaswhetherapredictionoranexplanationorneitherHertz(Thomsonwasclaiming)shouldhaveusedabetterselectionprocedure.ThisiswhatiscriticizableinHertz,notwhetherhewaspredictinganovelfactorexplainingaknownone.
Thesecondexampleinvolvesthenineteenthcenturyargumentforthewavetheoryoflightgiveninchapter7,section7.Theargumentbeganwithtwoclaims:(i)thatlighttravelsfromonepointtoanotherwithafinitevelocity;(ii)thatinotherknowncases,suchassoundwaves,waterwaves,andprojectiles,modesoftransferinvolvingfinitevelocitiesareviaclassicalwavesinamediumorclassicalparticles.From(i)and(ii),itisconcludedthat(h)lightiseitherawavemotioninamediumorastreamofparticles.Inprobabilisticterms,
(1)p(h/(i)&(ii))iscloseto1.
Indeed,itmightbeclaimedthat
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(2)p(thereisanexplanatoryconnectionbetweenhand(i)/(i)&(ii))>,
sothat(since(i)and(ii)weredeemedtrueinthenineteenthcentury),(i)constitutespotentialevidencethath,given(ii).Theclaimthatitdoesconstituteevidencethatlightiseitheraclassicalwaveoraclassicalstreamofparticleswasmadebywavetheoristsaspartoftheireliminativeargumentforthewavetheory.
Now,whatIbelievecanlegitimatelybesaidaboutthiscaseisthatalthough(p.226) (i)constitutesESevidencethath(wheretheepistemicsituationistheoneholdingforwavetheoristsduringthefirstfourdecadesofthenineteenthcentury),itisnotpotentialevidencethath.Thereasonitisnothasnothingtodowithwhetherwavetheoristsemployedhtoexplainthealreadyknownfactthatlighttravelsfromonepointtoanotherinafinitetimeortopredictnewcasesinwhichthisisso.Thereasonisthattheselectionprocedureisbiased,althoughthiswasnotknownbywavetheoristsandcouldnotbeknownuntilthetwentiethcentury.Theitemsselectedtodefendtheclaimthatmodesoftransferinvolveclassicalwavesandparticlessuchaswaterwaves,soundwaves,andprojectilesareallitemsfromthemacroworldofclassicalwavesandparticles;therearenonefromthesubatomicworldsubjecttothelawsofquantummechanicswhichprecludeclassicalwavesandparticles.Accordingly,(1)and(2)arecriticizable,notthefactthatwavetheoristswereexplainingknownfacts(orpredictingnewones).
IendthissectionwithaquotefromJohnMaynardKeynes,whosebookonprobabilitycontainslotsofinsights.Hereisone:
Thepeculiarvirtueofpredictionorpredesignationisaltogetherimaginary.Thenumberofinstancesexaminedandtheanalogybetweenthemaretheessentialpoints,andthequestionastowhetheraparticularhypothesishappenstobepropoundedbeforeoraftertheirexaminationisquiteirrelevant.26
9.TheProblemofOldEvidenceYearsagoClarkGlymourraisedafundamentalobjectiontothepopularpositiverelevancedefinitionofevidence.27Supposethattheprobabilityofeis1.Ifitis,then,assumingthattheprobabilityofhisgreaterthan0,p(e/h)=1.Now,accordingtoBayes'theorem,
(1)p(h/e)=p(h)xp(e/h)/p(e)).
Therefore,ifp(e)=1,thenfrom(1)weobtain(2)p(h/e)=p(h).
Thatis,theprobabilityofh,givene,isthesameasitspriorprobability.Nowonthepositiverelevancedefinition,
(3)eisevidencethathifandonlyifp(h/e)>p(h)
Soifp(e)=1andp(h)=0,thenfrom(2)and(3),ecannotbeevidencethath.
Whyisthisaproblem?Suppose,saysGlymour,thatpriortotheintroductionof
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hypothesish,ewasknownwithcertaintytoobtain.Glymourconcludesthate'sprobabilityisthen1.(Iwillreturntothisclaiminamoment.)Ife'struthwasknownpriortotheintroductionofh,theneisoldevidencewithrespecttoh.Itfollowsfromthepositiverelevancedefinitionofevidencethatoldevidencewithrespecttoahypothesiscannotbeevidencethatthehypothesisistrue,since(p.227) itcannotincreasetheprobabilityofthehypothesis.Thisstrikesmanyasabsurd,sincefrequentlyphenomenaconsideredevidenceinfavorofatheorywereknownwithcertaintytoobtainpriortotheformulationofthetheory.
Insection3ofthischapterweconsideredwhetherpredictionsofnovelfactsprovidestrongerevidenceforatheorythanexplanationsofoldones,orwhetherthereverseistrue.Predictionists(suchasWhewellandPopper)mightwelcomeGlymour'sproblembyrespondingthatifeisalreadyknowntobetruepriortotheformulationofh,thenitcannotbe(verymuch)evidenceforh,evenifitisderivablefromh.Forthem,inorderthatebeevidencethath,oratleastsubstantialevidence,itmustbeanewpredictionandnotafactknownpriortotheformulationofh.
Nowtomakethisclaimistoespousethehistoricalthesisofevidenceformulatedinsection1.Toknowwhethereisanewpredictionandnotafactknownpriortotheformulationofhonemustknowhistoricalfactsabouteandh,viz.whetherandwheneisknownandwhetheritisknownpriortowhenhwasproposed.InmyearlierdiscussionIrejectedthehistoricalthesisasauniversalprinciple.Tobesure,therearecasesinwhichdeterminingwhether,ortowhatextent,esupportshrequiresdeterminingthetruthofsomehistoricalfactaboute,h,ortheirrelationship.Buttherearealsocaseswherethisisnotsoatall.Toclaimthatonlypredictionsprovide(substantial)evidenceforahypothesisistoespouseamistakenhistoricalthesis.
Glymourconsideredtheproblemofoldevidencesufficienttoshowthatoneshouldnotaccept(3),thestandardpositiverelevancedefinitionofevidence.Sinceinchapter4Ihavegivenmanyotherreasonstorejectthisdefinition,whatIwanttofocusonhereisnotthequestionofwhetherGlymour'sobjectionisdevastatingtothepositiverelevanceaccountbutonwhatoneistosayaboutcasesinwhichtheprobabilityoftheputativeevidenceis1.
However,beforedoingsoIreturntoacentralclaimofGlymour'sargument,viz.thatifeisknowntobetrue(whetherornotthiswaspriortotheformulationofh),thenp(e)=1.Isthistrue?Itdoes,indeed,holdforsubjectiveprobability.IfatacertaintimetIknowthateistrue,thenattmysubjectiveprobabilityforeismaximal.28DoesitholdfortheconceptofepistemicprobabilitythatIespouse?
Myansweristhatitdependsonwhatrelativization,ifany,isbeingassumedintheprobabilitystatement.Ifeisknowntobetrue,thenitfollowsthatitistrue.Andiftheprobabilityinquestionisrelativizedtoe,ortothefactthateisknowntobetrue,then,tobesure,theprobabilityofeis1(pe(e)=1).Butthatistrivialanduninteresting.Withotherlesstrivial,moreinteresting,relativizationstheprobabilityisnot1,eventhoughitisknownthateistrue.Forexample,supposeeisasfollows:
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e:Thiscoin,whichwastossed100timesonJanuary1,2000,landedheads95%ofthetime.
(p.228) Supposesomepersonknowsthateistrue.Wemaywanttoconsidertheprobabilityofedisregardingthisknowledge,thatis,pd(Ke)(e),inwhichd(Ke)meansdisregardthefactthateisknowntobetrue.Inthiscasetheprobabilitymaybelessthan1,eventhoughitisknownthateistrue.29Accordingly,withepistemicprobability,fromthefactthateisknowntobetrue,itdoesnotnecessarilyfollowthattheprobabilityofeis1.Itdependsonwhatis,orisnot,beingassumed.
IturnnowtothequestionofwhetherGlymour'sproblemcanbeputinaformwhichdoesnotintroducetheideathateisafact(not)knownpriortotheformulationofh.Usingepistemicprobabilities,supposethatpb(e)=1.Thisisnotahistoricalclaim.Itdoesnotsaythatanyoneinfactknowsorcametoknowthateistrue,orthatanyonedidsobeforeoraftertheformulationofh.Allitsaysisthat,inviewofb,thedegreeofreasonablenessofbelievingeismaximal.Coulditbethecasethat,givenb,eispotentialorveridicalevidencethath?Ordoesthefactthatpb(e)=1precludethispossibility?
Mydefinitionsofpotentialandveridicalevidencedonotprecludethispossibility.Itispossibleforthefollowingconditionsforpotentialevidencetobesatisfied:
1.eandbaretrue2.edoesnotentailh3.pb(h/e)xpb(thereisanexplanatoryconnectionbetweenhande/h&e)>,
evenifpb(e)=1.
Todemonstratethissupposethebackgroundinformationbcontainsthefactthatthereisalotteryconsistingof1000ticketsoneofwhichwillbedrawnatrandom,andthatJohn'swifeMarypurchased950oftheseticketsasapresentforJohn,whodepositedtheminhissafedepositboxwithalegaldocumentsayingthattheticketsbelongtohim.LetebethatJohnowns950ticketsinthislottery.LethbethatJohnwillwin.Ifeandbaretrue,thentheaboveconditionsfore'sbeingpotentialevidencethatharesatisfied.Yetpb(e)=1,sinceinthiscasethebackgroundb(whichwemaysupposecontainsstandardlegalprinciplesofownership)entailse.Givenb,thedegreeofreasonablenessofbelievinge(thatJohnowns950ticketsinthislottery)ismaximal.Yetgivenb,eispotentialevidencethath.And,ifhistrue,itisveridicalevidenceaswell.Thefactthat,inthelightofthebackgroundinformationb,itismaximallyreasonabletobelieveedoesnotatallcountagainste'sbeingevidencethath.Indeed,inthiscase,inthelightofb,eisverystrongevidencethath.
Now,letusconsiderwhat,ifanything,happensifitisunreasonabletobelievee.Ifp(e)=0,thenp(h/e)isundefined.Soletusconsideracaseinwhichehaslow,butnonzero,probability.Predictionistschampionsuchcases,becausethelowertheprobabilityofthepredictione,thehighertheposteriorprobabilityofhone.
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Letthebackgroundinformationbbethatafaircoinwillberandomlytossed100times.Letebethatthecoinwilllandheads100timesinarow.LethbethehypothesisthattheDevilwillinterveneaftereachrandomtoss,causingthecoin(p.229) tolandheadseachtimeduringthefirst100tosses.Inthiscasethehypothesishentails(theprediction)e.Assumingb,theprobabilityofeisverylow: .Yet,givenb,itseemsfarfetchedtosaythate,iftrue,ispotentialevidencethath.Indeed,thedefinitionofpotentialevidenceprecludesthis,evenifeandbarebothtrue,since(givennormalbackgroundinformation)theprobabilityofthedevilhypothesish,evenontheassumptionofe,isextremelylow.Thefactthatthecoinwilllandheads100timesinarow,eveniftrue,isnotpotentialevidencethattheDevilwillintervene.
Canwefollowthepredictionistandsayatleastthis:Wherehentailssomepredictione,thelowertheprobabilityofethestrongertheevidencethateconfersuponh?No,wecannot.Allwecansayisthatthelowertheprobabilityofe,insuchacase,thehighertheprobabilityofhone.Butitdoesnotfollowfromthisthatthelowertheprobabilityofethestrongertheevidencethateconfersuponh,sinceemayconfernoevidenceuponh.Thus,inthepreviousexample,letuschangeeto
e:Thecoinwilllandheads1000timesinarow.
Lethandbbethesameasbefore.Nowwehave ,whichisamuchlowerprobabilitythan .Yetthatdoesnotmakeestrongerevidencethaththaneis,since,onmyconception,eisnotevidencethath.Thethresholdforhighprobabilityrequiredforevidencehasnotbeenreached.
Whatcanbesaidisthis.Ifeisevidencethath,thenthelowertheprobabilityofethestrongeristheevidencethateconfersuponh.Forexample,letbcontaintheinformationthatthiscoinisperfectlysymmetrical.Lethbethehypothesisthatitwilllandheadsapproximatelyhalfthetime.Lete1betheinformationthatwhentossedrandomlythefirst100timesitlandedheadsbetween45and55times.Lete2betheinformationthatwhentossedrandomlythenext1000timesitlandedheadsexactly500times.Wemightsaythat,givenb,bothe1ande2countasevidencethath.Nowinthiscasepb(e2)pb(h/e1).Butinthiscasealso,inthelightofb,e2isstrongerevidencethaththanise1.
Accordingly,whetherehasveryhighorverylowprobabilitydoesnotnecessarilyaffectwhether,ortheextenttowhich,eisevidencethath.Norisitingeneraltruethatifhpredictse,thelowerise'sprobabilitythestrongerise'sevidencethath.
10.Conclusions1.Accordingtothehistoricalthesisofevidence,whethereiftrueisevidencethath,orhowstrongthatevidenceis,dependsoncertainhistoricalfactsaboute,h,ortheirrelationship(forexample,onwhetherewasknownbeforeorafterhwasformulated).Althoughthisthesisholdsforsubjectiveevidence,itdoesnotholduniversallyfortheconceptsofobjectiveevidenceIhaveintroduced.Focusingonpotentialandveridical
(e) = ( )pb 12 100
(e') = ( )pb 12 1000(e) = ( )pb 12 100
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evidence,dependingontheparticularevidenceclaimandtheselectionprocedureemployed,insomecasesthehistoricalthesisholds,inothersitdoesnot.
(p.230) 2.Sometimesanovelfactthatispredictedprovidesbetterevidenceforahypothesisthanaknownfactthatisexplained.Sometimesthereverseistrue.Whichobtainshasnothingtodowithwhetheritisapredictionoranexplanation,butrather,inthecasesinquestion,withtheselectionprocedureusedtogeneratetheevidence.ThisisillustratedinthecaseofHertz'sclaimthattheresultsofhis1883cathoderayexperimentsprovideevidencethatcathoderaysarenotelectricallycharged.Itisalsoillustratedinthewavetheorists'argumentthatlightconsistsofclassicalwavesorclassicalparticles.
3.AresponseofPatrickMaherisexaminedwhichinvolvesformulatingtheinformationconcerningwhethereisapredictionorafactexplainedaspartoftheevidencestatementitselforaspartofthebackgroundinformation.Thisnewformulationwillnotsufficetoestablishthesuperiorpowerofpredictionoverexplanation(oraccommodation),orviceversa,intypicalscientificcasesofinteresttopredictionistsandexplanationists.
4.Glymourraisestheproblemofoldevidenceinordertorejectthepositiverelevancedefinition.Accordingtothatdefinition,ecannotbeevidencethathwheneisoldevidenceknowntobetrue.Predictionistsmaywelcomethisresult,sinceforthem,inorderforetobesubstantialevidencethath,emustbeanewprediction,notafactknownpriortotheformulationofh.Tosaythisistoespousethehistoricalthesisofevidenceasuniversal,whichIdonot.IdiscussGlymour'sprobleminaformthatdoesnotcommitonetothehistoricalthesis.Ifthedegreeofreasonablenessofbelievingtheputativeevidenceeismaximal(correspondingtooldevidence),canebepotential(orveridical)evidencethath?Idemonstratethatitcanbe.Sothereisnocomforthereforthepredictionist.(Nordoesthefactthattheprobabilityofeismaximal,whereeisexplainablebyderivationfromh,guaranteethateispotentialevidencethath;sothereisnocomfortherefortheexplanationisteither.)Asecondquestionisthis:Ifhentailssomepredictione,doesthelowertheprobabilityofemeanthestrongertheevidencethateconfersuponh?Theansweragainisno.Soagainthereisnocomfortforthepredictionist.
Notes:(1.)WilliamWhewell,ThePhilosophyoftheInductiveSciences(NewYork:Johnsonreprint,1967;fromthe1847ed.)KarlPopper,TheLogicofScientificDiscovery(London:Hutchinson,1959).
(2.)StephenBrush,PredictionandTheoryEvaluation:TheCaseofLightBending,Science,246(1989),11241129.
(3.)TowhatextentBrushwantstogeneralizethisexplanationistpositionisaquestionIleaveforhimtoanswer.Therearepassagesthatstronglysuggestamoregeneralposition.Forexample:Thereisevensomereasontosuspectthatasuccessfulexplanationofafactthatothertheorieshavealreadyfailedtoexplainsatisfactorily(forexample,theMercuryperihelion)ismoreconvincingthanthepredictionofanewfact,at
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leastuntilthecompetingtheorieshavehadtheirchance(andfailed)toexplainit(p.1127).Inwhatfollows,Iconsiderageneralizedexplanationistthesis.
(4.)eandhherearepropositions.Asnotedinchapter2,thisisthecustomarypracticeofphilosopherswhospeakofeasbeingevidencethath,eventhoughitisthefactthateistruethatisevidence.Thehistoricalthesis,then,concernsthepropositionseandhortheirrelationship.
(5.)AlanMusgrave,LogicalversusHistoricalTheoriesofConfirmation,BritishJournalforthePhilosophyofScience,25(1974),123.
(6.)RudolfCarnap,LogicalFoundationsofProbability.
(7.)CarlG.Hempel,AspectsofScientificExplanation.
(8.)ClarkGlymour,TheoryandEvidence(Princeton:PrincetonUniversityPress,1980).
(9.)SeeLauraJ.Snyder,IsEvidenceHistorical?,ScientificMethods:ConceptualandHistoricalProblems,PeterAchinsteinandLauraJ.Snyder,eds.(Malabar,Florida:Krieger,1994).
(10.)Itmightbeobjectedthatifthisisso,then,contrarytowhatIhavebeensaying,theconceptofevidenceinvolvedpotentialorveridicalevidencecannotbeobjective.Whethereispotential(orveridical)evidencethathinsuchcaseswilldependonwhatselectionprocedurewasemployed,which,inturn,dependsonwhatsomeperson(s)believedabouteandh,viz.uponwhatwasbelievedabouthowewasgenerated.Myreplyisthatinthecasesinquestionwhethereispotentialevidencethathdependsonwhatselectionprocedurewasinfactemployed,notonwhatbeliefsitsemployer(s)mayhavehadaboutit.
(11.)Microconditionsarebeingdisregarded.Seechapter5.
(12.)SeeEricBarnes,SocialPredictivism,Erkenntnis,45(1996),6989.
(13.)PatrickMaher,Prediction,Accommodation,andtheLogicofDiscovery,PSA,1988,vol.1,273285;HowPredictionEnhancesConfirmation,inJ.M.DunnandA.Gupta,eds.,TruthorConsequences(Netherlands:KluwerAcademicPublishers,1990),327343.
(14.)MaherconstruesphereasthesubjectiveprobabilityforsomerationalpersonwhohasnotyetlearnedthetruthvaluesofMh,e,andO.SeeHowPredictionEnhancesConfirmation,p.327.Withsuitablerelativization,probabilityin(PT)couldalsobeunderstoodinanobjectiveepistemicsenseofthesortintroducedinchapter5.
(15.)HowPredictionEnhancesConfirmation,p.328.
(16.)Prediction,Accommodation,andtheLogicofDiscovery,p.275.Seealsohis
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HowsonandFranklinonPrediction,PhilosophyofScience,60(1993),329340.
(17.)Theexperimenterisusingsomemethod,howeverrandom,forgeneratingpredictions.AlthoughMaherdoesnotsaysoexplicitly,Iassumehewouldclaimthatthemethodbeingusedcanbutneednotbegiveninb(e).
(18.)Ifafunctionhasafinitenumberofvalues,theexpectedvalueisthesumofallproductsconsistingofagivenvalueandtheprobabilityofthatvalue.Ifafunction(suchasprobability)hasinfinitelymanyvalues,theexpectedvalueisdefinedusinganintegral.
(19.)Personalcorrespondence.SeehisHowsonandFranklinonPrediction,PhilosophyofScience,60(June1993),329340;seepp.339340.IamindebtedtoPatrickMaherforhelpintryingtogetmetoexpresshisviewsaccuratelyinthesesections;IhopeIhavedoneso.
(20.)HowsonandFranklinonPrediction,p.340.
(21.)Withanobjectiveconceptofprobability,ofthesortIdefendinchapter5,thisconclusionwouldnotbepermitted,unlessaspecialrelativizationisintroducedthatentailsdisregardinginformationindicatingwhetherMyieldsthepredictioninquestion.
(22.)MahercitespredictionsfromMendeleyev'speriodictableoftheelements.Mendeleyevplacedtheelementsingroupsbasedonatomicweights(ratherthanonotherbases)andshowedthatperiodicrepetitionsofpropertiesoccurinthegroupsandcanbeusedtopredictnewelements.Hedidsoexplicitlyclaimingthatatomicweightsphysicallydetermineandcanexplainpropertiesoftheelementsandcompounds.(SeehisTheRelationsbetweenthePropertiesofElementsandtheirAtomicWeights,reprintedinHenryA.BoorseandLloydMotz,eds.,TheWorldoftheAtom(NewYork:BasicBooks,1966,vol.1),p.306.)Bycontrast,Balmermakesnophysicalorexplanatoryclaimsregardinghisformula.WhileBalmer'sformulaaccommodatesthedata,itismoreclearlynonexplanatorythanMaher'sexample,whilealsobeingpredictive.StephenBrush,bycontrasttoMaher,claimsthattheperiodictablewasregardedbyscientistsasprovidingstrongerexplanatorythanpredictiveevidence.
(23.)JohannJakobBalmer,TheHydrogenSpectralSeries,reprintedinWilliamFrancisMagie,ASourceBookinPhysics(Cambridge:HarvardUniversityPress,1965),360365;quoteonp.360.
(24.)MaherclaimsthathisMendeleyevexampleisoneinwhichhispresentuncertaintythesisholds.Hewrites:NobodywithoutthebenefitofhindsightcouldbecertainthatMendeleyevwouldproposethehypothesishedid(HowsonandFranklinonPrediction,p.340).But,again,thisisnottheprobabilityofconcern.Thequestioniswhether,withrespecttoapredictionMendeleyevmadeusinghisperiodictable,hewasinanydoubtaboutwhetherhistableyieldsthatprediction.Afterconstructinghisperiodictablebasedonatomicweights,Mendeleyevwrites:...itappearstobecertainwhenwelookattheproposedtable,thatinsomerowsthecorrespondingmembersare
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missing;thisappearsespeciallyclearly,e.g.,fortherowofcalcium;inwhichtherearemissingthemembersanalogoustosodiumandlithium(p.310).Also,attheendofhispaperhewrites:Thediscoveryofnumerousunknownelementsisstilltobeexpected,forinstance,ofelementssimilartoAlandSihavingatomicweightsfrom6575(p.312).Mendeleyevseemstohavenodoubtthatthesepredictionsaregeneratedbyhistable.
(25.)HeinrichHertz,MiscellaneousPapers(London:Macmillan,1896).
(26.)J.M.Keynes,ATreatiseonProbability(London:Macmillan,1921).
(27.)ClarkGlymour,TheoryandEvidence,p.86.
(28.)Althoughsubjectivistsacceptthis,somewishtosolveGlymour'sproblembyrefusingtouseone'sactualdegreeofbeliefineindeterminingp(e)butsomecounterfactualdegreeofbelief,suchasone'sdegreeofbeliefineoneverythingoneknowsminuse.SeeColinHowsonandPeterUrbach,ScientificReasoning,p.404.
(29.)ThisepistemicprobabilityisanalogoustoasubjectiveonethatwouldbeusedbysubjectivistsofferingacounterfactualsolutiontoGlymour'sproblemmentionedinthepreviousfootnote.
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