couldeveracquireany’experience’atall。SowhenMillsaysthattheburthenofproofrestswiththeintuitionist，heis，nodoubt，quiterightinthrowingtheburthenofproofuponthinkerswhosupposeparticulardoctrinestohavebeensomehowinsertedintothefabricofknowledgewithoutanyrelationtoothertruths；butitissurelynotagratuitousassumptionthatthemindwhichcombinesexperiencemusthavesomekindofpropertiesaswellasthethingscombined。Ifitknowsno’truths’exceptfromexperience，itisatleastpossiblethatitmayinsomewayreactuponthegivenexperience。This，atanyrate，shouldbeMill’sview，whotakes’mind’and’body’tobeunknowable，andallknowledgeoffacttobeacombinationof’sensations。’Heonlyrequirestoadmitthatknowledgemaybeincreasedeitherbyvaryingthedataorbyvaryingthemind’sactionuponfixeddata。
Inneithercasedowegetbeyond’experience。’Inmanyplaces，Millseemstointerprethisviewinconsistencywiththisdoctrine。Hisinvariablecandourleadshimtomakeadmissions，someofwhichIhavenoticed。Yethisprepossessionsleadhimtothesuperfluousparadoxeswhich，fortherest，hemaintainswithremarkablevigourandingenuity。
Oneotherdeviceoftheenemyraisedthetroublesomequestionofinconceivabilityasatestoftruth，whichbroughtMillintoconflictnotonlywithWhewellandHamilton，butwithMrHerbertSpencer。IwillonlynoticethecuriousillustrationwhichitaffordsofMill’stendencytoconfoundstatementsoffactwiththepurelylogicalassertionthattwomodesofstatingafactarepreciselyequivalent。TheexistenceofAntipodeans，inhisfavouriteillustration，55*wasdeclaredtobe’inconceivable。’
Disbeliefintheirexistenceinvolvedthestatementoffact：
gravityactshereandattheAntipodesinthesamedirection。
Thatstatementcouldofcoursebedisprovedbyevidence；andthereisnoreasontosupposethatthetruth，oncesuggested，wouldbeless’conceivable’toAugustineor，say，toArchimedes，thantoNewton。Italsoinvolvedtheassertion：menifthedirectionofgravitywereconstantwoulddropofftheearthattheAntipodesastheyheredropofftheceiling。Thedenialofthatstatementisstill’inconceivable，’thoughthestatementceasestobeapplicable。Mill，however，infersthat，asan’inconceivability’hasbeensurmounted，’inconceivability’ingeneralisnotestoftruth。’Thereis，’hesays，56*’nopropositionofwhichitcanbeassertedthateveryhumanmindmusteternallyandirrevocablybelieveit，’andhetries，asI
havesaid，toapplythiseventotheprincipleofcontradiction。
Inotherwords，becauseourlogicrequiresabasisinfact，andthefactmustbegivenbyexperience，thelogicisitselfdependentuponexperience。If’inconceivable’belimited，asI
thinkitshouldbelimitedinlogic，tothecontradictory，aninconceivablepropositionisincrediblebecauseitisreallynopropositionatall。Wemay，nodoubt，believestatementswhichareimplicitlycontradictory。butwhenthecontradictionismadeexplicit，thebeliefbecomesimpossible。Similarlywemaydisbelievestatementswhichappeartobecontradictory；andwhentheerrorisexposed，wemaybelievewhatwasonce’inconceivable。’Thatonlyshowsthatourthoughtsareofteninagreatmuddle，andingreatneedoflogicalunification。Itdoesnotproveanyincoherenceinthelogicalprocessitself。
IV。CAUSATION
Wecannowproceedtowhatmaybecalledtheconstructivepartofthelogic。Wehavegotridofproofsfromintuitions，fromdefinitions，andfrominconceivabilities，andthequestionremainshowwecanproveanything。Allknowledgeisinductive。Itisallderivedfromfacts；itproceedsfromparticularstoparticulars；thepreviouscoexistenceofsequenceswhichhavebeenobservedconstituteourwholerawmaterial。What，then，servestobindfactstogether？orhowarewetoknowthatfactsareboundtogether，orthatanytwogivenfactshavethisrelation？Thefundamentalpostulateofscienceistheso-called’uniformityofnature。’ButNature，asitisseenbytheunscientificmind，isanythingbutuniform。Thereare，itistrue，certainsimpleuniformitieswhichfrequentlyrecur。Fireburns，waterdrowns，stonesthrownupfalldown；andsuchobservationsarethegermsofwhatweafterwardscallscientific’laws。’Butthingsareconstantlyhappeningofwhichwecangivenoaccount。Catastrophesoccurwithoutanyassignable’antecedent’；stormandsunshineseemtocomeatrandom；andthesamecombinationofeventsneverrecursinallitsdetails。
Varietyisasmanifestasuniformity。Howcancosmosbemadeoutofchaos？Howdowecometotraceregularityinthisbewilderingworldofirregularities？Fromanyfacttakenbyitself，asHumehadfullyshown，wecandeducenonecessityforanyotherfact。
Thequestionis，whetherwearetoaccountforthebeliefinuniformitybyan’intuition’orbyJamesMill’suniversalsolventof’associationofideas。’J。S。Millwasfullyconvincedoftheefficacyofthispanacea，butheseesdifficultiesoverwhichhisfatherhadpassed。Ifassociationexplainseverything，thetiebetweenideasoughttobestronger，itmightbesupposed，inproportiontothefrequencyoftheirassociation。Theoftenertwofactshavebeenjoined，themoreconfidentlyweshouldexpectajunctionhereafter。Butthisdoesnotholdtrueuniversally。A
chemist，asMillobserves，analysesasubstance；andassumingtheaccuracyofhisresults，weatonceinferagenerallawofnaturefrom’asingleinstance。’Butifanyonefromthebeginningoftheworldhasseenthatcrowsareblack，andasinglecrediblewitnesssaysthathehasseenagreycrow，weabandonatonceaconjunctionwhichseemedtorestuponinvariableandsuperabundantevidence。Whyisa’singleinstance’sufficientinonecase，andanynumberofinstancesinsufficientintheother？
’Whoevercananswerthisquestion，’saysMill，’knowsmoreofthephilosophyoflogicthanthewisestoftheancients，andhassolvedtheproblemofinduction。’57*
HereMillagainprofessestosetmetaphysicsaside。Hehasnothingtodowith’ontology。’Hedealswith’physical，’not’efficient，’causes。Hedoesnotaskwhethertherebeorbenota’mysterious’tielyingbehindthephenomenaandactuallyproducingthem。58*Heiscontenttolaydownashisstatementofthe’lawofcausation’thatthereisaninvariablesuccessionbetween’everyfactinnature’and’someotherfactwhichhasprecededit。’This，heassumes，isatruth，whateverbethenatureofthingsinthemselves。Thetrueaccountisratherthathewillshowthat’ontology’isasetofmeaninglessphrases。Hecananswerhisproblemwithoutit。Causationis，infact，conceivedbyhimasitwasconceivedbyallthepsychologists，includingBrown；andhehassimplytoshowthatBrown’ssupposed’intuition’isasuperfluity。Histreatmentofthequestiongivesthereallycriticalpartofhisphilosophy。Itleadstosomeoftheresultswhichhavebeenmosthighlyand，asIthink，mostdeservedlypraised。Italsoleadstosomeofhisgreatesterrors，andshowstheweakpointofhismethod。
Mathematicalknowledge，asMillremarks，hasnothingtodowithcausation。Everygeometricalorarithmeticalformulaistruewithoutsupposingchange。Onetheoremdoesnot’cause’theothers；it’implies’them。Themostcomplexandthemostsimplearemutuallyinvolvedinthesingleperception，thoughourknowledgeofonemaybethecauseofourknowingtheothers。
Theirnecessityisanotherwayofstatingthisimplication。Wecanshowthattodenyonetheoremwhileadmittinganotheristobecontradictory。Thewholeofphysicalscience，however，fromfirsttolast，isaprocessofstatingthechangesofphenomenaintermsoftimeandplace，andthereforebringsthemallwithintherangeofmathematicalmethods。Scienceisnotfullyconstitutedtillitbecomesquantitativeorcanspeakintermsofdefiniterelationsofmagnitudes。How，then，areitslawsnecessary？Itiscontradictorytosaythatthesamethinghasdifferentspace-relationsatthesametime；butthereisnocontradictioninsayingthatitisherenowandsomewhereelseto-morrow。Theformulaofthe’uniformityofnature，’whatevermaybeitswarrant，transfersthenecessityofthegeometricaltheoremtothelawsofphenomena。Weassumethatthingsarecontinuousorretainidentityinchange。Wearenomorepermittedtosaythatthecombinationofthesameelementsmayproduceacompoundofdifferentproperties，thantosaythattheproductoftwonumbersmaysometimesgiveoneresultandsometimesanother。
Everychangeisregardedasregular，orashavinga’sufficientreason。’Thesameseriesofchangesthereforemusttakeplaceunderthesameconditions，oreverydifferenceimpliesadifferenceintheconditions。Sofaraswecarryoutthisassumption，weresolvetheshiftingandapparentlyirregularpanoramaintoasystemofuniformlaws。Eachlawmaybe，andifitbereallyalawmustbe，absolutelytrue，notinthesensethatitstatesafactunconditionally，butthatitisstatedsothattheconditionsunder。whichitisabsolutelytruearefullyspecified。Ifwecouldreachacompletescienceofallphysicalphenomena，weshouldhaveasystemofconnectedlawsasinfallibleandmutuallyconsistentasthoseofgeometryorarithmetic。Butinorderthustoorganiseourknowledge，wehavetoalter——notthefacts——buttheorderofgroupingandconceivingthem。Wehavetoseeidentitieswheretherewereapparentdifferences，anddifferencesinapparentidentities，andtoregardthewholeorderofnaturefromafreshpointofview。
Thefactremainsjustasitwas；butthelaws——thatis，theformulaewhichexpressthem——aregroupeduponanewsystem。Thequestionsremain，Whatistheprecisenatureofthescientificview？andWhatisourguaranteeforapostulatewhichiteverywhereimplies？