General MT09 4 Scepticism M
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GeneralGeneral PhilosophyPhilosophy DrDr PeterPeter MillicanMillican,, HertfordHertford CollegeCollege LectureLecture 4:4: TwoTwo CartesianCartesian TopicsTopics Scepticism,Scepticism, andand thethe MindMind LastLast TimeTime …… … we looked at scepticism about INDUCTION. ThisThis LectureLecture …… … will move on to SCEPTICISM concerning the external world, most famously exemplified in Descartes’ first Meditation, and his related claims about the nature of MIND AND BODY. TheThe NextNext LectureLecture …… … will say more about modern responses to SCEPTICISM, and focus on KNOWLEDGE. 2 TwoTwo KindsKinds ofof ScepticismScepticism VerticalVertical ScepticismScepticism – Inferring from one kind of thing to a different kind (e.g. inferring from one’s sensations or appearances, to the existence of real physical objects that cause them). HorizontalHorizontal ScepticismScepticism – Inferring things of the same kind as one has experienced (e.g. inferring from one’s sensations or appearances, to expect similar sensations or appearances in the future). 3 ExternalExternal WorldWorld ScepticismScepticism ItIt cancan seemseem thatthat ((““verticalvertical””)) externalexternal worldworld scepticismscepticism isis farfar moremore worryingworrying thanthan ((““horizontalhorizontal””)) inductiveinductive scepticism:scepticism: – Maybe I am just dreaming, and there is no external world at all. – Maybe an evil demon is causing me to have illusions of an external world. – Maybe a wicked scientist has my brain in a vat, and is creating these illusions. 4 DescartesDescartes’’ ApproachApproach TheThe onlyonly wayway toto defeatdefeat scepticismscepticism isis toto withholdwithhold assentassent fromfrom anythinganything thatthat isnisn’’tt completelycompletely certain.certain. WhenWhen II considerconsider ““II think,think, thereforetherefore II amam””,, itit isis quitequite impossibleimpossible forfor meme toto bebe mistaken.mistaken. SoSo II amam completelycompletely certaincertain ofof this,this, atat least.least. ByBy contemplatingcontemplating thisthis firstfirst certainty,certainty, II understandunderstand whatwhat makesmakes itit certaincertain isis thatthat II clearlyclearly andand distinctlydistinctly perceiveperceive itit toto bebe true.true. 5 DescartesDescartes andand GodGod HenceHence II cancan establishestablish asas aa generalgeneral rulerule thatthat anythinganything II clearlyclearly andand distinctlydistinctly perceiveperceive isis truetrue.. II clearlyclearly andand distinctlydistinctly perceiveperceive thatthat GodGod mustmust exist,exist, becausebecause onlyonly aa perfectperfect beingbeing couldcould bebe thethe ultimateultimate causecause ofof suchsuch aa perfectperfect ideaidea asas mymy ideaidea ofof God.God. AA perfectperfect GodGod cannotcannot deceive,deceive, soso II knowknow thatthat mymy facultiesfaculties areare essentiallyessentially reliable.reliable. 6 TheThe CartesianCartesian CircleCircle DescartesDescartes seemsseems toto bebe ““boot-strappingboot-strapping””:: – proving the existence of God by relying on his mental faculties. – then appealing to the existence of God to justify reliance on his mental faculties. IsnIsn’’tt thisthis viciouslyviciously circular?circular? – If my faculties might be defective, then how can I trust my proof of the existence of God in the first place? How can any anti-sceptical argument even get off the ground? 7 MooreMoore’’ss ResponseResponse G.E.G.E. MooreMoore famouslyfamously claimedclaimed toto refuterefute thisthis sortsort ofof scepticismscepticism byby appealappeal toto common-common- sensesense knowledge:knowledge: – Here’s one hand [he holds up a hand], and here’s another [he holds up the other]. – If this is a hand, then there is an external world. – Therefore there is an external world, and scepticism is refuted. 8 TwoTwo ArgumentsArguments fromfrom ““PP impliesimplies QQ”” ModusModus PonensPonens P implies Q P → Q P is true P therefore Q is true ∴ Q ModusModus TollensTollens P implies Q P → Q Q is false ¬Q therefore P is false ∴ ¬P 9 OneOne personperson’’ss modusmodus ponensponens …… Deuteronomy 20:16-17 commands multiple genocide to avoid religious pollution. TheThe religiousreligious fundamentalistfundamentalist mightmight say:say: Everything in the Bible is true. Therefore genocide is sometimes desirable. TheThe humanehumane philosopherphilosopher wouldwould say:say: Genocide is never desirable. Therefore not everything in the Bible is true. WhichWhich underlinedunderlined premisepremise isis moremore plausible?plausible? 10 …… isis anotheranother’’ss modusmodus tollenstollens …… – If this is a hand, then there is an external world. MooreMoore says:says: – We know this is a hand. – Therefore we know there is an external world. TheThe scepticsceptic says:says: – We don’t know that there is an external world. – Therefore we don’t know that this is a hand. MooreMoore willwill claimclaim thatthat hishis premisepremise isis moremore plausibleplausible thanthan thethe scepticsceptic’’s.s. 11 InternalismInternalism andand ExternalismExternalism WeWe’’dd likelike toto agreeagree withwith Moore,Moore, butbut itit seemsseems hardhard toto justifyjustify aa claimclaim toto knowledgeknowledge soso crudely:crudely: dondon’’tt wewe needneed somesome philosophicalphilosophical argumentargument ratherrather thanthan aa barebare common-sensecommon-sense claimclaim toto justifyjustify knowingknowing thatthat thisthis isis aa hand?hand? ButBut ““internalistinternalist”” arguments,arguments, likelike CartesianCartesian boot-strapping,boot-strapping, havehave difficultydifficulty doingdoing thethe job.job. SoSo manymany recentrecent philosophersphilosophers havehave movedmoved towardstowards externalismexternalism (next(next lecture,lecture, andand comparecompare MellorMellor’’ss approachapproach toto induction).induction). 12 CartesianCartesian DualismDualism TheThe viewview forfor whichwhich DescartesDescartes isis nownow bestbest known:known: – The body is material, composed of matter whose essence (i.e. fundamental property from which other properties follow) is extension. – The mind is composed of immaterial substance whose essence is thinking. ThisThis substancesubstance dualismdualism isis toto bebe contrastedcontrasted withwith propertyproperty dualismdualism (i.e.(i.e. therethere areare bothboth physicalphysical andand non-physicalnon-physical propertiesproperties).). 13 AA BadBad ArgumentArgument forfor DualismDualism InIn hishis DiscourseDiscourse,, DescartesDescartes arguesargues likelike this:this: I can doubt that my body exists. I cannot doubt that I exist. ∴ I am not identical with my body. Compare:Compare: I can doubt that Hesperus is Phosphorus. * I cannot doubt that Phosphorus is Phosphorus. ∴ Hesperus is not Phosphorus. * Hesperus = the Evening Star; Phosphorus = the Morning Star; in fact both are appearances of the planet Venus. 14 LeibnizLeibniz’’ss LawLaw IfIf aa andand bb areare thethe samesame thing,thing, thenthen anyany propertyproperty ofof aa mustmust alsoalso bebe aa propertyproperty ofof bb:: Fa, a=b ╞ Fb – If F is the property of being doubted by me to exist, a is me, and b is my body, we get Descartes’ argument from the Discourse. – Likewise F could be the property of being doubted by me to be Prime Minister (etc.) ToTo simplestsimplest wayway toto avoidavoid thethe fallacyfallacy isis toto denydeny thatthat thesethese areare genuinegenuine properties.properties. 15 AA BetterBetter ArgumentArgument forfor DualismDualism DescartesDescartes’’ argumentargument inin MeditationMeditation VIVI isis lessless fallacious,fallacious, butbut hashas questionablequestionable premises:premises: – I have a clear understanding of myself as (potentially) a thinking, non-extended thing. – I have a clear understanding of body as (potentially) extended and non-thinking. – Anything I clearly and distinctly understand could be created by God accordingly. – So I could exist separately from my body, and it follows that I am genuinely distinct from it. 16 FromFrom DoubtDoubt toto EssenceEssence EvenEven inin thethe MeditationsMeditations,, DescartesDescartes triestries toto motivatemotivate hishis claimclaim toto knowknow thethe essenceessence ofof mindmind (as(as thinking)thinking) fromfrom hishis doubtdoubt argument:argument: “what shall I now say that I am [when I might be deceived by an evil demon, or dreaming]? … At present I am not admitting anything except what is necessarily true. I am, then, in a strict sense only a thing that thinks; that is, I am a mind, or intelligence, or intellect, or reason … what kind of thing? … a thinking thing. 17 EpistemologyEpistemology →→ Metaphysics?Metaphysics? TheThe wayway inin whichwhich wewe comecome toto knowknow,, oror bebe certain,certain, ofof somethingsomething needneed notnot reflectreflect itsits ultimateultimate naturenature (or(or whywhy itit isis thatthat wayway).). – From I am thinking, it plausibly follows that (in at least one sense) I am a thing that thinks. – But it does not necessarily follow that I am something whose essence is to think. – Nor does it follow that the thing that thinks could exist without being extended. (Imagine if a piece of matter were made able to think.) 18 PossiblyPossibly DistinctDistinct →→ ActuallyActually Distinct?Distinct? TheThe finalfinal movemove ofof DescartesDescartes’’ argumentargument seemsseems moremore defensible,defensible, inin aa sense:sense: God could have created my mind and body as separate entities. ∴ It is possible for my mind and body to exist separately. ∴ My mind and body