General Philosophy General Philosophy Dr Peter Millican Millican, Hertford College , Hertford College Dr Peter Lecture 4: Lecture 4: Two Cartesian Topics Two Cartesian Topics
Scepticism, and the Mind Scepticism, and the Mind Last Time … … Last Time … we looked at scepticism about INDUCTION. we looked at scepticism about INDUCTION. … This Lecture … … This Lecture … will move on to SCEPTICISM concerning will move on to SCEPTICISM concerning … the external world, most famously exemplified the external world, most famously exemplified in Descartes’ ’ first first Meditation Meditation , and his related , and his related in Descartes claims about the nature of MIND AND BODY. claims about the nature of MIND AND BODY. The Next Lecture … … The Next Lecture … will say more about modern responses to will say more about modern responses to … SCEPTICISM, and focus on KNOWLEDGE. SCEPTICISM, and focus on KNOWLEDGE. 2 2
Two Kinds of Scepticism Two Kinds of Scepticism Vertical Scepticism Vertical Scepticism – Inferring from one kind of thing to Inferring from one kind of thing to a different a different – kind (e.g. inferring from one (e.g. inferring from one’ ’s sensations or s sensations or kind appearances, to the existence of real physical appearances, to the existence of real physical objects that cause them). objects that cause them). Horizontal Scepticism Horizontal Scepticism – Inferring things Inferring things of the same kind of the same kind as one has as one has – experienced (e.g. inferring from one’ ’s s experienced (e.g. inferring from one sensations or appearances, to expect similar sensations or appearances, to expect similar sensations or appearances in the future). sensations or appearances in the future). 3 3
External World Scepticism External World Scepticism It can seem that (“ “vertical vertical” ”) external world ) external world It can seem that ( scepticism is far more worrying than scepticism is far more worrying than (“ “horizontal horizontal” ”) inductive scepticism: ) inductive scepticism: ( – Maybe I am just dreaming, and there is no Maybe I am just dreaming, and there is no – external world at all. external world at all. – Maybe an evil demon is causing me to have – Maybe an evil demon is causing me to have illusions of an external world. illusions of an external world. – Maybe a wicked scientist has my brain in a Maybe a wicked scientist has my brain in a – vat, and is creating these illusions. vat, and is creating these illusions. 4 4
Descartes’ ’ Approach Approach Descartes The only way to defeat scepticism is to The only way to defeat scepticism is to withhold assent from anything that isn’ ’t t withhold assent from anything that isn completely certain. completely certain. When I consider “ “I think, therefore I am I think, therefore I am” ”, it , it When I consider is quite impossible for me to be mistaken. is quite impossible for me to be mistaken. So I am completely certain of this, at least. So I am completely certain of this, at least. By contemplating this first certainty, I By contemplating this first certainty, I understand what makes it certain is that I understand what makes it certain is that I clearly and distinctly perceive it to be true. to be true. clearly and distinctly perceive it 5 5
Descartes and God Descartes and God Hence I can establish as a general rule Hence I can establish as a general rule that anything I clearly and distinctly anything I clearly and distinctly that perceive is true . . perceive is true I clearly and distinctly perceive that God I clearly and distinctly perceive that God must exist, because only a perfect being must exist, because only a perfect being could be the ultimate cause of such a could be the ultimate cause of such a perfect idea as my idea of God. perfect idea as my idea of God. A perfect God cannot deceive, so I know A perfect God cannot deceive, so I know that my faculties are essentially reliable. that my faculties are essentially reliable. 6 6
The Cartesian Circle The Cartesian Circle Descartes seems to be “ “boot-strapping boot-strapping” ”: : Descartes seems to be – proving the existence of God by relying on his proving the existence of God by relying on his – mental faculties. mental faculties. – then appealing to the existence of God to then appealing to the existence of God to – justify reliance on his mental faculties. justify reliance on his mental faculties. Isn’ ’t this viciously circular? t this viciously circular? Isn – If my faculties might be defective, then how If my faculties might be defective, then how – can I trust my proof of the existence of God in can I trust my proof of the existence of God in the first place? How can any any anti-sceptical anti-sceptical the first place? How can argument even get off the ground? argument even get off the ground? 7 7
Moore’ ’s Response s Response Moore G.E. Moore famously claimed to refute this G.E. Moore famously claimed to refute this sort of scepticism by appeal to common- sort of scepticism by appeal to common- sense knowledge: sense knowledge: – Here Here’ ’s one hand [he holds up a hand], and s one hand [he holds up a hand], and – here’ ’s another [he holds up the other]. s another [he holds up the other]. here – If this is a hand, then there is an external – If this is a hand, then there is an external world. world. – Therefore there is an external world, and Therefore there is an external world, and – scepticism is refuted. scepticism is refuted. 8 8
Two Arguments from “ “P implies Q P implies Q” ” Two Arguments from Modus Ponens Modus Ponens P implies Q P → Q P implies Q P → Q P is true P P is true P therefore Q is true Q is true Q therefore Q ∴ ∴ Modus Tollens Modus Tollens P implies Q P → Q P implies Q P → Q Q is false Q Q is false ¬ Q ¬ therefore P is false P is false P therefore ¬ P ∴ ∴ ¬ 9 9
One person’ ’s s modus ponens modus ponens … … One person Deuteronomy 20:16-17 commands multiple commands multiple Deuteronomy 20:16-17 genocide to avoid religious pollution. genocide to avoid religious pollution. The religious fundamentalist might say: The religious fundamentalist might say: Everything in the Bible is true. . Everything in the Bible is true Therefore genocide is sometimes desirable. Therefore genocide is sometimes desirable. The humane philosopher would say: The humane philosopher would say: Genocide is never desirable. . Genocide is never desirable Therefore not everything in the Bible is true. Therefore not everything in the Bible is true. Which underlined premise is more plausible? Which underlined premise is more plausible? 10 10
… is another is another’ ’s s modus tollens modus tollens … … … – If this is a hand, then there is an external world. If this is a hand, then there is an external world. – Moore says: Moore says: – We know this is a hand We know this is a hand. . – – Therefore we know there is an external world. Therefore we know there is an external world. – The sceptic says: The sceptic says: – We don We don’ ’t know that there is an external world t know that there is an external world. . – – Therefore we don Therefore we don’ ’t know that this is a hand. t know that this is a hand. – Moore will claim that his premise is more Moore will claim that his premise is more plausible than the sceptic’ ’s. s. plausible than the sceptic 11 11
Internalism and Externalism Internalism and Externalism We’ ’d like to agree with Moore, but it seems d like to agree with Moore, but it seems We hard to justify a claim to knowledge so hard to justify a claim to knowledge so crudely: don’ ’t we need some t we need some philosophical philosophical crudely: don argument rather than a bare common-sense rather than a bare common-sense argument claim to justify knowing that this is a hand? claim to justify knowing that this is a hand? But “ “internalist internalist” ” arguments, like Cartesian arguments, like Cartesian But boot-strapping, have difficulty doing the job. boot-strapping, have difficulty doing the job. So many recent philosophers have moved So many recent philosophers have moved towards externalism externalism (next lecture, and (next lecture, and towards compare Mellor’ ’s approach to induction). s approach to induction). compare Mellor 12 12
Cartesian Dualism Cartesian Dualism The view for which Descartes The view for which Descartes is now best known: is now best known: – The body is The body is material material , composed of matter , composed of matter – whose essence (i.e. fundamental property from whose essence (i.e. fundamental property from which other properties follow) is extension extension . . which other properties follow) is – The mind is composed of The mind is composed of immaterial substance immaterial substance – whose essence is thinking thinking . . whose essence is This substance dualism substance dualism is to be contrasted is to be contrasted This with property dualism property dualism (i.e. there are both (i.e. there are both with physical and non-physical properties properties ). ). physical and non-physical 13 13
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