Descartes’ rationalism

1. Descartes’ rationalism Michael Lacewing enquiries@alevelphilosophy.co.uk

2. Descartes’ cogito • Descartes asks what he can know if he were being deceived by an evil demon. • I cannot doubt that I exist. • What am I? I am a thing that thinks. I cannot doubt this. • ‘I think’ is the first certainty. • I can doubt whether I have a body • So my existence doesn’t depend on whether or not I have a body.

3. Thinking • ‘Think’: doubt, understand, affirm, deny, want, refuse, and also imagine and sense • Doesn’t sense experience depend on having a body? • Not taken ‘narrowly’ – e.g. I have sense experiences in my dreams • The further question is what causes these experiences.

4. Do ‘I’ exist? • ‘I think’ - is there an ‘I’? What does this mean? • If I exist - as a substance - from one thought to the next, Descartes has not shown this; only that ‘there are thoughts’. • If I exist as that which thinks this thought, Descartes has not shown I exist for more than one thought.

5. Clear and distinct ideas • On the cogito: ‘In this first item of knowledge there is simply a clear and distinct perception of what I am asserting’ • While thinking it, I cannot doubt it. • If clarity and distinctness do not guarantee truth, then I cannot know that I exist. • I do know that I exist. • Therefore, ‘as a general rule… whatever I perceive very clearly and distinctly is true.’ • Descartes’ theory of ‘rational intuition’

6. Clear and distinct • (from Principles of Philosophy) • An idea is clear ‘when it is present and accessible to the attentive mind – just as we say that we see something clearly when it is present to the eye’s gaze and stimulates it with a sufficient degree of strength and accessibility.’ • An idea is distinct if it is clear and ‘it is so sharply separated from all other ideas that every part of it is clear’. • Analogy with vision: truths revealed by ‘the natural light’.

7. On God • Clear and distinct ideas are only certainly true when we are considering them • How do we know that we aren’t deceived about them when we are not focusing on them? • Descartes argues a priori that God exists • Trademark argument • Ontological argument • He also needs to argue that God wouldn’t deceive us.

8. God is not a deceiver • God exists. • By definition, God is supremely perfect. • ‘The natural light makes it clear that all fraud and deception depend on some defect’ (p. 17). • (By definition, something that is supremely perfect can have no defects.) • Therefore, it is not possible for God to deceive us. • (This only means that we have the ability to correct our mistakes, not that we can’t make mistakes.)

9. God v. the evil demon • God is supremely powerful. • If God is supremely powerful, then an evil demon could only deceive us if God allowed it. • If an evil demon is deceiving me, then I have no way of correcting my false opinions. • If I have no way of correcting my false opinions, then God is a deceiver. • Therefore, if God permits an evil demon to deceive me, then God is a deceiver. • God is not a deceiver. • Therefore, God will not permit an evil demon to deceive me.

10. Physical objects • Perception doesn’t show that physical objects exist • I cannot know, from perception, that I am not being deceived by an evil demon. • First show that it is possible that physical objects exist.

11. The possibility of physical objects • I have a clear and distinct idea of what a physical object is. • (God exists and is supremely powerful.) • The only reason for thinking that God cannot make something is that the concept of it is contradictory. • Therefore, God can make physical objects. • Therefore, it is possible that physical objects exist.

12. The existence of physical objects • I have involuntary perceptual experiences of physical objects. • (These experiences are caused by some substance.) • If the cause of my perceptual experiences is my own mind, my perceptual experiences are voluntary. • Because I know my mind, I would know if my perceptual experiences are voluntary. • Therefore, because I know that my perceptual experiences are involuntary, I know that the cause of my perceptual experiences is not my own mind.

13. The existence of physical objects • Therefore, the cause must be some substance outside me – either God or physical objects. • If the cause is God, then God has created me with a very strong tendency to have a false belief (that physical objects exist) that I cannot correct. • If God has created me with such a tendency, then God is a deceiver. • (God is perfect by definition.) • (Therefore,) God is not a deceiver.

14. The existence of physical objects • (Therefore, God did not create me with a tendency to have false beliefs that I cannot correct.) • (Therefore, if God exists, I do not have such a tendency.) • Therefore, if God exists, the cause of my perceptual experiences of physical objects is the existence of physical objects. • (God exists.) • Therefore, there is an external world of physical objects that causes our perceptual experiences.

15. Objections • On the self: this doesn’t count as synthetic a priori knowledge, because it derives from the experience of my existence • It has nothing to do with ‘clear and distinct’ ideas. • Objections to arguments for the existence of God • If Descartes can’t prove that God exists, then his proof that physical objects exist fails.

16. The Cartesian circle • I am certain that God exists only because I am certain of whatever I clearly and distinctly perceive; and yet • I am certain of whatever I clearly and distinctly perceive only because I am certain that God exists. • Reply: I can be certain of what I clearly and distinctly perceive without knowing that God exists, but only at the time that I focus on that specific thought • Once he has shown that God exists, Descartes can know the general principle that whatever is clear and distinct is true.

17. Clear and distinct ideas • What guarantees that clear and distinct ideas are true, even when the evil demon may exist? • To deny a clear and distinct idea is a ‘plain contradiction’ • The demon cannot bring about contradictions. • In modern terms: clear and distinct ideas are necessarily true (at the time one thinks them) • Empiricists will then argue that necessary truths are analytic or made true by thinking them (e.g. ‘I think’).