1. Descartes’ rationalism
Michael Lacewing

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’).