1 The possibility of bodies

Descartes has argued: I exist as a thinking thing.

He then argues that God exists, and one part of that argument is his view that “clear and distinct ideas” are trustworthy.

From that basic setup you get this certainty about the (note) possible existence of material things right up front: See p. 40.

2 The mind-body distinction

Imagination seems to be a distinct power from the mind, and for a few reasons. First: I have no problem thinking of a triangle and a chiliagon; but, whereas I likewise have no problem imagining a triangle, imagining a chiliagon suddenly introduces problems. I imagine something, but it is not actually a chiliagon. Thus p. 41.

It seems also that imagination is not “required for my own essence,” since it is distinct from thinking, but I am a thinking thing above all. Hence imagination seems to depend on something else, namely a body. See p. 41 green.

This still only gives me grounds for belief in the possibility of the existence of bodies.

3 Actual existence of bodies

p. 43: My clear and distinct idea ideas can be made by God
p. 44 yellow: I have a clear and distinct idea of myself (as a thinking thing) and a distinct idea of a body; hence I must be distinct from my body
p. 44 green: I have faculties of imagining and sensing, which give me the impression of arising in connection with physical things; these must be separate from me
pp. 44–45 pink: the objects of pure mathematics (bodies having extension, shape, motion, etc.) exist

4 The mind-body union

“I am taught by nature” that I have a body; “I should not doubt that there is some truth in this.” See p. 45 yellow.

Note how this “commingling of the mind with the body” stands in tension with Descartes’ “real distinction” between mind and body.

5 Sensory error

How is error possible? Descartes seems to have described a situation in which only the most pristinely true ideas are possible. How does “a certain habit of making reckless judgments” arise? He has two particular errors he’s interested in explaining:

phantom limb
thirst in dropsy (edema) patients

He offers a physiological explanation. First, the mind isn’t influenced by every part of the body but specifically by the brain, and the brain is habit-forming. See p. 48 yellow. The brain moreover can receive the same stimulus from the body along any point of a nerve. See p. 48 green. This explains, to his mind, the phantom limb. See pp. 48–49 blue.

6 Restoration of the ordinary

This explanation depends on a mechanism—something like an inference to the explanation most likely conducive to living well—that seems general. See p. 49 yellow and green, where Descartes claims the phantom limb explanation can be extended to the dropsy (edema) problem.

So really there is nothing to be worried about; see p. 50 blue.