Meditation II

(I), pp. 22-23

    1.  I can doubt that my body exists.

    2.  I cannot doubt that my mind exists.
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 So   3.  My mind is distinct from my body.   [1,2]
 

As I said in class, we can substitute any physical thing for “my body” in this argument; so Descartes thinks he has shown that his mind is distinct from any physical thing; so his mind is simply not physical.  The same goes for each of the following arguments.
 

(II), p. 24

    1.  My body is not strictly perceived by the senses, but is indirectly understood by the intellect.   [Defended by his reasoning about the famous blob of wax.]

    2.  My mind is perceived or encountered directly (by me, or itself).

    3.  If X is directly perceived or encountered by me, while Y is not strictly perceived but only indirectly understood, then “I can achieve an easier and more evident perception of... [X] than of... [Y].”

So 4.  I can achieve an easier and more evident perception of my mind than of my body.   [1,2,3]

    5.  If I can achieve an easier and more evident perception of X than of Y, then X is distinct from Y.
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So 6.  My mind is distinct from my body.   [4,5]
 

Meditation VI

(III), p. 26

    1.  “[E]verything which I clearly and distinctly understand is capable of being created by God so as to correspond exactly with my understanding of it.”  (I.e., minus the theology:  Everything which I can genuinely and coherently conceive  not just shallowly seem to imagine  is a metaphysical possibility.)

So 2.  If I can genuinely and coherently conceive X apart from Y, then X and Y “are capable of being separated, at least by God,” i.e., then it is metaphysically possible for X to exist without Y and vice versa.   [Special case of 1]

    3.  If it is metaphysically possible for X to exist without Y and vice versa, then X and Y are distinct.

    4.  I can genuinely and coherently conceive my mind apart from my body.
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So 5.  My mind is distinct from my body.   [2,3,4]
 

(IV), p. 26

    1.  My mind’s essence is to think.  [I.e., what it is for something to be a mind is for it to think; because I cannot genuinely and coherently conceive my mind apart from its activity of thinking.]

    2.  Being extended in (physical) space is not part of my mind’s essence.  [Because I can genuinely and coherently conceive my mind apart from any spatial property it may have.]

    3.  Being extended in space is part of the essence of my (or any) body.

    4.  Thinking is not part of the essence of my (or any) body.
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So 5.  My mind is distinct from my body.   [1,2,3,4, Leibniz’ Law?]
 

Arguments (III) and (IV) are cheek by jowl in the text; it’s possible that Descartes intended them as combined in such a way as to make the combined argument more powerful than either (III) or (IV) alone.  For example, that the mind’s essence includes thinking but not extension and the body’s essence includes extension but not thinking  might be taken to support premise 4 of (III), that I can genuinely and coherently conceive my mind apart from my body.
 

(V), p. 28

    1.  My body is “by its very nature always divisible”; it has parts.

    2.  My mind does not have parts; it is “quite single and complete.”  [The various “faculties” of the mind are not parts of it, but only something like modes or activities of it.]
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So 3.  My mind is distinct from my body.   [1,2, Leibniz’ Law]
 

Discourse on Method

(VI), p. 34

    1.  Suppose God [or some natural process] were to create a human body out of the usual sort of chemicals, but did not give it a “rational soul.”  (Such a body, Descartes grants, would have genuine biological functions like ours, and could even move around in the automaton-like way that a nonhuman animal does.)

    2.  But the soulless body would not be able to think.  [Because no heap of mere chemicals, no matter how large or complex, could think.  Consider: A group of three or four organic molecules cannot think. If I were to add a few more molecules, that would not help the original group to think; just adding more stupid molecules would never produce a thinker.]

So 3.  My body alone cannot think; it must have a “rational soul” or mind specially added.   [1,2]

    4.  My mind thinks.   [Duhhh.]
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So 5.  My mind is distinct from my body.   [3,4]

(Descartes then anticipates an obvious objection and answers it.)