Descartes' epistemology in a nutshell
1) We should doubt all that can be doubted, to find only that which is certain (build with firm foundation)
2) Senses can deceive us, but perhaps some sensory knowledge is certain?
3) In dreams there are clearly no senses that are certain; all is possible -- except math.
4) Even in dreams, math is certain -- no five sided squares.
5) There could be a God so powerful that he puts even math illusions into my head. Lets take this "worst case scenario."
6) But even if I am being deceived, at least I know I exist -- cogito, ergo sum.
7) Can we get to truth from that? He waxes philosophic, but gets nowhere.
8) Categorizing thoughts, he finds they vary in formal (real) vs objective (mental) reality.
9) Just as a stone cannot be heated by something will less heat, objective reality of an idea must be less then the formal reality of an idea.
10) Of all the ideas I have, the only one with so much objective reality that it cannot have come from me is an infinitely powerful God.
11) The idea of infinite perfection cannot come from me (less than perfect) or some combination of others (only unity is perfection).
12) God, being perfect and all, is not a deceiver. Thus he has given us free will, and thus the ability to avoid deception: restrain our will such that we only accept clear and distinct ideas.
13) The difference between body and mind is clear and distinct, and we can trust the clear and distinct aspects of both.
14) The body is like a machine, thus its occasional deceptions (thirst when actually full of water) are necessary because
bodies must be made that way.