Summary of The Third Meditation of Descartes
At the beginning of the third mediations,
Descartes has proved to himself that, for the time being the only thing he can prove with certainty, is that he exists, as the rest of his perceptions can be a deception. To prove his own existence, Descartes says that every time he doubts his own existence, he must necessarily exist as he is the doubting one. This leave him then to ponder what sort of thing he is, but he quickly resolves that he is a thinking thing, as if he is doubting, he is thinking, and if he is wondering about his own existence, he is thinking, etc. Everything he does is based on his thoughts, so he can only be certain that he is a thinking thing.
So, at the beginning of the third meditation, Descartes has proven to himself that the only thing he can be certain of is that he exists and that he is a thinking thing, but in proving this, he assumes that he would know truth, as he knows these things for certain. If things at this time seem to be perceived as clearly and distinctly true, why could not it be in his nature to only perceive them that way? Here, Descartes says that his nature is not the issue as if he has a nature, he has a self, and that is still no valid objection. By the
light of nature, not by his nature, he seems to find these answers, and so he goes on to think about the nature of ideas.
If he is only in himself, since that is the only thing he can prove, then everything he has perceived must be in him, though coming from outside as he does not will it, so it must once it reaches him, become an idea. An idea can not be true of false, as only judgments can, for ideas are perfectly unique - nor can a will or desire be false, regardless of its moral implications. Judgments, however, can be false, as applying that idea to a thing can be faulty. These ideas then, Descartes asserts, must come from something, and that something must be outside the individual, since something can not come from nothing. Cause can not exist without effect, and in the realm of ideas, all ideas are effects of a cause. With this spectral understanding, Descartes then proves to himself G-d must exist, he says if he is being deceived, or not understanding truth, then truth must still exist and someone must never have been deceived. If someone does however know all of truth, they must be infinite and have always known truth, and if Descartes can slowly gain an understanding of truth, be is still finite, as he has not always had it. This person - or thing, rather - he says is G-d.