Why Can’t Descartes Doubt his Own Existence?

In his First Meditation, Descartes wants to more or less abandon any belief that he can doubt at all. This is a way of finding a belief that is completely certain, a belief that he can be absolutely sure is true. If he cannot find any reason to doubt a belief, even the slightest bit, then it's certain in this sense. And the reason he wants to find such a belief (or such beliefs) is that he thinks that he can then base the rest of his beliefs on them, and he will be able to be sure that his beliefs rest on a secure foundation.

The problem is that he seems to have found skeptical arguments that show that there is no absolutely certain belief, that every belief he has is open to some (perhaps very slight) doubt. These are the skeptical arguments he gives in the First Meditation. It might seem, then, that he's doomed to be without an absolutely certain foundation for the rest of his beliefs. And this, clearly, would be a grave problem for his aims in the Meditations.

But this is where the fact that he cannot doubt his own existence comes in. It turns out that he really cannot doubt the belief that he himself exists. For the mere fact of his trying to doubt that he exists, or of his thinking anything at all, clearly shows him that he does exist: “I think therefore I am.” So there is no doubt, not even a very slight one, that he exists. Consequently, there is at least one belief that is absolutely certain: that he himself exists. So it turns out that there is a belief of the sort he needs to ground the rest of his beliefs on a secure foundation.