It is true that "Cogito, ergo sum" is woefully misunderstood because it is so often taken out of context. I would like to say here that I do not have any educational background in philosophy, but I did undertake a casual read of Descartes' Meditations last weekend, wherein this idea is developed. If I make any kind of mistake, I trust the E2 philosophers will let me know.

The argument goes something like this. We all have sensory experiences, and it is through our senses that we interpret things in the world. But these senses can be easily deceived--haven't you ever had a dream that seemed more real than reality? Knowing this, it's easy to argue that our entire lives may be shams, à la everything from Plato's cave to The Matrix. How do we deal with this?

Descartes suggests the possibility of a benevolent, omnipotent God, in the best Christian tradition. Would this God really allow us to live out our lives deceiving ourselves, believing that our senses are telling us the truth when we are, in actual fact, brains in a vat? This makes no sense to ol' Rene, who would presumably argue that a good God means that we exist and we're seeing true things.

Then he suggests that there may be, instead, a malevolent god (or no god at all, though I don't think he mentions this) keeping the wool over our eyes. If this is the case, how then do we know that we exist at all? Because we can think. Even if we are brains in a vat, we have the faculty of thought, which implies that we are, at the very least, brains in a vat. See how neatly that works?