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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?

Descartes asked the question: how can we know anything? Doesn't all our knowledge depend on other knowledge? How exactly? Is there anything we can be absolutely sure of, without depending on any assumptions?

And his answer: regardless of anything else, what I cannot doubt is that I'm pondering this question! Cogito, ergo sum.

From this starting point, Descartes proceeded to 'prove' the existence of many other things; God among them.

I agree with Little Tony that this whole enterprise is a gross overestimation of the powers of introspection and logical reasoning. Descartes puts human reason at the center of the universe. While he was not the first or last person to do so, he was very influential. Maybe, just maybe, thought isn't the start and end of everything. And if you hate Descartes for making people believe that it is, taking him to court won't help: if any place is built on Cartesian thinking it's the courtroom. Come to think of it, there's no such thing as truth anyway, all we do in courtrooms is make up excuses. Don't bother with all that crap, just take the guy out and shoot him.

OK, calm down, calm down. Logical argument isn't all-powerful but it does have its place. Descartes truly wasn't interested in his ego, in putting his own mind above anyone else's, but in the opposite: he was looking for objective communicable truth, knowledge we can all share about the world. While it was a little one-sided of him to rely on logical argument only, this attitude has led him and others to many powerful insights about the world.

In particular, it leads to mathematics. To take his famous phrase in context, you have to know that Descartes was a mathematician; his interest was in the power of deductive reasoning, as practiced in Euclidean geometry. Descartes was an excellent mathematician: we all learn some of his work in school, and it has many important practical applications, mostly in physics. He may have overplayed his hand with the cogito, ergo sum business, but that doesn't take away any of the merits of his approach in other areas.

Using this expression, people put themselves in the center of the universe. Opening them to the possibility that they are the only things that are real. That, in my opinion, is a very arrogant way of looking the entire universe. It took humanity centuries to figure out that the earth was not the center of the universe, and here comes this weird French guy trying to convince us all that he is!

What Descartes says is that just because we think we see stuff doesn't mean it's real. It's arrogant to say that just because we think something is true, it's unquestionable. That's raising yourself to omniscience. That's what Descartes is saying, and so he establishes doubt about everything he perceives. The one thing that he can prove without his senses is that for there to be doubt there has to be a 'doubter'--himself, in this case. All he knows about himself is that he is something that can doubt. (and think.)

He's not telling us he exists, he's telling himself that he exists, and at the same time that if any of us exist (which he tries to prove) then we can only be certain that we exist. This doesn't make us the centre of the universe; he's establishing himself lower than you establish yourself: he says he could be wrong about everything except his own existence while you say you're right about your existence, my existence, his existence, and the existence of nearly everything you see. (I assume. If not you're making an awful fuss over something you seemingly agree with.)

Thinking does not prove existence. I've seen many people who haven't thought a day in their lives, and yet unfortunately... they still exist.

By the same logic, if today is Wednesday, that doesn't prove that today isn't Friday. I've seen many days that aren't Wednesday and yet, unfortunately... they still aren't Friday. (Monday isn't Friday, therefore Wednesday is Friday.) WRONG!

(Anyway, some people may not think a lot, but they certainly think. Any action whatsoever takes thought.)

In my opinion, establishing the universal doubt may be the greatest thing Descartes ever did in the field of philosophy, and his extremely lame (I can not put into words how awful it is) attempt to eliminate it was the worst. :-)

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