[Guille]

Descartes said that "Cogito, ergo sum" because the only thing of which you can be sure, really sure, is that you think. Do you deny this? If you deny this, then you must be thinking to deny, and so, you think.

Philosophers state things for reasons, why do you think that this sentence has become the most famous out of all philosophy? It was developped very intelligently, and in a way that proved itself.