Cogito Ergo Sum XIII

Descartes, Principles of Philosophy I, 7:

So, if we reject everything we can doubt in any way, and even imagine it all to be false, we can readily suppose that there is no God, no sky, and no bodies — and even that we ourselves have no hands, no feet, and indeed no body at all. However, this does not allow us to suppose that we who are thinking such things are nothing, since it is a contradiction to believe that something which thinks does not exist at the very time when it is thinking. So the knowledge that I think therefore I am is the first and most certain of all items of knowledge which anyone will arrive at if they philosophise in the right order.