Descartes resolved to doubt everything, to assume that everything one believes is false until one can prove it to be true. But, Descartes reasoned, suppose I try to doubt the existence of my own mind? What I find is that I doubt that I am now doubting. But if I doubt that I am doubting, then, as a matter of necessity, I must indeed be doubting. So, I cannot doubt that I'm doubting. And if that is true, then I am thinking, and if I think, I exist (Cogito ergo sum).