" But I have convinced myself that there is absolutely nothing in the world, no sky, no earth, no minds, no bodies. Does it now follow that I, too, do not exist ? No. If I convinced myself of something (or thought anything at all), then I certainly existed. But there is a deceiver of supreme power and cunning who deliberately and constantly deceives me. In that case, I, too, undoubtedly exist, if he deceives me ; and let him deceive me as much as he can, he will never bring it about that I am nothing, so long as I think that I am something. So, after considering everything very thoroughly, I must finally conclude that the proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind. "

- Discourse on the Method (1637), René Descartes -