The basic story is well-known. Descartes goes looking for something absolutely certain, beyond even the slightest, most unreasonable doubt, to serve as the permanent foundation for his knowledge. He dismisses the propositions evidenced by his senses. The traditional skeptical worries about hallucinations, madness, dreams and deceiving gods convince him that there is no certainty there. He lands on a bedrock certainty capable of withstanding even his worries about a deceptive god: He exists.
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 then I certainly existed. But there is a deceiver of supreme power and cunning who is deliberately and constantly deceiving 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 this proposition, I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind.(Second Meditation: AT VII 25: CSM II 16-17)
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