This philosophy of Descarte has been brought up numerous times, in the old PFs. I'm just starting it up again. Descarte gave an illustration that went (somewhat) as follows: And Evil Demon sought to convince a man that everything he (the man) had ever believed, was false. The Demon had such power that it almost succeeded. The only thing that the Demon could not prove to the man was that the man himself did not exist. It could not do this because you cannot convince someone that doesn't exist, of anything. From this came the saying, "I think, therefore, I am". What is your opinion?