French philosophy could be placed and progressively built. Thus he rejected anything which appeared uncertain and decided to only accept apodeictic knowledge as truth. After invoking the possibility of an omnipotent deceiver to reject the external world, the information given to him from his senses, mathematics and logic, Descartes discovered at least one thing could be known apodictically. If he himself was doubting, then he had to exist. Thus Cogito Ergo Sum - I think, therefore I am - became the first principle of Cartesianism. Descartes then claimed that because he discovered the Cogito through perceiving it clearly and distinctly, anything he
Cogito, ergo sum ("If I am mistaken, I am") (book XI, 26), and also anticipates modern refutations of the concept. Furthermore, in the "Enchiridion" Augustine attempts to refute skepticism by stating, "[B]y not positively affirming that they are alive, the skeptics ward off the appearance of error in themselves, yet they do make errors simply by showing themselves alive; one cannot err who is not alive. That we live is therefore not only true, but it is altogether certain as well" (Chapter 7 section 20). In 1640 correspondence, Descartes thanked two colleagues for drawing his attention to Augustine and notes similarity and difference.