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 and that "I" know what "thinking" is. He suggested a more appropriate phrase would be "it thinks" wherein the "it" could be an impersonal subject as in the sentence "It is raining." The Danish philosopher Søren Kierkegaard calls the phrase a tautology in his "Concluding Unscientific Postscript". He argues that the "cogito" already presupposes the existence of "I", and therefore concluding with existence is logically trivial. Kierkegaard's argument can be made clearer if one extracts the premise "I think" into the premises "'x' thinks" and "I am that 'x'", where "x" is used as a placeholder in order to disambiguate