Cogito, ergo sum Cogito, ergo sum This proposition became a fundamental element of Western philosophy, as it purported to form a secure foundation for knowledge in the face of radical doubt. While other knowledge could be a figment of imagination, deception, or mistake, Descartes asserted that the very act of doubting one's own existence served—at minimum—as proof of the reality of one's own mind; there must be a thinking entity—in this case the self—for there to be a thought. Descartes first wrote the phrase in French in his 1637 "Discourse on the Method". He referred to it in Latin without explicitly stating the
Cogito, ergo sum one's belief in their own existence would be secure, for there is no way one could be deceived unless one existed in order to be deceived. 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;