A Latin philosophical proposition by René Descartes usually translated into English as "I think, therefore I am." The phrase originally appeared in his Discourse on the Method. 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.