Cogito ergo sum (French: Je pense donc je suis; English: I think, therefore I am) is a Latin statement French philosopher René Descartes (1596-1650) used to illustrate a philosophical point in his Discourse on Method (1637). The simple meaning of the phrase is that if a person contemplates (or doubts) whether or not he exists, then that in and of itself is proof that he does exist.
Although the idea expressed in Cogito ergo sum is widely attributed to Descartes, he was not the first to mention it. Plato spoke about the “knowledge of knowledge” (Greek νόησις νοήσεως – nóesis noéseos) and Aristotle explains the idea in full length (Nicomachean Ethics, 1170a25 ff.) Other predecessors include Augustine in De Civitate Dei, and Avicenna‘s Floating Man thought experiment on human self-awareness and self-consciousness.
Descartes however, was original in expanding the idea as a defense against skepticism and using it as a foundation of his whole metaphysics.