When did Descartes write "I think, therefore I am"?
Cogito, ergo sum
1637
Descartes first wrote the phrase in French in his 1637 Discourse on the Method. He referred to it in Latin without explicitly stating the familiar form of the phrase in his 1641 Meditations on First Philosophy. The earliest written record of the phrase in Latin is in his 1644 Principles of Philosophy, where, in a margin note (see below), he provides a clear explanation of his intent: "[W]e cannot doubt of our existence while we doubt". Fuller forms of the phrase are attributable to other authors.