Descartes' "I think, therefore I am" comes from his 1637 work *Discourse on the Method*. The idea is that the act of doubting or thinking is itself proof of one's existence. It's the starting point of his entire philosophical system: the one thing that can't be doubted is that there's a mind doing the doubting.

A commonly missed detail: Descartes wrote it in French, not Latin. The original line is "je pense, donc je suis." The famous Latin version, "Cogito, ergo sum," came from a later translation. The *Discourse* was written in French precisely because Descartes wanted it to be accessible beyond the Latin-reading academic world.