Descartes' Discourse marks a watershed in European thought; in it, the author sets out in brief his radical new philosophy, which begins with a proof of the existence of the self (the famous "cogito ergo sum"). Next he deduces from it the existence and nature of God, and ends by offering a radical new account of the physical world and of human and animal nature. Written in everyday language and meant to be read by common people of the day, it swept away all previous philosophical traditions.