Discourse on The Method

With a full title of "Discourse on the Method of Rightly Conducting One's Reason and of Seeking Truth in the Sciences", this philosophical treatise tackles the problem of skepticism.

His initial position is to first doubt everything, which allows him to assess the world from a fresh perspective, clear of any preconceived notions.

From this he concludes that the one thing he cannot doubt, due to the fact that he is perceiving his own thoughts, is that he himself exists. "I think therefore I am" is his first point of reference from which he then proceeds.