As he writes in his Discourse on Method, having made some stunning discoveries, he felt that at age 22, it was too premature for him to reveal his findings. Instead he traveled, joined the army for a brief time, and saw some battles. In 1637 Descartes published Optics, Meteorology, and Geometry, a collection of essays. The preface to the collection is titled Discourse on the Method of Rightly Conducting the Reason and Seeking Truth in the Sciences. Descartes' Discourse alters the course of philosophy, by starting first with a proof of the existence of the self - cogito ergo sum- and subsequently deducing from it the existence and nature of God. That is to say, he offers a radical modern account not only of the physical world, but also of the fauna and flora. Written in plain, straight forward French, he made his method accessible not only to the learned, but all who wanted to pursue truth in knowledge.