Descartes: A VeryShort Introduction explores the life and work of a man who made fundamental contributions to physics, mathematics, and optics, and reported useful observations in meteorology and physiology. Had he confined himself to the natural sciences, his achievement would have been remarkable enough. But his range was considerably wider. His famous statement Cogito, ergo sum is the first principle of his metaphysics, his theory of what has to be known for stable and exact science to be possible. This VSI shows that Descartes was, essentially, an advocate and practitioner of a new mathematical approach to physics, and that he developed his metaphysics to support his scientific programme.