Descartes' The World offers the most comprehensive vision of the nature of the world since Aristotle, and is crucial for an understanding of his later writings, in particular the Meditations and Principles of Philosophy. Above all, it provides an insight into how Descartes conceived of natural philosophy before he started to reformulate his doctrines in terms of a sceptically driven epistemology. Of its two parts, the Treatise on Light introduced the first comprehensive, quantitative version of a mechanistic natural philosophy, supplying a theory of matter, a physical optics, and a cosmology. The Treatise on Man provided the first comprehensive mechanist physiology. This volume also includes translations of material important for an understanding of the work: related sections from the Dioptrics and the Meteors, and the first English translation of the complete text of The Description of the Human Body.