Natural philosophy lay at the core of Descartes' philosophical enterprise, and he instituted the most comprehensive reform of philosophy that has ever been attempted. His achievement was wide-ranging: he completely reformulated metaphysics by exploring its epistemological credentials in a wholly novel and indeed unprecedented fashion; he led the way in seventeenth-century cosmology up until Newton; he was one of the founders of modern geometrical optics; his contribution to mathematics was second to none in the seventeenth century; and he not only discovered reflex action, but developed a mechanistic approach to physiology which set the parameters for much thinking about physiology in the eighteenth and nineteenth centuries. The variety of Descartes' interests is not always immediately evident, however. This is partly because – unlike philosophers of similar standing such as Plato, Aristotle, and Kant – Descartes is usually approached through a single concern, namely the foundationalist metaphysics that is set out in similar ways in La Discours de la Méthode, the Meditationes, and the Principia Philosophiæ. It is also partly because we can discern a plausible systematic connection between many of the parts of the Platonic or Aristotelian or Kantian corpus, which we cannot do in the case of Descartes. These two points are connected: Descartes' foundationalist metaphysics is so notoriously problematic that it is difficult to get beyond it to what it is supposed to provide the foundation for, and, in any case, if the foundations are not viable, there would seem to be little to be gained in asking what plausible systematic connection there could be between them and what is built upon them.