  It should be stressed that the three attached essays are important independent of the Discourse, for they contain much worth studying. In the Optics, for example, Descartes works out his laws of refraction, and within this context, what would later be called Snell’s Law (which Descartes seems to have worked out as early as 1632). Further, although the Geometry would seem to have come out of nowhere, there is evidence in Descartes’s notes to himself, from which Clerselier reconstructed some of Descartes’s correspondence, that he had been working on some version of it as early as 1619. In a letter to Beeckman, dated 26 March 1619, for example, Descartes discusses the subject matter that is found in the Geometry, and in a letter dated 23 April 1619, he explicitly mentions the book’s title. It is in this work that Descartes shows how certain geometrical problems can be solved by way of algebraic equations.