The purpose of this Descartes Dictionary is to bring together as many as possible of the technical and special terms in Descartes’ writings with their definitions in Descartes’ own words. There are also implicit characterizations of the meanings of many words, and a handful of entries were included simply for their own sake—because Descartes had something interesting to say about his life and world. All of the entries, or almost all of them, have been newly translated for this volume. There are many reasons for preparing new translations: the language in the older translations is archaic and often inaccurate; many of Descartes’ most important works have never been translated into English; and modern translations vary in quality and style. What is most important, though, is that the old translations simply failed to pay attention to the technical language that Descartes used, and so blurred some of the most important distinctions that he made.