 Ideas are among the most important items in Descartes’ philosophy. They serve to unify his ontology and epistemology. As he says in a letter to Guillaume Gibieuf (1583–1650), dated 19 January 1642, “I am certain that I can have no knowledge of what is outside me except by means of the ideas I have within  me.”[1]  Descartes never published anything that specifically worked out a theory of ideas. Even so, he said enough in published and unpublished work, as well as in correspondence, that allows for a basic reconstruction of a theory. This entry will focus principally on the theory of ideas and how it relates to Descartes’ ontology, though in Section 6 of this entry, which includes discussion of simple natures and Descartes’ concepts of clarity and distinctness, certain components of his epistemology are briefly considered. For more on the epistemology, see the related entry  Descartes, René: epistemology.