  Like Francisco Suárez, his most immediate scholastic predecessor, Descartes sides with the proponents of a rational distinction between essence and existence. His position is unique, however, insofar as it springs from a more general theory of “attributes”. Articulating this theory in an important passage in the Principles of Philosophy, Descartes claims that there is merely a distinction of reason between a substance and any one of its attributes or between any two attributes of a single substance (1:62, AT 8A:30; CSM 1:214). For Descartes’ purposes, the most significant instance of a rational distinction is that which obtains between a substance and its essence — or what he sometimes refers to as its “principal attribute” (1:53, AT 8A:25; CSM 1:210). Since thought and extension constitute the essence of mind and body, respectively, a mind is merely rationally distinct from its thinking and a body is merely rationally distinct from its extension (1:63, AT 8A:31; CSM 1:215). But Descartes insists that a rational distinction also obtains between any two attributes of a substance. Since existence qualifies as an attribute in this technical sense, the essence and existence of a substance are also distinct merely by reason (1:56, AT 8A:26; CSM 1:211). Descartes reaffirms this conclusion in a letter intended to elucidate his account of the relation between essence and existence: