Descartes is commonly called a “rationalist” who prized reason above the senses. Yet though the Meditations repeatedly describe their aim as “leading the mind away from the senses,” it is striking how seldom they refer to reason by name. Descartes does have a distinctive conception of reason, and it plays a fundamental role in the argument of that work. But it operates through some other basic concepts that figure more prominently in this and in others of his writings.

The term ratio, though occurring frequently in the Meditations, generally means an argument or a reason to believe or do something. Only rarely does it designate the faculty of reason. One such passage, reviewing what it is like to suppose that knowledge is based on sensation, says that since “I remembered having used my senses before my reason … I easily persuaded myself that I had no idea in the intellect that I did not have previously in the senses” (AT VII 75, CSM II 52). Here, however, Descartes is referring to the Scholastic doctrine – “nothing in the intellect if not first in the senses” – that the Meditations are geared to overthrow. Precisely because the idea of reason was fraught with unwanted associations, he may have been reluctant to invoke it as he laid out anew “the foundations [initia] of all first philosophy” (AT VII 9, CSM II 8). Such is indeed the attitude evinced in an earlier passage. Having shown that the proposition “I exist” is necessarily true whenever he conceives it and turning to the question of what this “I” is, Descartes sets aside the stock Scholastic response, a “rational animal,” since it would require a detour through the difficult question of “what an animal is, and what rational [rationale] is” (AT VII 25, CSM II 17). When he goes on to conclude that his only certainty so far concerning his existence is that he is a “thinking thing,” he suggests that this truth is the proper basis on which the idea of reason should be defined: “a thing that thinks, that is, a mind, or intelligence, or intellect, or reason – words whose meaning I have been ignorant of until now” (AT VII 27, CSM II 18). Yet nowhere in the Meditations or in the accompanying Replies to Objections does Descartes offer anything like an explicit definition of reason.