In his early Rules for the Direction of the Mind, Descartes recognizes only two sources of knowledge: intuition and deduction (AT X 368, CSM I 14; cf. AT X 372, 400, 425; CSM I 16, 33, 48). Intuitus is not “the fluctuating testimony of the senses” (empirical intuition or sense perception) but “the conception of a clear and attentive mind, which is so easy and distinct that there can be no room for doubt…. Alternatively, intuition is the indubitable conception of a clear and attentive mind which proceeds solely from the light of reason (rationis lux)” (AT X 368, CSM I 14) (for “intuition,” see clarity and distinctness). Since the examples Descartes gives here include not just the contingent truths cogito and sum (“everyone can mentally intuit that he exists, that he is thinking”), but also necessary truths regarding the “simple natures” of both “spiritual” or “intellectual” and “corporeal” or “material” things (AT X 399, CSM I 32; cf. AT X 419, CSM I 44), intuition cannot be confined to immediate logical insights, be they intrapropositional, like “a triangle is bounded by just three lines” (AT X 368, CSM I 14), or interpropositional, like “all A is B, all B is C, therefore all A is C” (AT X 439, CSM I 57). The former (the immediate apprehension of a logical relation between concepts, or between the properties they denote) are, however, part of what Descartes means by intuition. As for logical relations of an interpropositional nature, they may be intuitions or deductions, depending on how long a chain of inferences is involved.

Deductio is defined as “the inference of something as following necessarily from some other propositions which are known with certainty” (AT X 369, CSM I 15). Although it differs from “pure intuition” in being mediate, the boundary between the two is quite fluid: