  Descartes’s effort to resolve a second difficulty is more promising, and also exemplifies the rationalistic character of his thought. The problem is to show how the mind can know something such as a material thing that is different in kind from it, given a longstanding principle that only like can know like. He rejected this essentially Aristotelian principle, but still had to give an account of such knowledge. From scholastic sources, Descartes was able to construct a theory of ideas according to which to know something is to have an idea of it, the idea being the very thing known in so far as it is known. He saw the term “idea” as ambiguous: taken materially, it has formal reality, as a mode of the mind; taken in another sense, it has objective reality, as the thing represented. But there is no real distinction between these realities, only a rational distinction. They are really the same thing considered differently. A welcome epistemological upshot of this rationalist gambit is that Descartes has no skeptical problems generated by ideas standing as a tertium quid between the knower and what is known.