In the Regulae, in first part of Rule XII, Descartes characterizes “ideas” in terms of “figures” or “shapes” formed in the imagination (AT X 414: CSM I 41), thus reworking in a fairly precise, if critical, fashion the doctrines of Aristotle's De Anima. But in the second part of Rule XII, he abandons this seemingly cautious use of the traditional framework, and introduces an utterly new concept, that of the “simple nature” (natura simplicissima; res simplex). This is not only, or primarily, a terminological innovation; what is involved is an epistemological revolution.
A simple nature has two characteristic features: it is neither simple, nor a nature. It is, first of all, opposed to "nature," since in place of the thing considered in itself, according to its ousia (essence), or physis (nature), it denotes the thing considered in respect of our knowledge: "when we consider things in the order that corresponds to our knowledge of them (in ordine ad cognitionem nostram) our view of them must be different from what it would be if it were speaking of them in accordance with how they exist in reality" (AT X 418: CSM 144).