The concept of “simple natures” is specific to the Rules for the Direction of the Mind. The expression itself does not appear in any of Descartes’ other known writings. Only in the Conversation with Burman (1648) do we find a reference to God as “the cause not only of what is actual and to come, but also of what is possible and of the simple natures” (AT V 160, CSMK 343). That said, there is an obvious continuity between the doctrine of simple natures in the Rules and the content of some later texts (e.g., Principles I.47–48; letter to Elisabeth, May 21, 1643) devoted to “the simple notions of which all our thoughts are composed.” Therefore, to find the expression again in Descartes’ replies some years later is perhaps puzzling but not so surprising.

In the text of the Rules itself, the concept of simple natures is introduced in two or three distinct stages. The first is to be found in Rule 6, where it is said that all things, insofar as some may be known on the basis of others, can be arranged sequentially: for any question that we have to solve, the first term (“absolute”) of the series will have within it “the pure and simple nature in question” (AT X 381, CSM I 21). The notion appears again in Rule 8 and has its most complete exposition in Rule 12: in both cases, the question has become that of the composition and limits of human knowledge in general. In Rule 12I, Descartes distinguishes between three sorts of simple natures, the composition of which constitutes all the knowledge we can have about other things. There are (a) the purely intellectual natures, which relate only to spirits, such as knowledge, doubt, ignorance, or volition; (b) the purely material natures, which relate only to bodies, such as extension, shape, and motion; (c) the “common” natures, which are ascribed indifferently to two kinds of things, namely, to any existing thing: of this kind are found existence, unity, and duration. One has to add to this last category “those common notions which are, as it were, links which connect other simple natures together.” These notions are axioms of the following sort: “Two things which are identical to a same third thing are the same as each other,” etc.