 Descartes recognizes two forms of conjunction found among the simple natures: necessary and contingent conjunction. (AT X 421f; CSM I 45f) Two simple natures are said to be necessarily conjoined whenever one presupposes (entails) the other. (Ibid.) So, for instance, the simple nature shape is necessarily conjoined with the simple nature extension insofar as the former presupposes (or entails) the latter. An idea is said to be clear whenever the necessary conjunction between simple natures in the idea is exhibited or made explicit. Descartes’ procedure for making an idea “clearer” is to compare the simple natures in the idea. He writes that the procedure: 