 Simple natures form an ordered, hierarchical system. Upon analysis they appear to be sorted into two basic groups or classes, which not surprisingly corresponds to Descartes’ mind-body dualism. (AT X 399; CSM I 32) Descartes refers to this partition of simple natures as the enumeration. The basic classes of this enumeration will also be partitioned. In light of this, this ultimate enumeration—the partition of the simple natures into the classes of thinking and extended things—can be referred to as the master enumeration. As laid out in the Rules, the hierarchy is not understood in terms of the ontology, but in terms of what must be known in terms of what. (AT X 381; CSM I 21) These groups or classes are formed in light of epistemic priority. One group includes those simple natures that presuppose the simple nature thought or thinking, while the other group includes those simple natures that presuppose the simple nature extension. The view is that the simple nature shape, for instance, presupposes the simple nature extension in that the former is known (understood) on the basis of the latter. As Descartes puts it later in the Principles, “shape is unintelligible except in an extended thing.” (AT VIIIA 25; CSM I 210) No extension, no shape. The same holds for the other class. The simple nature hot, a sensible quality, presupposes the simple nature thought or thinking in that the former is known (or understood) on the basis of the latter. No thought or thinking, no (feeling of) hotness. 