 According to Descartes’ ontology there are substances, attributes, and modes. These are understood relative to one another, in terms of ontological dependence. Modes depend on attributes, and attributes depend on substances. The dependence relation is transitive; thus, modes depend ultimately on substances. No substances, no modes. In Principles, Part I, Article 53, Descartes says that a mode “presupposes” an attribute (AT VIIIA 25; CSM I 210), and in Article 52 he says that an attribute “presupposes” an existing substance. A mode of some thing was understood by Descartes as a a way of being that thing. So, where X is some substance, a mode M is a way of being X. That said, in Article 52 he also says that a substance minus its attributes cannot be known to the human mind. Attributes are in fact what make existing substances intelligible to the human mind. He reaffirms this in Article 62, where he says that there is only a distinction in reason between an attribute and an existing substance. (AT VIIIA 30; CSM I 214) This strongly suggests that although one may draw a conceptual distinction between an attribute and existing substance, the two are not distinct in reality. They are really one and the same thing. (Nolan 1997, Hoffman 2002) Subsequently, if mode M is a way of being X, where X is the substance, the intelligibility of X requires that we conceive some attribute A. And so, strictly speaking, mode M is understood as a mode of attribute A, where A is the attribute through which the existing substance X is conceived (where in reality A and X are presumably identical).