Descartes inherited from the Aristotelian tradition a model of reality comprising substances and affections or modes that inhere in substances. But he reduced the kinds of substance to just two and made the inherence relation strong enough to block what he considered the ontological excesses of Scholasticism. In a general sense, modes are the various ways of qualifying or conceiving of substances. We can speak or conceive of the duration, number, or shape of a given object; so all of these are its modes. In a more precise sense, only those features of an object that admit of variation are modes. In this sense, shape is a mode while duration and number are classified as “attributes” since a thing can change its shape but cannot cease to endure (or become more or fewer) without ceasing to exist (AT VIIIA 30, CSM I 214). And since God is absolutely immutable, he has no modes, only attributes (AT VIIIA 26, CSM I 211). Order, duration, and number may be termed “generic attributes” since they “extend to all classes of things” (AT VIIIA 23, CSM I 208). But there are also two “principal attributes,”thought and extension, which bifurcate the world of substance into two natural kinds: minds and bodies. The principal attributes have their characteristic modes: minds change as thought takes on various perceptions and volitions; bodies change as extension takes on various shapes and motions. And these modes account for everything we perceive in things: “Each substance has one principal property which constitutes its nature and essence, and to which all its other properties are referred” (AT VIIIA 25, CSM I 210).

Descartes would dismiss the question whether modes properly pertain to a substance or to its principal attribute since there is merely a conceptual, rather than a real or modal, distinction between a substance and its attributes (see distinction [real, modal, and rational]). That is, “we are unable to form a clear and distinct idea of the substance if we exclude from it the attribute in question” (AT VIIIA 30, CSM I 214). In reality, the principal attributes thought and extension just are minds and bodies: “They must be considered as thinking substance itself and extended substance itself, that is as mind and body” (AT VIIIA 30–31, CSM I 215). There is also merely a conceptual distinction among the various attributes of a single substance.