Descartes introduces the notion of material falsity in the Third Meditation. An idea is materially false, he says, whenever it represents a nonthing as a thing. He contrasts material falsity to another kind of falsity, namely, formal falsity, which, he says, “can occur only in judgments” (AT VII 43, CSM II 30). The difference, then, is that whereas material falsity has its origin in ideation (or in the faculty of representation), specifically in sensory ideas, formal falsity, in being related to judgment, has its origin in the will. To help clarify the notion, Descartes offers the example of the sensory idea of cold. Suppose that one is holding an ice cube in one's hand and that the sensory idea of this ice cube presents cold as a real and positive quality. “If it is true that cold is nothing but the absence of heat [i.e., cold is a privation; so it is nothing at all], the idea which represents it to me as something real and positive deserves to be called false” (AT VII 44; CSM II 30). This kind of falsity is present in the idea even before judgment, and because this is so, it could lead one to judge falsely that the ice cube is cold.

Several related interpretations of the notion of material falsity have emerged in the secondary literature. One interpretation emphasizes Descartes’ mention of a thing (res) in his discussion of the notion, which in this context scholars have interpreted as substance (Wilson 1978, Menn 1995). Here, an idea is said to be materially false whenever it represents something that is not a thing or substance as though it were a thing or substance. In the case of the idea of cold, then, the idea is taken to be materially false whenever it represents cold, which is a mode (or quality), as though it were a substance. A second interpretation emphasizes the conceptual or logical relation that holds between modes and their principal attributes (Field 1993). On this view, an idea is said to be materially false whenever it represents cold, for example, which strictly speaking is a mode of mind, as though it were a mode of body or extension (in terms of the example, it represents cold to be a mode of the ice cube). Here, the idea “represents” a conceptual or logical impossibility.