 The nature of a mind, Descartes says, is to think. If a thing does not think, it is not a mind. In terms of his ontology, the mind is an existing (finite) substance, and thought or thinking is its attribute. Insofar as the nature of a mind is to think, where thought is the mind’s defining feature, Descartes calls it the mind’s principal attribute (AT VIIIA 25; CSM I 210–11). An idea is a mode of thinking. In being a mode of thinking, an idea is understood as a way of being (an instance of) thinking, or an idea is way in which an instance of thinking is manifested. This is similar to what Descartes says about a body, its principal attribute, and its modes. The nature of a body is to be extended (in length, breadth, and depth). A body is a (finite) substance, and extension is its attribute. Since extension is the defining feature of a body, Descartes refers to it as a body’s principal attribute. Shape is a mode of extension. What this means is that shape is a way of being extended, or a way in which an instance of extension is manifested. Thus, shape is to extension as idea is to thought. Thus a shape presupposes extension and an idea presupposes thinking, where each principal attribute presupposes an existing substance.