 All operations of the mind involve ideas. The Logic characterizes ideas in the Cartesian way. They are, in the first place, individuated by their objects. Ideas are “as it were the images of things” as Descartes wrote (Meditations, Third meditation [CSM II, 25]). The notion of an image is taken generally; some very important ideas, the idea of God for example, are not acquired by sensory perception. The Logic takes pains to emphasize this aspect of rationalist philosophy Thinking about a thing requires an idea that represents the thing. Because ideas are in this sense objective, i.e., of objects, they can be classified according to the metaphysical classification of their objects. In the Cartesian metaphysics of the Logic, there are three fundamental kinds of objects, substances, unchanging aspects of substances called “attributes”, and features that change over time called “modes”. Although Arnauld elsewhere takes up metaphysical questions about the relation between ideas and their objects, then are not taken up here (see Nadler 1989).