My project in this paper is to fill a gap in Spinoza's theory of metaphysical individuation. In a few brief passages of the Ethics, Spinoza manages to explain his views on the nature of composition and the part-whole relation, the metaphysical facts which ground the individuation of simple bodies and the extended individuals they compose, and the persistence of one and the same individual through time and mereological change. Yet Spinoza nowhere presents a corresponding account of the individuation of simple ideas, or the minds such ideas compose. While it is initially tempting to locate the details of such an account in Spinoza's views on the relation between the mental and physical domains, I argue here that such approaches fail, in conflicting with Spinoza's insistence that the mental and the physical are conceptually and explanatorily independent. By contrast, I show that for Spinoza, each idea essentially possesses the property of affirming the existence of its object, and that such properties are well-suited to serve as the principle of ideal individuation Spinoza never explicitly provided.
