 Leibniz’s Russellian monist theory may be found in his critique of René Descartes’ theory of matter (Pereboom 2011: 92–97; cf. 1991a,b). In Descartes’ theory, material substance is essentially just extension in three spatial dimensions (Descartes Principia Philosophiæ, “Part Two: The Principles of Material Things” 1644 [1984: v. 1, 223–247; AT VIII 40–79]; see the entry  Descartes’ physics).  Leibniz contends that Descartes’ theory is unsatisfactory because, first, the extension of a physical object is (in the terminology introduced in  section 1.1,  which is Kantian (Kant 1781/1787: A277/B333)) a relatively intrinsic property of it, since it resolves into the plurality, spatial continuity, and temporal coexistence of its parts, and the extension of those parts resolves into those extrinsic properties of their parts, ad infinitum; and, second, any real thing, that is, any actual, concrete entity, cannot possess only extrinsic or relatively intrinsic properties: it must possess an absolutely intrinsic property as well. Leibniz writes: “there is no denomination so extrinsic that it does not have an intrinsic denomination at its basis” (Leibniz to De Volder April 1702 [Leibniz G II, 240; 1969: 526–527]). In his view, a conception of the physical world that does not include absolutely intrinsic properties is in an important sense incomplete. In effect, his complaint is that Descartes’ theory of matter describes only structure without describing a quiddistic ground for that structure, or even acknowledging the possibility of or the need for such a ground. 