In Spinoza’s substance monism, radically different attributes constitute the essence of one and the same substance qua a strongly unified whole. Showing how this is possible poses a formidable Cartesian challenge to Spinoza’s metaphysics. In this paper I suggest a reconstruction of Spinoza’s notion of substance that meets this challenge and explains a major feature of this notion. I then show how this reconstruction can be used to resolve two fundamental problems of the Cartesian framework that pertain to Spinoza’s metaphysics. On this basis, I then explain two further major features of Spinoza’s notion of substance. While my suggested reconstruction has all these advantages and accords well with Spinoza’s conceptual framework, it goes beyond what he explicitly says concerning substance monism. It is in the spirit of his metaphysical framework rather than in its letter. Thus, all I intend to show is that Spinoza has the conceptual resources for coming to terms with some of the deep problems that beset his metaphysics.