How many things are there in the world? Spinoza’s answer: one. What might seem to be other things are merely ways in which the one thing exists. In this chapter, I will explain Spinoza’s conception of this one thing-which Spinoza calls a substance-and of the ways in which it exists. I will also unpack his powerful argument for this monism-this oneness-of substance. It cannot be overemphasized how the rest of Spinoza’s philosophy-his philosophy of mind, his epistemology, his psychology, his moral philosophy, his political philosophy, and his philosophy of religion-flows more or less directly from the metaphysical underpinnings in Part I of the Ethics. Spinoza’s understanding of substance is, in many ways, a prin-

cipled transformation and criticism of Descartes’s conception. So it will be easier to understand Spinoza’s conception if we first briefly sketch that of Descartes. The main theme here is this: Descartes’s conception incorporates some guiding rationalist motivations butSpinoza can be seen as implicitly saying-Descartes does not carry out these rationalist motivations consistently or far enough. Once you take the rationalist motivations in Descartes and follow through on them clear-headedly, you will arrive at something like Spinoza’s more controversial account.1