In Spinoza’s Argument for Substance Monism: Why There Is Only One Thing, Christopher Martin provides an interpretation and defense of this argument, using speculative metaphysics as a method to show how the key terms and concepts are employed and fit together. Specifically, Martin argues that (i) Spinoza’s understanding of substance and attribute departs only slightly from dominant historic notions; (ii) his definition of God in terms of attributes instead of perfections is quite helpful and (mostly) compatible with more traditional definitions; and (iii) Spinoza’s pairing of causal and conceptual relations is more intuitive than we think. Martin also shows how these essences function as causes and explains why, with Spinoza’s understanding of emanation and conceptual independence, any substance must have every attribute. These features of Spinoza’s argument explain and defend his ultimate claim that God/Nature is the only substantial being in existence. This book demonstrates how approachable and compelling Spinoza’s argument is and illustrates the practice and potential of speculative metaphysics for specialists and non-specialists alike.