Near the beginning of Part III of the Ethics, Spinoza introduces a notion that guides him throughout the rest of that work and that also structures his thought in his political writings. This is the notion of the universal striving for self-preservation, or as Spinoza puts it in 3p6: “Each thing, insofar as it is in itself, strives to persevere in its being.” In this and the next two chapters, this principle will structure our thought just as it structures Spinoza’s: we will investigate the meaning of and argument for this claim as well as the multifarious uses to which Spinoza puts it. We will find that with 3p6 Spinoza takes a notion that Descartes and Hobbes employ, deepens it, and extends it in ways that his predecessors never dreamed of.