This essay explores the metaphysical foundations of Spinoza's psychology. A particular focus is Spinoza's conatus principle according to which each thing strives not only to persevere in existence but also to increase its power of acting. This striving is, for Spinoza, the actual essence of each thing, and it forms the basis of the three fundamental affects desire, joy, and sadness--which are central to Spinoza's accounts of weakness of the will, self-deception, the imitation of the affects, egoism, altruism, and teleology. Throughout, the essay emphasizes the ways in which Spinoza's psychology manifests his naturalism, his view that everything –including human beings and their various affects or emotions – is governed by the same laws that are found throughout nature.