  In Part III of his Ethics, “On the Origin and Nature of the Affects,” which is the subject of this article, Spinoza addresses two of the most serious challenges facing his thoroughgoing naturalism. First, he attempts to show that human beings follow the order of nature. Human beings, on Spinoza’s view, have causal natures similar in kind to other ordinary objects, other “finite modes” in the technical language of the Ethics, so they ought to be analyzed and understood in the same way as the rest of nature. Second, Spinoza attempts to show that moral concepts, such as the concepts of good and evil, virtue, and perfection, have a basis in human psychology. Just as human beings are no different from the rest of nature, so moral concepts are no different from other concepts. Spinoza’s detailed account of the human affects—the actions and passions of the human mind—is crucial to both tasks. For his argument to succeed, the theory of the affects must be both a plausible account of human psychology and also a plausible basis for ethics. 