This chapter gives a careful exposition of G. E. Moore’s ethical views, setting Principia Ethica in context within the late nineteenth-century drive toward a “science of ethics,” and comparing it to other contributors in that field. The norm in the “science of ethics” movement was to focus on conduct, but Moore saw this as an instance of “the naturalistic fallacy.” He thus turned away from “conduct” and toward abstract goodness as the primary subject of moral analysis. But Moore’s portrayal of goodness as simple, undefinable, and non-natural made it impossible for him to construct a plausible moral epistemology. This set the stage for the outbreak of moral nihilism that was to come in the form of “emotivism” and other varieties of Noncognitivism. In this way, Moore had a powerful effect on subsequent ethical theorizing and the disappearance of moral knowledge.