 Immanuel Kant (1724–1804) argued that the supreme principle of morality is a principle of practical rationality that he dubbed the “Categorical Imperative” (CI). Kant characterized the CI as an objective, rationally necessary and unconditional principle that we must follow despite any natural desires we may have to the contrary. All specific moral requirements, according to Kant, are justified by this principle, which means that all immoral actions are irrational because they violate the CI. Other philosophers, such as Hobbes, Locke and Aquinas, had also argued that moral requirements are based on standards of rationality. However, these standards were either instrumental principles of rationality for satisfying one’s desires, as in Hobbes, or external rational principles that are discoverable by reason, as in Locke and Aquinas. Kant agreed with many of his predecessors that an analysis of practical reason reveals the requirement that rational agents must conform to instrumental principles. Yet he also argued that conformity to the CI (a non-instrumental principle), and hence to moral requirements themselves, can nevertheless be shown to be essential to rational agency. This argument was based on his striking doctrine that a rational will must be regarded as autonomous, or free, in the sense of being the author of the law that binds it. The fundamental principle of morality — the CI — is none other than the law of an autonomous will. Thus, at the heart of Kant’s moral philosophy is a conception of reason whose reach in practical affairs goes well beyond that of a Humean ‘slave’ to the passions. Moreover, it is the presence of this self-governing reason in each person that Kant thought offered decisive grounds for viewing each as possessed of equal worth and deserving of equal respect.