First published Mon Feb 23, 2004; substantive revision Sun Apr 6, 2008

Immanuel Kant (1724–1804) argued that moral requirements are based on a standard of rationality he dubbed the “Categorical Imperative” (CI). Immorality thus involves a violation of the CI and is thereby irrational. Other philosophers, such as Locke and Hobbes, had also argued that moral requirements are based on standards of rationality. However, these standards were either desire-based instrumental principles of rationality or based on sui generis rational intuitions. Kant agreed with many of his predecessors that an analysis of practical reason will reveal only the requirement that rational agents must conform to instrumental principles. Yet he 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.

Kant's most influential positions are found in The Groundwork of the Metaphysics of Morals (hereafter, “Groundwork”) but he developed, enriched, and in some cases modified those views in later works such as The Critique of Practical Reason, The Metaphysics of Morals, Anthropology from a Pragmatic Point of View and Religion within the Boundaries of Mere Reason. I will focus on the foundational doctrines of the Groundwork, even though in recent years some scholars have become dissatisfied with this standard approach to Kant's views and have turned their attention to the later works. I myself still find the standard approach most illuminating, though I will highlight important positions from the later works where needed.