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 desirebased 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. (shrink)