The Metaphysics of Morals

Kant's central claim is that the system of moral duties divides into two distinct domains: juridical duties, which concern external actions and admit of coercive enforcement, and duties of virtue, which concern internal ends and are subject only to self-constraint. He rejects the idea that all moral duties are of one kind. As the text states, 'The doctrine of Right and the doctrine of virtue are therefore distinguished not so much by their different duties as by the difference in their lawgiving' . Legality and morality share a common ground in the autonomy of the will but differ in the incentives they attach to law.

The Groundwork (1785) searches for and establishes the supreme principle of morality—the categorical imperative—in abstract form . The Metaphysics of Morals (1797) is the positive system that the Groundwork was meant to ground. Its focus is not rational beings as such but human beings in particular, with their duties, rights, and morally practical relations . The later work is less abstract and richer in anthropological detail, applying the moral law to concrete conditions of property, contract, punishment, and the duties of virtue.

The work introduces the systematic division between the Doctrine of Right (Rechtslehre) and the Doctrine of Virtue (Tugendlehre). The Doctrine of Right defines rights as conditions under which one's freedom can coexist with the freedom of others under a universal law, and it grounds property, contract, and the state . The Doctrine of Virtue treats duties to adopt ends, such as one's own perfection and the happiness of others. This division structures contemporary Kantian legal and political philosophy, and the Doctrine of Right's theory of public right influenced Hegel's Philosophy of Right and modern theories of international law .

The Doctrine of Right includes a section on the theory of public right that addresses relations between states. Kant sets forth a rational idea of a peaceful community of all nations, developing themes later elaborated in Perpetual Peace . The work grounds international right in the same a priori principles that govern domestic right, arguing that lawful relations among states are a demand of practical reason. For international relations, the text provides the juridical framework underlying Kant's cosmopolitanism and his critique of war as a means of settling disputes.

Kant defines conscience as an 'inner moral court of judgement' in human beings, a metaphor drawn from the external court of law . This inner court involves a judge (conscience), a prosecutor, and a defendant—the self divided into two aspects. Some interpreters argue the metaphor is merely fictional, but scholarly analysis suggests the concept has historical roots in St. Paul and Baumgarten, and that the court metaphor reflects a real structure of moral self-examination . The conscience is the subjective condition of all moral accountability.