Metaphysics of MoralsImmanuel Kant

The Metaphysics of Morals is Kant's attempt to lay out the complete system of duties that follow from the moral law he established in the Groundwork of the Metaphysics of Morals and the Critique of Practical Reason. It divides into two parts: the Doctrine of Right, which concerns external legislation and the conditions under which one person's freedom can coexist with another's, and the Doctrine of Virtue, which concerns the internal ends that every rational being is obligated to set.

The Doctrine of Right covers property, contract, marriage, inheritance, and the state. Kant argues that a rightful civil condition is not optional but morally required: without a public authority to enforce external law, private rights remain provisional. The state exists to make freedom actual, not to promote happiness. Punishment is retributive, proportioned to the crime, never used merely as a deterrent. The Doctrine of Virtue takes up duties to oneself and others, grounding them in the obligation to treat humanity as an end. Duties of virtue, unlike duties of right, cannot be externally enforced; they require the cultivation of moral dispositions, including beneficence, gratitude, and respect.

The work completes Kant's practical philosophy. Where the Groundwork establishes the supreme principle of morality and the second Critique defends the freedom that makes morality possible, the Metaphysics of Morals descends into the specific obligations that bind rational agents in a shared world.