The Metaphysics of Morals

Context and Aims

Immanuel Kant’s The Metaphysics of Morals (Die Metaphysik der Sitten, 1797) is his most systematic treatment of concrete duties in both law and ethics. Whereas the earlier Groundwork of the Metaphysics of Morals (1785) and Critique of Practical Reason (1788) develop the foundations of moral philosophy—most notably the categorical imperative—The Metaphysics of Morals attempts to present a comprehensive doctrine of duties grounded in those foundations.

Kant’s aim is to provide a “metaphysics” of morals, meaning a purely rational, a priori account of moral principles that does not rest on empirical psychology or contingent social facts. Yet the work is more applied than his earlier foundational writings: it specifies particular rights, laws, and virtues while still claiming that these are derivable from pure practical reason.

The book is divided into two main parts:

1. The Doctrine of Right (Rechtslehre), which deals with external laws, rights, and political institutions.
2. The Doctrine of Virtue (Tugendlehre), which concerns internal ethical duties and moral character.

Doctrine of Right

The Doctrine of Right examines those duties that concern external actions and that may, in principle, be enforced by coercion. Kant defines Right (Recht) as the system of conditions under which the freedom of each can coexist with the freedom of all according to a universal law. This part of the work is foundational for modern Kantian legal and political philosophy.

A core claim is that every person possesses an innate right to freedom: the right to be independent from being constrained by others, insofar as this is consistent with the same freedom of everyone else. From this basic right, Kant aims to derive the legitimacy of various forms of private and public right:

• Private Right: Concerned with relations between individuals and includes:

  • Property (Right to External Objects): Kant argues that rightful possession of external objects is possible only within a system of public laws, even though the concept of property is rationally required to secure external freedom.
  • Contract: Promises that create legally binding obligations; contracts are understood as exercises of external freedom under universalizable rules.
  • Status and Family Relations: Including marriage, parental authority, and domestic service, analyzed as juridical relations of persons under law.

• Public Right: Concerned with the political community as a whole:

  • State (Civic Condition): Kant insists that a condition of merely private right (a “state of nature”) is unstable and inconsistent with full external freedom. A republican civil state, governed by public laws, is required.
  • Constitution and Sovereignty: He defends principles of separation of powers and representative government, though not full democracy in the modern sense.
  • International Right: Addresses relations between states, anticipating ideas of international law and federation of states (developed further in Perpetual Peace).
  • Cosmopolitan Right: Limited rights of individuals as citizens of the world, including rights of hospitality.

The coercibility of juridical duties is crucial: juridical laws legitimately use external coercion to secure conditions of equal freedom, but they do not aim to make people virtuous or morally good in their inner motives. This distinction between Right and ethics is one of the central structural claims of the book.

Doctrine of Virtue

The Doctrine of Virtue (Tugendlehre) concerns ethical duties that regulate inner maxims, intentions, and character. Unlike juridical duties, ethical duties cannot be coercively enforced without contradiction, because genuine virtue requires that the agent act from the moral law, not merely in outward conformity to it.

Kant defines virtue (Tugend) as the strength of will to fulfill one’s duties despite contrary inclinations. Ethical duties are thus linked to the cultivation of a stable moral disposition. A distinctive feature of these duties is their latitude: they specify what must be adopted as an end (for example, helping others), but leave room for judgment about how and when to act.

Kant’s system of ethical duties is divided into:

• Duties to oneself:
  • Duties to oneself as an animal being (e.g., against self-harm, suicide, and neglect of bodily capacities).
  • Duties to oneself as a moral being (e.g., maintaining self-respect, cultivating one’s talents, avoiding servility and moral self-deception).
• Duties to others:
  • Perfect duties (strict, no-latitude duties) such as duties not to lie, not to coerce, and to respect the dignity of others as ends in themselves.
  • Imperfect duties (meritorious duties with latitude), such as beneficence, gratitude, and sympathetic participation in the joys and sorrows of others.

Kant emphasizes that the categorical imperative underlies all these duties. For instance, using humanity—one’s own or another’s—merely as a means violates the requirement to treat humanity always as an end in itself. Similarly, failing to develop one’s talents or to assist others in need violates rationally required ends.

The Doctrine of Virtue also discusses the methods of ethical cultivation, including reflection, moral examples (while warning against hero-worship), and practices that strengthen the will against inclination. However, Kant maintains that such empirical and pedagogical considerations remain subordinate to the a priori structure of duties.

Reception and Influence

The Metaphysics of Morals was initially less influential than Kant’s Critiques and the Groundwork, in part due to its density and the complexity of its organization. Some early commentators regarded it as an unsystematic or unfinished application of his earlier moral theory. Others questioned whether Kant successfully derived such a detailed catalog of duties from purely a priori principles, suggesting that many points reflect contingent cultural assumptions.

In the 19th and early 20th centuries, the work attracted the attention of legal and political theorists, particularly in German philosophy of law, who drew on Kant’s concept of Right and his justification of the modern state. At the same time, critics from utilitarian and Hegelian backgrounds contended that Kant’s sharp distinction between Right and ethics was artificial or that his account of property and the state was overly abstract.

From the late 20th century onward, The Metaphysics of Morals has become a central text for contemporary Kantian ethics and political philosophy. Scholars such as John Rawls, Thomas Pogge, and Onora O’Neill, among others, have engaged with its ideas about human rights, institutional justice, and duties of beneficence. The Doctrine of Right now figures prominently in debates about liberalism, autonomy, and international law, while the Doctrine of Virtue shapes ongoing discussions about duties to self, the nature of moral character, and the relationship between moral principles and moral psychology.

Overall, The Metaphysics of Morals is widely regarded as Kant’s most comprehensive attempt to systematize the domain of moral duties, providing a bridge between abstract moral principles and the concrete norms of law, politics, and everyday ethical life. Its enduring influence lies in its rigorous distinction between coercible legal rights and non-coercible ethical obligations, and in its effort to ground both in a unified conception of human freedom.