Kant opens the preface by insisting that any practical philosophy, including ethics, requires a system of pure rational concepts—a metaphysic—if it is to be presented as a true science rather than a fragmentary aggregate. He immediately distinguishes ethics from jurisprudence: jurisprudence concerns only the formal limitation of external freedom according to laws, without regard to ends, whereas ethics introduces ends that are also duties, demanding internal self-constraint. This foundational move shapes the entire treatise, as Kant repeatedly contrasts the analytical principle of jurisprudence with the synthetic principle of ethics.

The Synthetic Principle of Ethics

In section X, Kant declares that the supreme principle of jurisprudence is analytical, because it follows from the concept of external freedom by contradiction alone. Ethics, by contrast, is synthetic: it connects an end with the notion of freedom, making that end a duty. This enlargement of duty beyond external constraint introduces what Kant calls 'freedom within'—the power of self-constraint by pure practical reason, not by other inclinations. The synthetic character means the principle cannot be proved analytically but requires a deduction (provided in section IX). Kant's diction here is precise: he speaks of 'setting up freedom within' and 'self-constraint' as the distinctive mark of ethical obligation.

Ends That Are Also Duties

Kant defines a duty of virtue as one whose matter—the end—is also a duty. He identifies two such ends: one's own perfection and the happiness of others. These are not optional but commanded by the categorical imperative, which requires that the ends of one's maxims could be a universal law for everyone. The imperative itself is not proved but justified from pure practical reason. Kant's phrasing is careful: 'a man is an end to himself as well as others,' and it is a duty to make mankind in general one's end. This section shows how Kant moves from formal maxims to material ends, a crucial step in his ethical system.

Duties of Virtue vs. Juridical Duties

Kant repeatedly distinguishes duties of virtue from juridical duties. Juridical duties are strict and admit of external constraint; duties of virtue are indeterminate and subject only to free self-constraint. He writes that the doctrine of virtue commands us to regard the rights of men as holy, but this does not make every virtuous act a duty of virtue—only those that involve an end that is also a duty. The distinction hinges on the form of maxims versus their matter. Kant's vocabulary reflects this: he uses 'officia honestatis' for duties of virtue, emphasizing their connection to inner worth rather than external coercion.

The Role of Moral Feeling and Conscience

In the preliminary notions, Kant discusses the susceptibility of the mind for duty, including moral feeling, conscience, and love of men. These are not the basis of morality but natural predispositions that can be cultivated. Conscience, in particular, is treated in a remark as an internal court that judges actions. Kant's tone is systematic: he does not appeal to sentiment but to rational principles. The section on apathy (section XVII) argues that virtue requires command over oneself and presupposes apathy as strength—a striking claim that contrasts with common notions of virtue as emotional engagement.

Kant's ethics is a demanding system that prioritizes rational self-constraint over external conformity. Readers should attend to his careful definitions of key terms—maxim, end, duty of virtue—and the way he builds from formal principles to material obligations. The synthetic principle at the heart of ethics requires a shift in perspective from what is lawful to what is good in itself, a move that remains central to Kantian moral philosophy.