Book: Critique of Practical Reason

Aim and Structure
Published in 1788 as the second of Kant’s three Critiques, Critique of Practical Reason clarifies and defends the authority of morality by examining the capacity of pure practical reason, reason that legislates action independently of empirical desires. It is organized into an Analytic (expounding the principles of pure practical reason), a Dialectic (diagnosing its characteristic illusions), and a Methodology (guidance for moral cultivation and application).

Pure Practical Reason and the Moral Law
Kant argues that practical reason supplies a supreme principle of morality that is a priori and categorical, binding independently of any ends we happen to desire. This principle, the moral law, grounds the categorical imperative, requiring that maxims be fit for universal legislation. Unlike hypothetical imperatives, which depend on contingent ends, the moral law commands with necessity. Kant treats the moral law as a “fact of reason”: our consciousness of its authority is immediately evident in practical deliberation, not inferred from psychology or theology. This consciousness reveals the autonomy of the will: reason gives the law to itself, rather than receiving it from external authorities or internal inclinations.

Freedom and the Two Standpoints
The reality of moral obligation presupposes freedom, understood as the capacity to initiate a causal series independently of natural determination. Within his critical philosophy, humans occupy two standpoints. As appearances in nature, our actions are subject to causal laws; as things in themselves, we can be free. The moral law is the ratio cognoscendi (reason for knowing) of our freedom, since our awareness of duty discloses our independence from inclination. Conversely, freedom is the ratio essendi (reason for being) of the moral law, since only a free will can be bound by a self-given universal law. Thus the practical point of view secures what speculative reason could not prove without contradiction: genuine freedom.

Incentives and Respect
Kant rejects the view that moral motivation must come from empirical desire. The proper incentive is respect for the moral law, a distinctive feeling produced by reason’s constraint on self-love and inclination. Respect is not the foundation of morality but its subjective effect in finite agents, explaining how a pure principle can move a sensible will without reducing morality to emotion or interest.

Application: The Typic of the Moral Law
To apply the abstract law to concrete cases, practical judgment employs a “typic”: the idea of a universal law of nature. A maxim is tested by asking whether one could will it as a law governing all rational agents, analogous to laws of nature. This frames moral assessment while preserving the law’s a priori character.

The Highest Good and the Postulates
Reason demands a final end: the highest good, in which virtue is proportionately accompanied by happiness. Nature alone does not secure this harmony, and virtue’s ideal, holiness, is unattainable in finite life. Pure practical reason therefore postulates, as necessary conditions of the highest good, the immortality of the soul (for unending progress in moral improvement) and the existence of God (as moral author of the world who can harmonize happiness with worthiness). These postulates extend belief practically, without constituting theoretical knowledge, and exhibit the primacy of practical over speculative reason.

Dialectic, Method, and Significance
The Dialectic defuses moral fanaticism and eudaimonist distortions by subordinating happiness to duty while licensing rational hope through the postulates. The Methodology addresses education and character formation, insisting on the purity of the moral motive and the disciplined use of examples. The work secures the autonomy and universality of the moral law, vindicates freedom within the critical system, and grounds a practical faith oriented by the highest good, reshaping modern ethics around dignity, duty, and the self-legislation of reason.