Critique of Practical ReasonImmanuel Kant

The Critique of Practical Reason completes what the Groundwork of the Metaphysics of Morals began. Where the Groundwork identifies the moral law and its supreme principle, the second Critique asks how pure reason can be practical: how the moral law can determine the will without relying on desire, inclination, or empirical incentive.

Kant's answer turns on the "fact of reason." The moral law is not derived from anything more fundamental. It presents itself to rational agents as an unconditional command, and our consciousness of this command is itself the proof that pure reason can move us to act. No theoretical deduction is needed; the law's authority is immediately given in moral experience.

The Analytic works out the implications. Happiness cannot ground morality, because making the good depend on what satisfies desire would make morality contingent on temperament. Duty, not inclination, is the proper motive. Yet Kant does not despise happiness. The Dialectic introduces the "highest good," in which virtue and happiness are proportioned to each other, and argues that this proportioning requires us to postulate God's existence and the immortality of the soul. These are not theoretical proofs but practical necessities: without them, the moral life would be incoherent.

The second Critique establishes practical reason's primacy over theoretical reason. What we cannot know through speculation, we must believe through moral commitment.