Critique of Practical Reason

The second Critique is the central statement of Kantian moral philosophy. Every later Kantian ethical position takes its bearings from this work.

"Two things fill the mind with ever new and increasing admiration and reverence: the starry heavens above me and the moral law within me." (KpV, Conclusion)

Practical reason is pure reason in its practical use — the moral law is a priori, binding by necessity rather than by inclination. The rationalist tradition's moral philosophy finds its most sophisticated statement here.

"Act so that the maxim of your will could always hold at the same time as a principle of universal legislation." (KpV §7)

The postulates of practical reason (freedom, immortality, God) are conditions of moral life that exceed theoretical demonstration but are rationally required. The German Idealist tradition develops this framework.

"The moral law leads, through the concept of the highest good, to religion." (KpV, Method)

Kant's religion within the limits of reason alone — built on the postulates of practical reason — has been the principal source of liberal Protestant moral theology since Schleiermacher.

"It is necessary to assume the existence of God." (KpV, Dialectic, on the postulate of God)

Modern Kantian constructivism (Rawls, Korsgaard, O'Neill) develops the second Critique's autonomy doctrine into a fully constructivist meta-ethics.

"Autonomy of the will is the sole principle of all moral laws." (KpV §8)

Kant's formula of humanity — that persons must be treated as ends in themselves, never merely as means — is the philosophical core of twentieth-century Christian personalism.

"Always treat humanity, whether in your own person or in the person of any other, never merely as a means but always at the same time as an end." (Groundwork, paraphrased; consistent with KpV)