Critique of Practical Reason

Kant's second Critique, which follows the Groundwork of the Metaphysics of Morals and asks how pure reason can by itself determine the will. The Analytic argues that material principles, which aim at pleasure or happiness, can never ground a moral law; only the form of universal law can. The moral law is presented as a 'fact of reason' of which we are directly conscious, and through it we learn that we are free: freedom is the condition of the moral law, and the moral law is how we come to know our freedom. Respect for the law is the one moral feeling. The Dialectic takes up the highest good, virtue joined with happiness in proportion to it. Since we cannot bring this about ourselves, reason must postulate the immortality of the soul and the existence of God, as beliefs required by practical rather than theoretical reason. The book ends with Kant's famous wonder at the starry heavens above and the moral law within.

It shows how Kant ties morality to freedom and why he thought belief in God and immortality is rational, completing the ethics sketched in the Groundwork.