The practical use of reason is developed briefly near the end of The Critique of Pure Reason, and more fully in The Critique of Practical Reason (1786). The argument is that the moral law demands justice, i.e., happiness proportional to virtue. Only Providence can insure this, and has evidently not insured it in this life. Therefore there is a God and a future life; and there must be freedom, since otherwise there would be no such thing as virtue.
