Kant drew a distinction between the "Theoretical Reason," which his Critique of Pure Reason had dealt with, and the "Practical Reason," which he discusses in his Critique of Practical Reason (1788).
The "Practical Reason."—By the "practical reason" Kant meant the moral consciousness, and the law of the "practical reason" is the moral law, the fulfilment of which constitutes duty. This law springs neither from outside authority nor from experience; it is autonomous. And it is upon the existence of this autonomous moral consciousness that Kant plants his foothold in his endeavour to find a refuge from the philosophic agnosticism to which his analysis of the "theoretical reason" had led; and upon this rock he founded his belief in "God, Freedom, and Immortality."
By means of his "practical reason," man gets into touch with that real world, which his "theoretical reason" is unable to reach. In fact, the "practical reason" itself (or moral consciousness) is an element in man's nature which belongs to the real, as opposed to the phenomenal world. For man himself is a citizen of both worlds, and has (so to speak) a dual nature, a foot on either shore. He is an inhabitant both of the world of mechanical phenomena, and of the "timeless world of freedom," which lies altogether outside of all mechanical conceptions.
Kant and Religion.—"Religion we must seek in ourselves, not outside ourselves," is a saying of Kant's that gives the clue to his general attitude.
It is only in that world which cannot be interpreted mechanically (i.e. the inner world of freedom of which we never cease to be conscious) that we may seek, or can hope to find the source of religion. It is not the spectacle of the mechanically determined world of nature, but the demands of the moral consciousness that create religion.
For instance, it is the gulf that yawns between the ideal commands of the moral law, and the actual possibilities (so poor and meagre) of fulfilling and satisfying them, that creates, in the view of Kant, the need of God and immortality. These alone can guarantee the realisation of the ideal claims of the moral consciousness.
Religious Faith.—Thus the "practical reason" leads on to convictions concerning what lies beyond the limits defined by the "theoretical reason." The nature of the demands of the moral consciousness give us an insight into the nature of the super-phenomenal (transcendental, noumenal) world. That world must be of such a kind as to sanction and guarantee our moral ideals; it must be friendly and not hostile or indifferent to those ideals which man cherishes, but which his "phenomenal" experience seems to contradict. Thus we see the truth of the saying that "The universe as a moral system is the last word of the Kantian philosophy."[24]
Kant's Influence.—Kant was one of those thinkers who are responsible for turning the stream of thought into fresh channels. Through his researches into the nature of human knowledge, he discovered the conditions upon which it rests, and defined the limits beyond which it cannot pass. Thus, once for all, he put an end to dogmatism.
And to Kant also belongs the credit of having established the reality and validity of inner experience. The rock upon which his philosophy is built is no external fact or event—nothing in time or space—but the moral consciousness itself. And thus he restores, as the central interest of philosophy, the human individual, with all his experiences of need, of hope, and of insight. Personality is the principle of his philosophy. In this he is the true successor of the Reformation.