Since Kant won so great a prominence in modern philosophy for the idea of pure reason by his famous _Critique_ (1781), it has been much discussed, especially in the modern metaphysical theory of knowledge. It has, however, like all other ideas, undergone considerable changes of meaning in the course of time. Kant himself at first understood by pure reason "reason independent of all experience." But impartial modern psychology based on the physiology of the brain and the phylogeny of its functions, has shown that there is no such thing as this pure _a priori_ knowledge, independent of all experience. Those principles of reason which at present seem to be a priori in this sense have been attained in virtue of thousands of experiences. In so far as this is a question of real knowledge of the truth, Kant himself has frequently recognized the point. He says expressly in his _Prolegomena to any future metaphysic that can be regarded as Science_ (1783, p. 204): "A knowledge of things by pure reason or pure intelligence is nothing but an empty appearance; only in experience is there truth." In subscribing to this empirical theory of knowledge of Kant I. and rejecting the transcendental theory of Kant II., we may on our side understand by pure reason "knowledge without prejudices," free from all dogma--all fictions of faith. The familiar cry of modern metaphysicians, "Return to Kant," has become so general in Germany that not only nearly all metaphysicians--the official representatives of "philosophy" at our universities--but also many distinguished scientists, regard Kant's dualistic theory of knowledge as a necessary condition for the attainment of truth. Kant dominated philosophy in the nineteenth century much as Aristotle did in the Middle Ages. His authority became especially powerful when the prevailing Christian faith believed that his "practical reason" fully supported its own three fundamental dogmas--the personality of God, the immortality of the soul, and the freedom of the will. It overlooked the fact that Kant had utterly failed to find proofs of these dogmas in his _Critique of Pure Reason_. Even conservative governments found favorable features in this dualistic philosophy. We are, therefore, forced to return once more to this mischievous system; though Kant's antinomy of the two reasons has now been refuted so often and so thoroughly that we need not dwell any further on this point.