The most interesting example of such an entire change of objective and subjective psychological opinions is found in the case of the most influential leader of German philosophy, Immanuel Kant. The young, severely _critical_ Kant came to the conclusion that the three great buttresses of mysticism--"God, freedom, and immortality"--were untenable in the light of "pure reason"; the older, _dogmatic_ Kant found that these three great hallucinations were postulates of "practical reason," and were, as such, indispensable. The more the distinguished modern school of "Neokantians" urges a "return to Kant" as the only possible salvation from the frightful jumble of modern metaphysics, the more clearly do we perceive the undeniable and fatal contradiction between the fundamental opinions of the young and the older Kant. We shall return to this point later on. Other interesting examples of this change of views are found in two of the most famous living scientists, R. Virchow and E. du Bois-Reymond; the metamorphoses of their fundamental views on psychology cannot be overlooked, as both these Berlin biologists have played a most important part at Germany's greatest university for more than forty years, and have, therefore, directly and indirectly, had a most profound influence on the modern mind. Rudolph Virchow, the eminent founder of cellular pathology, was a _pure monist_ in the best days of his scientific activity, about the middle of the century; he passed at that time as one of the most distinguished representatives of the newly awakened _materialism_, which appeared in 1855, especially through two famous works, almost contemporaneous in appearance--Ludwig Büchner's _Matter and Force_ and Carl Vogt's _Superstition and Science_. Virchow published his general biological views on the vital processes in man--which he takes to be purely mechanical natural phenomena--in a series of distinguished papers in the first volumes of the _Archiv für pathologische Anatomie_, which he founded. The most important of these articles, and the one in which he most clearly expresses his monistic views of that period, is that on "The Tendencies Towards Unity in Scientific Medicine" (1849). It was certainly not without careful thought, and a conviction of its philosophic value, that Virchow put this "medical confession of faith" at the head of his _Collected Essays on Scientific Medicine_ in 1856. He defended in it, clearly and definitely, the fundamental principles of monism, which I am presenting here with a view to the solution of the world-problem; he vindicated the exclusive title of empirical science, of which the only reliable sources are sense and brain activity; he vigorously attacked anthropological dualism, the alleged "revelation," and the transcendental philosophy, with their two methods--"faith and anthropomorphism." Above all, he emphasized the monistic character of anthropology, the inseparable connection of spirit and body, of force and matter.