It is said to be the chief merit of Kant's system that he first clearly stated the problem: "How is knowledge possible?" In trying to solve this problem introspectively, by a subtle analysis of his own mental activity, he reached the conviction that the most important and soundest of all knowledge--namely, mathematical--consists of synthetic _a priori_ judgments, and that pure science is only possible on condition that there are strict _a priori_ ideas, independent of all experience, without _a posteriori_ judgments. Kant regarded this highest faculty of the human mind as innate, and made no inquiry into its development, its physiological mechanism, and its anatomic organ, the brain. Seeing the very imperfect knowledge which human anatomy had of the complicated structure of the brain at the beginning of the nineteenth century, it was impossible to have at that time a correct idea of its physiological function. What seems to us to-day to be an innate capacity, or an _a priori_ quality, of our phronema, is really a phylogenetic result of a long series of brain-adaptations, formed by _a posteriori_ sense-perceptions and experiences. Kant's much-lauded critical theory of knowledge is therefore just as dogmatic as his idea of "the thing in itself," the unintelligible entity that lurks behind the phenomena. This dogma is erroneously built on the correct idea that our knowledge, obtained through the senses, is imperfect; it extends only so far as the specific energy of the senses and the structure of the phronema admit. But it by no means follows that it is a mere illusion, and least of all that the external world exists only in our ideas. All sound men believe, when they use their senses of touch and space, that the stone they feel fills a certain part of space, and this space does really exist. When all men who can see agree that the sun rises and sets every day, this proves a relative motion of the two heavenly bodies, and so the real existence of time. Space and time are not merely necessary forms of intuition for human knowledge, but real features of things, existing quite independently of perception.