Kant's chief error, which proved so injurious to the whole of subsequent philosophy, lay in the absence of any physiological and phylogenetic base to his theory of knowledge; this was only provided sixty years after his death by Darwin's reform of the science of evolution, and by the discoveries of cerebral physiologists. He regarded the human mind, with its innate quality of reason, as a completely formed entity from the first, and made no inquiry into its historical development. Hence, he defended its immortality as a practical postulate, incapable of proof; he had no suspicion of the evolution of man's soul from that of the nearest related mammals. The curious predisposition to _a priori_ knowledge is really the effect of the inheritance of certain structures of the brain, which have been formed in man's vertebrate ancestors slowly and gradually, by adaptation to an association of experiences, and therefore of _a posteriori_ knowledge. Even the absolutely certain truths of mathematics and physics, which Kant described as synthetic judgments _a priori_, were originally attained by the phyletic development of the judgment, and may be reduced to constantly repeated experiences and _a priori_ conclusions derived therefrom. The "necessity" which Kant considered to be a special feature of these _a priori_ propositions would be found in all other judgments if we were fully acquainted with the phenomena and their conditions. Among the censures which the academic metaphysicians, especially in Germany, have passed on my _Riddle of the Universe_, the heaviest is perhaps the charge that I know nothing whatever about the theory of knowledge. The charge is correct to this extent, that I do _not_ understand the current dualistic theory of knowledge which is based on Kant's metaphysics; I cannot understand how their introspective psychological methods--disdaining all physiological, histological, or phylogenetic foundations--can satisfy the demands of "pure reason." My monistic theory of knowledge is assuredly very different from this. It is firmly and thoroughly based on the splendid advances of modern physiology, histology, and phytogeny--on the remarkable results of these empirical sciences in the last forty years, which are entirely ignored by the prevailing system of metaphysics. It is on the ground of these experiences that I have adopted the views on the nature of the human mind which are expounded in the second part of _The Riddle of the Universe_ (chapters vi.-xi.). The following are the chief points: 1. The soul of man is--objectively considered--essentially similar to that of all other vertebrates; it is the physiological action or function of the brain. 2. Like the functions of all other organs, those of the brain are effected by the cells, which make up the organ. 3. These brain-cells, which are also known as soul-cells, ganglionic cells, or neurona, are real nucleated cells of a very elaborate structure.