was from the ghastly history of the Middle Ages, with its Inquisition, religious wars, instruments of torture, and drowning of witches. In the face of the current enthusiasm for the romantic side of mediævalism, the Crusades and Church art, we cannot lay too much stress on these dark and bloody pages of its chronicles.
An impartial study of the immense progress made by science in the course of the nineteenth century shows convincingly that the three central metaphysical dogmas established by Plato have become untenable for pure reason. Our clear modern insight into the regularity and causative character of natural processes, and especially our knowledge of the universal reign of the law of substance, are inconsistent with belief in a personal God, the immortality of the soul, and the freedom of the will. If we find this threefold superstition still widely prevalent, and even retained by academic philosophers as an unshakable consequence of "critical philosophy," we must trace this remarkable fact chiefly to the great prestige of Immanuel Kant. His so-called critical system—really a hybrid product of the crossing of pure reason with practical superstition—has enjoyed a greater popularity than any other philosophy, and we must stop to consider it for a moment.
I have described in chapters xiv. and xx. of the Riddle the profound opposition between my monistic system and Kant's dualistic philosophy. In the appendix to the popular edition, especially, I have pointed out the glaring contradictions of his system, which other philosophers have often detected and criticised. Whenever there is question of his teaching one must ask: "Which Kant do you mean? Kant I., the founder of the monistic cosmogony, the critical formulator of pure reason; or Kant II., the author of the dualistic criticism of judgment, the dogmatic discoverer of practical reason?" These contradictions are partly due to the psychological metamorphoses which Kant underwent (Riddle, chapter vi.), partly to the perennial conflict between his scientific bias towards a mechanical explanation of this world and his religious craving (an outcome of heredity and education) and mystic belief in a life beyond. This culminates in the distinction between the world of sense and the world of spirit. The sense world (mundus sensibilis) lies open to our senses and our intellect, and is empirically knowable within certain limits. But behind it there is the spiritual world (mundus intelligibilis) of which we know, and can know, nothing; its existence (as the thing in itself) is, however, assured by our emotional needs. In this transcendental world dwells the power of mysticism.
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.
The increasing recognition of fixed natural laws which accompanied the growth of science in the nineteenth century was bound to restrict more and more the blind faith in miracles. There are three chief reasons why we find this, nevertheless, still so prevalent—the continued influence of dualistic metaphysics, the authority of the Christian Church, and the pressure of the modern state in allying itself with the Church. These three strong bulwarks of superstition are so hostile to pure reason and the truth it seeks that we must devote special attention to them. It is a question of the highest interests of humanity. The struggle against superstition and ignorance is a fight for civilization. Our modern civilization will only emerge from it in triumph, and we shall only eliminate the last barbaric features from our social and political life, when the light of true knowledge has driven out the belief in miracles and the prejudices of dualism.
The remarkable history of philosophy in the nineteenth century, which has not yet been written with complete impartiality and knowledge, shows us in the first place an ever-increasing struggle between the rising young sciences and the paramount authority of tradition and dogma. In the first half of the century the various branches of biology made progress without coming into direct collision with natural philosophy. The great advance of comparative anatomy, physiology, embryology, paleontology, the cell-theory, and classification, provided scientists with such ample material that they attached little importance to speculative metaphysics. It was otherwise in the second half of the nineteenth century. Soon after its commencement the controversy about the immortality of the soul broke out, in which Moleschott (1852), Büchner, and Carl Vogt (1854) contended for the physiological dependence of the soul on the brain, while Rudolph Wagner endeavored to maintain the prevailing metaphysical idea of its supernatural character. Then Darwin especially initiated in 1859 that vast reform in biology which brought to light the natural origin of species and shattered the miracle of creation. When the application of the theory of descent and the biogenetic law to man was made by anthropogeny (1874), and his evolution from a series of other mammals was proved, the belief in the immortality of the soul, the freedom of the will, and an anthropomorphic deity lost its last support. Nevertheless, these three fundamental dogmas continued to find favor in academic philosophy, which mostly followed the paths opened out by Kant. Most of the representatives of philosophy at the universities are narrow metaphysicians and idealists, who think more of the fiction of the "intelligible world" than of the truth of the world of sense. They ignore the vast progress made by modern biology, especially in the science of evolution; and they endeavor to meet the difficulties which it creates for their transcendental idealism by a sort of verbal gymnastic and sophistry. Behind all these metaphysical struggles there is still the personal element—the desire to save one's immortality from the wreck. In this it comes into line with the prevailing theology, which again builds on Kant. The pitiful condition of modern psychology is a characteristic result of this state of things. While the empirical physiology and pathology of the brain have made the greatest discoveries, the comparative anatomy and histology of the brain have thrown light on the details of its elaborate structure, and the ontogeny and phylogeny of the brain have proved its natural origin, the speculative philosophy of the schools stands aside from it all, and in its introspective analysis of the functions of the brain will not hear a word about the brain itself. It would explain the working of a most complicated machine without paying any attention to its structure. It is, therefore, not surprising to find that the dualistic theories established by Kant flourish at our universities as they did in the Middle Ages.
If the official philosophers, whose formal duty it is to study truth and natural law, still cling to the belief in miracles in spite of all the advance of empirical science, we shall not be surprised to find this in the case of official theology. Nevertheless, the sense of truth has prompted many unprejudiced and honorable theologians to look critically at the venerable structure of dogma, and open their minds to the streaming light of modern science. In the first third of the nineteenth century a rationalistic section of the Protestant Church attempted to rid itself of the fetters of dogma and reconcile its ideas with pure reason. Its chief leader, Schleiermacher,