Especially notable are those great Free-thinkers of classic antiquity who taught a monistic view of life in the sixth century before Christ—the Ionic natural philosophers, Thales, Anaximander, and Anaximenes; and a little later, Heraclitus, Empedocles, and Democritus. They made the first thorough attempt to explain the world on rational principles, independently of all mythological tradition and theological dogmas. However, these remarkable efforts to found a primitive monism, which found so finished an expression in the De rerum natura of the great poet-philosopher, Lucretius Carus (98-54 B.C.), were shortly thrust out by the spread—through Plato's curious dualism—of the belief in the immortality of the soul and the transcendental world of ideas.
The Eleatics, Parmenides and Zeno, had foreshadowed in the fifth century the division of philosophy into two branches; but Plato and his pupil Aristotle (in the fourth century B.C.) succeeded in gaining general acceptance for this dualism and antithesis of physics and metaphysics. Physics devoted itself on the ground of experience to the study of the phenomena of things, leaving their real essences (or noumena) that lay behind the phenomena to metaphysics. These inner essences are transcendental and inaccessible to empirical research; they form the metaphysical world of eternal ideas, which is independent of the real world, and has its highest unity in God, as the Absolute. The soul, an eternal idea that dwells for a time in the passing human body, is immortal. This consistent dualism of Plato's system, with its sharp antithesis of this world and the next, of body and soul, of world and God, is its chief characteristic. It became all the more influential when Plato's pupil Aristotle blended it with his empirical metaphysics, based on ample scientific experience, and pointed out the idea in the entelechy, or purposively acting principle, of every being; and especially when Christianity (three hundred years afterwards) found in this dualism a welcome philosophic support of its own transcendental tendency.
In the course of the thousand years which historians call the Middle Ages, and which are usually dated from the fall of the Roman Empire (476) to the discovery of America (1492), the superstition of civilized races reached its highest development. The authority of Aristotle was paramount in philosophy; it was used by the dominant Church for its own purposes. But the influence of the Christian faith, with all the gay coloring which the fairy-tales of the Bible added to its structure of dogmas, was seen much more in practical life. In the foreground of belief were the three central dogmas of metaphysics, to which Plato had first given complete expression—the personal God as creator of the world, the immortality of the soul, and the freedom of the human will. As Christianity laid the greatest theoretical stress on the first two dogmas and the greatest practical stress on the third, metaphysical dualism soon prevailed on all sides. Especially inimical to scientific inquiry was the Christian contempt of nature and its belittlement of earthly life in view of the eternal life to come. As long as the light of philosophical criticism in any form was extinguished, the flower-garden of religious poetry flourished exceedingly and the idea of miracle was taken as self-evident. We know what the practical result of this superstition 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.