and the theory of knowledge, I need only refer the reader thereto. There is just one point I may touch here, as it has been attacked with particular vehemence by my critics. I had made several allusions to the works of the distinguished English zoologist, Romanes, who had made a careful comparative study of mental development in the animal and man, and had continued the work of Darwin. Romanes partly retracted his monistic convictions shortly before his death, and adopted mystic religious views. As this conversion was only known at first through one of his friends, a zealous English theologian [Dr. Gore], it was natural to retain a certain reserve. However, it turned out that there had really been in this case (just as in the case of the aged Baer) one of those interesting psychological metamorphoses which I have described in chapter vi. of the Riddle. Romanes suffered a good deal from illness and grief at the loss of friends in his last years. In this condition of extreme depression and melancholy he fell under mystic influences which promised him rest and hope by belief in the supernatural. It is hardly necessary to point out to impartial readers that such a conversion as this does not shake his earlier monistic views. As in similar cases where deep emotional disturbance, painful experiences, and exuberant hope have clouded the judgment, we must still hold that it is the place of the latter, and not of the emotions or of any supernatural revelation, to attain a knowledge of the truth. But for such attainment it is necessary for the organ of mind, the phronema, to be in a normal condition.[3]
Of all the wonders of life, consciousness may be said to be the greatest and most astounding. It is true that to-day most physiologists are agreed that man's consciousness, like all his other mental powers, is a function of the brain, and may be reduced to physical and chemical processes in the cells of the cortex. Nevertheless, some biologists still cling to the metaphysical view that this "central mystery of psychology" is an insoluble enigma, and not a natural phenomenon. In face of this, I must refer the reader to the monistic theory of consciousness which I have given in chapter x. of the Riddle, and must insist that in this case again embryology is the best guide to a comprehension of the subject. Sight is next to consciousness, in many respects, as one of the wonders of life. The well-known embryology of the eye teaches us how sight—the perception of images from the external world—has been gradually evolved from the simple sensitiveness to light of the lower animals, by the development of a transparent lens. In the same way the conscious soul, the internal mirror of the mind's own action, has been produced as a new wonder of life out of the unconscious associations in the phronema of our earlier vertebrate ancestors.
From this thorough and unprejudiced appreciation of the biology of the phronema it follows that the knowledge of truth, the aim of all science, is a natural physiological process, and that it must have its organs like all other psychic functions. These organs have been revealed to us so fully in the advance of biology during the last half-century that we may be said to have a generally satisfactory idea of the natural character of their organization and action, though we are still far from enjoying a complete anatomical and physiological insight into their details. The most important acquisition we have made is the conviction that all knowledge was originally acquired a posteriori and from experience, and that its first sources are the impressions made on our organs of sense. Both these—the peripheral sense-organs—and the phronema, or central psychic organ, are subject to the law of substance; and the action of the phronema is just as reducible to chemical and physical processes as the action of the organs of sense.
In diametrical opposition to our monistic and empirical theory of knowledge, the prevailing dualistic metaphysics assumes that our knowledge is only partly empirical and a posteriori, and is partly quite independent of experience and a priori, or due to the original constitution of our "immaterial" mind. The powerful authority of Kant has lent enormous prestige to this mystic and supernatural view, and the academic philosophers of our time are endeavoring to maintain it. A "return to Kant" is held to be the only means of salvation for philosophy; in my opinion it should be a return to nature. As a fact, the return to Kant and his famous theory of knowledge is an unfortunate "crab-walk" on the part of philosophy. Our modern metaphysicians regard the brain, as Kant did one hundred and twenty years ago, as a mysterious, whitish-gray, pulpy mass, the significance of which as an instrument of the mind is very enigmatic and obscure. But for modern biology the brain is the most wonderful structure in nature, a compound of innumerable soul-cells or neurona. These have a most elaborate finer structure, are combined in a vast psychic apparatus by thousands of interlacing nerve-fibrils, and are thus fitted to accomplish the highest mental functions.
First Table
ANTITHESIS OF THE TWO WAYS OF ATTAINING THE TRUTH
| Monistic Theory of Knowledge | Dualistic Theory of Knowledge | |
| 1. Knowledge is a natural process, not a miracle. | 1. Knowledge is a supernatural process, a miracle. | |
| 2. Knowledge, as a natural process, is subject to the general law of substance. | 2. Knowledge, as a transcendental process, is not subject to the law of substance. | |
| 3. Knowledge is a physiological process, with the brain for its anatomic organ. | 3. Knowledge is not a physiological, but a purely spiritual, process. | |
| 4. The part of the human brain in which knowledge is exclusively engendered is a definite and limited part of the cortex, the phronema. | 4. The part of the human brain which seems to act as organ of knowledge is really only the instrument that allows the spiritual process to appear | |
| 5. The organ of knowledge, or the phronema, consists of the association-centres, and differs by its special histological structure from the neighboring sensory and motor centres in the cortex, and it is in close relation with these. | 5. The organ of knowledge, or the phronema (the sum of the association-centres), is merely a part of the instrument of mind, like the neighboring and correlated sensory and motor-centres. | |
| 6. The innumerable cells which make up the phronema—the phronetal cells—are the elementary organs of the cognitive process: the possibility of knowledge depends on their normal physical texture and chemical composition. | 6. The innumerable phronetal cells, as the microscopic elementary parts of the phronema, are, it is true, indispensable instruments of the cognitive process, but not its real factors—merely finer parts of its instrument. | |
| 7. The physical process of knowledge consists in the combination or association of presentations, the first sources of which are the impressions transmitted to the sense-centres. | 7. The metaphysical process of knowledge consists in the combination or association of presentations, which are only partly traceable to sense-impressions, and are partly supersensual, transcendental processes. | |
| 8. Hence all knowledge originally comes from experience, by means of the organs of sense; partly directly (direct experience, observation, and experiment of the present), partly indirectly (historical and indirectly transmitted past experiences). All knowledge (even mathematical) is of empirical origin and a posteriori. | 8. Hence knowledge is of two kinds: empirical and a posteriori knowledge, obtained by experience, and transcendental a priori knowledge, independent of experience. Mathematics especially belongs to the latter class, its axioms differing from empirical truths by their absolute certainty. |
II
LIFE
Definition of life—Comparison with a flame—Organism and organization—Machine theory of life—Organisms without organs: monera—Organization and life of the chromacea—Stages of organization—Complex organisms—Symbolic organisms—Organic compounds—Organisms and inorganic bodies compared