Personally, I have never subscribed to this hypothesis of atomic consciousness. I emphasize the point because Emil du Bois-Reymond has attributed it to me. In the controversy I had with him (1880) he violently attacked my “pernicious and false philosophy,” and contended that I had, in my paper on “The Perigenesis of the Plastidule,” “laid it down as a metaphysical axiom that every atom has its individual consciousness.” On the contrary, I explicitly stated that I conceive the elementary psychic qualities of sensation and will, which may be attributed to atoms, to be unconscious—just as unconscious as the elementary memory which I, in company with that distinguished physiologist, Ewald Hering, consider to be “a common function of all organized matter”—or, more correctly, “living substance.” Du Bois-Reymond curiously confuses “soul” and “consciousness”; whether from oversight or not I cannot say. Since he considers consciousness to be a transcendental phenomenon (as we shall see presently), while denying that character to other psychic functions—the action of the senses, for example—I must infer that he recognizes the difference of the two ideas. Other parts of his eloquent speeches contain quite the opposite view, for the famous orator not infrequently contradicts himself on important questions of principle. However, I repeat that, in my opinion, consciousness is only part of the psychic phenomena which we find in man and the higher animals; the great majority of them are unconscious.
However divergent are the different views as to the nature and origin of consciousness, they may, nevertheless, on a clear and logical examination, all be reduced to two fundamental theories—the transcendental (or dualistic) and the physiological (or monistic). I have myself always held the latter view, in the light of my evolutionary principles, and it is now shared by a great number of distinguished scientists, though it is by no means generally accepted. The transcendental theory is the older and much more common; it has recently come once more into prominence, principally through Du Bois-Reymond, and it has acquired a great importance in modern discussions of cosmic problems through his famous “Ignorabimus speech.” On account of the extreme importance of this fundamental question we must touch briefly on its main features.
In the celebrated discourse on “The Limits of Natural Science,” which E. du Bois-Reymond gave on August 14, 1872, at the Scientific Congress at Leipzig, he spoke of two “absolute limits” to our possible knowledge of nature which the human mind will never transcend in its most advanced science—never, as the oft-quoted termination of the address, “Ignorabimus,” emphatically pronounces. The first absolutely insoluble “world-enigma” is the “connection of matter and force,” and the distinctive character of these fundamental natural phenomena; we shall go more fully into this “problem of substance” in the [twelfth chapter]. The second insuperable difficulty of philosophy is given as the problem of consciousness—the question how our mental activity is to be explained by material conditions, especially movements, how “substance [the substance which underlies matter and force] comes, under certain conditions, to feel, to desire, and to think.”
For brevity, and in order to give a characteristic name to the Leipzig discourse, I have called it the “Ignorabimus speech”; this is the more permissible, as E. du Bois-Reymond himself, with a just pride, eight years afterwards, speaking of the extraordinary consequences of his discourse, said: “Criticism sounded every possible note, from friendly praise to the severest censure, and the word ‘Ignorabimus,’ which was the culmination of my inquiry, was at once transformed into a kind of scientific shibboleth.” It is quite true that loud praise and approbation resounded in the halls of the dualistic and spiritualistic philosophy, and especially in the camp of the “Church militant”; even the spiritists and the host of believers, who thought the immortality of their precious souls was saved by the “Ignorabimus,” joined in the chorus. The “severest censure” came at first only from a few scientists and philosophers—from the few who had sufficient scientific knowledge and moral courage to oppose the dogmatism of the all-powerful secretary and dictator of the Berlin Academy of Science.
Towards the end, however, the author of the “Ignorabimus speech” briefly alluded to the question whether these two great “world-enigmas,” the general problem of substance and the special problem of consciousness, are not two aspects of one and the same problem. “This idea,” he said, “is certainly the simplest, and preferable to the one which makes the world doubly incomprehensible. Such, however, is the nature of things that even here we can obtain no clear knowledge, and it is useless to speak further of the question.” The latter sentiment I have always stoutly contested, and have endeavored to prove that the two great questions are not two distinct problems. “The neurological problem of consciousness is but a particular aspect of the all-pervading cosmological problem of substance.”
The peculiar phenomenon of consciousness is not, as Du Bois-Reymond and the dualistic school would have us believe, a completely “transcendental” problem; it is, as I showed thirty-three years ago, a physiological problem, and, as such, must be reduced to the phenomena of physics and chemistry. I subsequently gave it the more definite title of a neurological problem, as I share the view that true consciousness (thought and reason) is only present in those higher animals which have a centralized nervous system and organs of sense of a certain degree of development. Those conditions are certainly found in the higher vertebrates, especially in the placental mammals, the class from which man has sprung. The consciousness of the highest apes, dogs, elephants, etc., differs from that of man in degree only, not in kind, and the graduated interval between the consciousness of these “rational” placentals and that of the lowest races of men (the Veddahs, etc.) is less than the corresponding interval between these uncivilized races and the highest specimens of thoughtful humanity (Spinoza, Goethe, Lamarck, Darwin, etc.). Consciousness is but a part of the higher activity of the soul, and as such it is dependent on the normal structure of the corresponding psychic organ, the brain.
Physiological observation and experiment determined twenty years ago that the particular portion of the mammal-brain which we call the seat (preferably the organ) of consciousness is a part of the cerebrum, an area in the late-developed gray bed, or cortex, which is evolved out of the convex dorsal portion of the primary cerebral vesicle, the “fore-brain.” Now, the morphological proof of this physiological thesis has been successfully given by the remarkable progress of the microscopic anatomy of the brain, which we owe to the perfect methods of research of modern science (Kölliker, Flechsig, Golgi, Edinger, Weigert, and others).
The most important development is the discovery of the organs of thought by Paul Flechsig, of Leipzig; he proved that in the gray bed of the brain are found the four seats of the central sense-organs, or four “inner spheres of sensation”—the sphere of touch in the vertical lobe, the sphere of smell in the frontal lobe, the sphere of sight in the occipital lobe, and the sphere of hearing in the temporal lobe. Between these four “sense-centres” lie the four great “thought-centres,” or centres of association, the real organs of mental life; they are those highest instruments of psychic activity that produce thought and consciousness. In front we have the frontal brain or centre of association; behind, on top there is the vertical brain, or parietal centre of association, and underneath the principal brain, or “the great occipito-temporal centre of association” (the most important of all); lower down, and internally, the insular brain or the insula of Reil, the insular centre of association. These four “thought-centres,” distinguished from the intermediate “sense-centres” by a peculiar and elaborate nerve-structure, are the true and sole organs of thought and consciousness. Flechsig has recently pointed out that, in the case of man, very specific structures are found in one part of them; these structures are wanting in the other mammals, and they, therefore, afford an explanation of the superiority of man’s mental powers.
The momentous announcement of modern physiology, that the cerebrum is the organ of consciousness and mental action in man and the higher mammals, is illustrated and confirmed by the pathological study of its diseases. When parts of the cortex are destroyed by disease their respective functions are affected, and thus we are enabled, to some extent, to localize the activities of the brain; when certain parts of the area are diseased, that portion of thought and consciousness disappears which depends on those particular sections. Pathological experiment yields the same result; the decay of some known area (for instance, the centre of speech) extinguishes its function (speech). In fact, there is proof enough in the most familiar phenomena of consciousness of their complete dependence on chemical changes in the substance of the brain. Many beverages (such as coffee and tea) stimulate our powers of thought; others (such as wine and beer) intensify feeling; musk and camphor reanimate the fainting consciousness; ether and chloroform deaden it, and so forth. How would that be possible if consciousness were an immaterial entity, independent of these anatomical organs? And what becomes of the consciousness of the “immortal soul” when it no longer has the use of these organs?
These and other familiar facts prove that man’s consciousness—and that of the nearest mammals—is changeable, and that its activity is always open to modification from inner (alimentation, circulation, etc.) and outer causes (lesion of the brain, stimulation, etc.). Very instructive, too, are the facts of double and intermittent consciousness, which remind us of “alternate generations of presentations.” The same individual has an entirely different consciousness on different days, with a change of circumstances; he does not know to-day what he did yesterday: yesterday he could say, “I am I”; to-day he must say, “I am another being.” Such intermittence of consciousness may last not only days, but months, and even years; the change may even become permanent.