Even as to the elementary idea of consciousness, its contents and extension, the views of the most distinguished philosophers and scientists are widely divergent. Perhaps the meaning of consciousness is best conceived as an internal perception, and compared with the action of a mirror. As its two chief departments we distinguish objective and subjective consciousness—consciousness of the world, the non-ego, and of the ego. By far the greater part of our conscious activity, as Schopenhauer justly remarked, belongs to the consciousness of the outer world, or the non-ego: this world-consciousness embraces all possible phenomena of the outer world which are in any sense accessible to our minds. Much more contracted is the sphere of self-consciousness, the internal mirror of all our own psychic activity, all our presentations, sensations, and volitions.
Many distinguished thinkers, especially on the physiological side (Wundt and Ziehen, for instance) take the ideas of consciousness and psychic function to be identical—“all psychic action is conscious”; the province of psychic life, they say, is coextensive with that of consciousness. In our opinion, such a definition gives an undue extension to the meaning of consciousness, and occasions many errors and misunderstandings. We share, rather, the view of other philosophers (Romanes, Fritz Schultze, and Paulsen), that even our unconscious presentations, sensations, and volitions pertain to our psychic life; indeed, the province of these unconscious psychic actions (reflex action, and so forth) is far more extensive than that of consciousness. Moreover, the two provinces are intimately connected, and are separated by no sharp line of demarcation. An unconscious presentation may become conscious at any moment; let our attention be withdrawn from it by some other object, and forthwith it disappears from consciousness once more.
The only source of our knowledge of consciousness is that faculty itself; that is the chief cause of the extraordinary difficulty of subjecting it to scientific research. Subject and object are one and the same in it: the perceptive subject mirrors itself in its own inner nature, which is to be the object of our inquiry. Thus we can never have a complete objective certainty of the consciousness of others; we can only proceed by a comparison of their psychic condition with our own. As long as this comparison is restricted to normal people we are justified in drawing certain conclusions as to their consciousness, the validity of which is unchallenged. But when we pass on to consider abnormal individuals (the genius, the eccentric, the stupid, or the insane) our conclusions from analogy are either unsafe or entirely erroneous. The same must be said with even greater truth when we attempt to compare human consciousness with that of the animals (even the higher, but especially the lower). In that case such grave difficulties arise that the views of physiologists and philosophers diverge as widely as the poles on the subject. We shall briefly enumerate the most important of these views.
I. The anthropistic theory of consciousness.—It is peculiar to man. To Descartes we must trace the widespread notion that consciousness and thought are man’s exclusive prerogative, and that he alone is blessed with an “immortal soul.” This famous French philosopher and mathematician (educated in a Jesuit College) established a rigid partition between the psychic activity of man and that of the brute. In his opinion the human soul, a thinking, immaterial being, is completely separated from the body, which is extended and material. Yet it is united to the body at a certain point in the brain (the glandula pinealis) for the purpose of receiving impressions from the outer world and effecting muscular movements. The animals, not being endowed with thought, have no soul: they are mere automata, or cleverly constructed machines, whose sensations, presentations, and volitions are purely mechanical, and take place according to the ordinary laws of physics. Hence Descartes was a dualist in human psychology, and a monist in the psychology of the brute. This open contradiction in so clear and acute a thinker is very striking; in explaining it, it is not unnatural to suppose that he concealed his real opinion, and left the discovery of it to independent scholars. As a pupil of the Jesuits, Descartes had been taught to deny the truth in the face of his better insight; and perhaps he dreaded the power and the fires of the Church. Besides, his sceptical principle, that every sincere effort to attain the truth must start with a doubt of the traditional dogma had already drawn upon him fanatical accusations of scepticism and atheism. The great influence which Descartes had on subsequent philosophy was very remarkable, and entirely in harmony with his “book-keeping by double entry.” The materialists of the seventeenth and eighteenth centuries appealed to the Cartesian theory of the animal soul and its purely mechanical activity in support of their monistic psychology. The spiritualists, on the other hand, asserted that their dogma of the immortality of the soul and its independence of the body was firmly established by Descartes’ theory of the human soul. This view is still prevalent in the camp of the theologians and dualistic metaphysicians. The scientific conception of nature, however, which has been built up in the nineteenth century, has, with the aid of empirical progress, in physiological and comparative psychology, completely falsified it.
II. Neurological theory of consciousness.—It is present only in man and those higher animals which have a centralized nervous system and organs of sense. The conviction that a large number of animals—at least the higher mammals—are not less endowed than man with a thinking soul and consciousness prevails in modern zoology, exact physiology, and the monistic psychology. The immense progress we have made in the various branches of biology has contributed to bring about a recognition of this important truth. We confine ourselves for the present to the higher vertebrates, and especially the mammals. That these most intelligent specimens of these highly developed vertebrates—apes and dogs, in particular—have a strong resemblance to man in their whole psychic life has been recognized and speculated on for thousands of years. Their faculty of presentation and sensation, of feeling and desire, is so like that of man that we need adduce no proof of our thesis. But even the higher associational activity of the brain, the formation of judgments and their connection into chains of reasoning, thought, and consciousness in the narrower sense, are developed in them after the same fashion as in man: they differ only in degree, not in kind. Moreover, we learn from comparative anatomy and histology that the intricate structure of the brain (both in general and in detail) is substantially the same in the mammals as it is in man. The same lesson is enforced by comparative ontogeny with regard to the origin of these psychic organs. Comparative physiology teaches us that the various states of consciousness are just the same in these highest placentals as in man; and we learn by experiment that there is the same reaction to external stimuli. The higher animals can be narcotized by alcohol, chloroform, ether, etc., and may be hypnotized by the usual methods, just as in the case of man.
It is, however, impossible to determine mathematically at what stage of animal life consciousness is to be first recognized as such. Some zoologists draw the line very high in the scale, others very low. Darwin, who most accurately distinguishes the various stages of consciousness, intelligence, and emotion in the higher animals, and explains them by progressive evolution, points out how difficult, or even impossible, it is to determine the first beginning of this supreme psychic faculty in the lower animals. Personally, out of the many contradictory theories, I take that to be most probable which holds the centralization of the nervous system to be a condition of consciousness; and that is wanting in the lower classes of animals. The presence of a central nervous organ, of highly developed sense-organs, and an elaborate association of groups of presentations, seem to me to be required before the unity of consciousness is possible.
III. Animal theory of consciousness.—All animals, and they alone, have consciousness. This theory would draw a sharp distinction between the psychic life of the animal and of the plant. Such a distinction was urged by many of the older writers, and was clearly formulated by Linné in his celebrated Systema Naturae; the two great kingdoms of the organic world are, in his opinion, divided by the fact that animals have sensation and consciousness, and the plants are devoid of them. Later on Schopenhauer laid stress on the same distinction: “Consciousness is only known to us as a feature of animal nature. Even though it extend upwards through the whole animal kingdom, even to man and his reason, the unconsciousness of the plant, from which it started, remains as the basic feature. In the lowest animals we have but the dawn of it.” The inaccuracy of this view was obvious by about the middle of the present century, when a deeper study was made of the psychic activity of the lower animal forms, especially the cœlenterates (sponges and cnidaria): they are undoubtedly animals, yet there is no more trace of a definite consciousness in them than in most of the plants. The distinction between the two kingdoms was still further obliterated when more careful research was made into their unicellular forms. There is no psychological difference between the plasmophagous protozoa and the plasmodomous protophyta, even in respect of their consciousness.
IV. Biological theory of consciousness.—It is found in all organisms, animal or vegetal, but not in lifeless bodies (such as crystals). This opinion is usually associated with the idea that all organisms (as distinguished from inorganic substances) have souls: the three ideas—life, soul, and consciousness—are then taken to be coextensive. Another modification of this view holds that, though these fundamental phenomena of organic life are inseparably connected, yet consciousness is only a part of the activity of the soul, and of the vital activity. Fechner, in particular, has endeavored to prove that the plant has a “soul,” in the same sense as an animal is said to have one; and many credit the vegetal soul with a consciousness similar to that of the animal soul. In truth, the remarkable stimulated movements of the leaves of the sensitive plants (the mimosa, drosera, and dionæa), the automatic movements of other plants (the clover and wood-sorrel, and especially the hedysarum), the movements of the “sleeping plants” (particularly the papilionacea), etc., are strikingly similar to the movements of the lower animal forms: whoever ascribes consciousness to the latter cannot refuse it to such vegetal forms.
V. Cellular theory of consciousness.—It is a vital property of every cell. The application of the cellular theory to every branch of biology involved its extension to psychology. Just as we take the living cell to be the “elementary organism” in anatomy and physiology, and derive the whole system of the multicellular animal or plant from it, so, with equal right, we may consider the “cell-soul” to be the psychological unit, and the complex psychic activity of the higher organism to be the result of the combination of the psychic activity of the cells which compose it. I gave the outlines of this cellular psychology in my General Morphology in 1866, and entered more fully into the subject in my paper on “Cell-Souls and Soul-Cells.” I was led to a deeper study of this “elementary psychology” by my protracted research into the unicellular forms of life. Many of these tiny (generally microscopic) protists show similar expressions of sensation and will, and similar instincts and movements, to those of higher animals; that is especially true of the very sensitive and lively infusoria. In the relation of these sensitive cell-organisms to their environment, and in many other of their vital expressions (for instance, in the wonderful architecture of the rhizopods, the thalamophoræ, and the infusoria), we seemed to have clear indications of conscious psychic action. If, then, we accept the biological theory of consciousness (No. IV.), and credit every psychic function with a share of that faculty, we shall be compelled to ascribe it to each independent protist cell. In that case its material basis would be either the entire protoplasm of the cell, or its nucleus, or a portion of it. In the “psychade theory” of Fritz Schultze the elementary consciousness of the psychade would have the same relation to the individual cells as personal consciousness has to the multicellular organism of the personality in the higher animals and man. It is impossible definitively to disprove this theory, which I held at one time. Still, I now feel compelled to agree with Max Verworn, in his belief that none of the protists have a developed self-consciousness, but that their sensations and movements are of an unconscious character.
VI. Atomistic theory of consciousness.—It is an elementary property of all atoms. This atomistic hypothesis goes furthest of all the different views as to the extension of consciousness. It certainly escapes the difficulty which so many philosophers and biologists experience in solving the problem of the first origin of consciousness. It is a phenomenon of so peculiar a character that a derivation of it from other psychic functions seems extremely hazardous. It seemed, therefore, the easiest way out of the difficulty to conceive it as an inherent property of all matter, like gravitation or chemical affinity. On that hypothesis there would be as many forms of this original consciousness as there are chemical elements; each atom of hydrogen would have its hydrogenic consciousness, each atom of carbon its carbonic consciousness, and so forth. There are philosophers, even, who ascribe consciousness to the four elements of Empedocles, the union of which, by “love and hate,” produces the totality of things.