the fore-brain, the earliest and foremost of the five embryonic brain-vesicles; they are confined to that part of the surface of the fore-brain which anatomists call the cortex, or gray bed, of the brain.
6. Within the cortex we have localized a number of different mental activities, or traced them to certain regions; if the latter are destroyed, their functions are extinguished.
7. These regions are so distributed in the cortex that one part of them is directly connected with the organs of sense, and receives and elaborates the impressions from these: these are the inner sense-centres, or sensoria.
8. Between these central organs of sense lie the intellectual or thought-organs, the instruments of presentation and thought, judgment and consciousness, intellect and reason; they are called the thought-centres, or association-centres, because the various impressions received from the sense-centres are associated, combined, and united in harmonious thought by them.[2]
The anatomic distinction between the two regions of the cortex which we oppose to each other as the internal sense-centres and the thought or association-centres seems to me of the highest importance. Certain physiological considerations had for some time suggested this distinction, but the sound anatomic proof of it has only been furnished during the last ten years. In 1894 Flechsig showed that there are four central sense-regions ("internal sense-spheres," or æstheta) in the gray cortex of the brain, and four thought-centres ("association-centres" or phroneta) between these: the most important of the latter, from the psychological point of view, is the "principal brain," or the "great occipito-temporal association-centre." The anatomic determination of the two "psychic regions" which Flechsig first introduced was afterwards modified by himself and substantially altered by others. The distinguished works of Edinger, Weigert, Hitzig, and others, lead to somewhat discrepant conclusions. But for the general conception of psychic action, and especially of the cognitive functions, which interests us at present, it is not necessary to have this delimitation of the regions. The chief point holds, that we can to-day anatomically distinguish between the two most important organs of mental life; that the neurona, which compose both, differ histologically (or in finer structure) and ontogenetically (or in origin); and that even chemical differences (or a different relation to certain coloring matters) may be perceived. We may conclude from this that the neurona or psychic cells which compose both organs also differ in their finer structure; there is probably a difference in the complicated fibrils which extend in the cytoplasm of both organs, although our coarse means of investigation have not yet succeeded in detecting this difference. In order to distinguish properly between the two sets of neurona, I propose to call the sensory-cells or sense-centres æsthetal cells, and the thought-cells or thought-centres phronetal cells. The former are, anatomically and physiologically, the intermediaries between the external sense-organs and the internal thought-organs.
To this anatomic delimitation of the internal sense-centres and thought-organs in the cortex corresponds their physiological differentiation. The sensorium, or sense-centre, works up the external sense-impressions that are conveyed by the peripheral sense-organs and the specific energy of their sensory nerves; the æstheta, or the central sense-instruments that make up the sensorium, and their organic units, the æsthetal cells, prepare the sense-impressions for thought and judgment in the proper sense. This work of "pure reason" is accomplished by the phronema of the thought-centres, the phroneta (or the various thought-organs that compose it) and their histological elements, the phronetal cells, bringing about an association or combination of the prepared impressions. By this important distinction we avoid the error of the older sensualism (of Hume, Condillac, etc.)—namely, that all knowledge depends on sense-action alone. It is true that the senses are the original source of all knowledge; but, in order to have real knowledge and thought, the specific task of reason, the impressions received from the external world by the sense-organs, and their nerves and centres, must be combined in the association-centres and elaborated in the conscious thought-centres. Then there is the important, but frequently overlooked, circumstance that there is in advance in the phronetal cells of the civilized man a valuable quality in the shape of inherited potential nerve-energy, which was originally engendered by the actual sense-action of the æsthetal cells in the course of many generations.
An impartial and critical study of the action of the brain in various scientific leaders shows that, as a rule, there is a certain opposition, or an antagonistic correlation, between the two sections of the highest mental power. The empirical representatives of science, or those who are devoted to physical studies, have a preponderant development of the sensorium, which means a greater disposition and capacity for the observation of phenomena in detail. On the other hand, the speculative representatives of what is called mental science and philosophy, or of metaphysical studies, have the phronema more strongly developed, which means a preponderant tendency to, and capacity for, a comprehensive perception of the universal in particulars. Hence it is that metaphysicians usually look with disdain on "materialistic" scientists and observers; while the latter regard the play of ideas of the former as an unscientific and speculative dissipation. This physiological antagonism may be traced histologically to the comparative development of the æsthetal and the phronetal cells in the two cases. It is only in natural philosophers of the first rank, such as Copernicus, Newton, Lamarck, Darwin, and Johannes Müller, that both sections are harmoniously developed, and thus the individual is equipped for the highest mental achievements.
If we take the ambiguous term "soul" (psyche or anima) in the narrower sense of the higher mental power, we may assign as its "seat" (or, more correctly, its organ), in man and the other mammals, that part of the cortex which contains the phroneta and is made up of the phronetal cells; a short and convenient name for this is the phronema. According to our monistic theory, the phronema is the organ of thought in the same sense in which we consider the eye the organ of vision, or the heart the central organ of circulation. With the destruction of the organ its function disappears. In opposition to this biological and empirically grounded theory, the current metaphysical psychology regards the brain as the seat of the soul, only in a very different sense. It has a strictly dualistic conception of the human soul as a being apart, only dwelling in the brain (like a snail in its shell) for a time. At the death of the brain it is supposed to live on, and indeed for all eternity. The immortal soul, on this theory (which we can trace to Plato), is an immaterial entity, feeling, thinking, and acting independently, and only using the material body as a temporary implement. The well-known "piano-theory" compares the soul to a musician who plays an interesting piece (the individual life) on the instrument of the body, and then deserts it, to live forever on its own account. According to Descartes, who insured the widest acceptance for Plato's dualistic mysticism, the proper habitation of the soul in the brain—in the music-room—is the pineal gland, a posterior section of the middle-brain (the second embryonic cerebral vesicle). The famous pineal gland has lately been recognized by comparative anatomists as the rudiment of a single organ of vision, the pineal eye (which is still found in certain reptiles). Moreover, not one of the innumerable psychologists who seek the seat of the soul in some part of the body, after the fashion of Plato, has yet formulated a plausible theory of the connection of mind and body and the nature of their reciprocal action. On our monistic principles the answer to this question is very simple, and consonant with experience. In view of its extreme importance, it is advisable to devote at least a few lines to the consideration of the phronema in the light of anatomy, physiology, ontogeny, and phylogeny.
When we conceive the phronema as the real "organ of the soul" in the strict sense—that is to say, as the central instrument of thought, knowledge, reason, and consciousness—we may at once lay down the principle that there is an anatomical unity of organ corresponding to the physiological and generally admitted unity of thought and consciousness. As we assign to this phronema a most elaborate anatomical structure, we may call it the organic apparatus of the soul, in the same sense in which we conceive the eye as a purposively arranged apparatus of vision. It is true that we have as yet only made a beginning of the finer anatomic analysis of the phronema, and are not yet able to mark off its field decisively from the neighboring spheres of sense and motion. With the most improved means of modern histology, the most perfect microscopes and coloring methods, we are only just beginning to penetrate into the marvellous structure of the phronetal cells and their complicated grouping. Yet we have advanced far enough to regard