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Table of Contents

Truth and the riddle of the universe—Experience and thought—Empiricism and speculation—Natural philosophy—Science—Empirical science—Descriptive science—Observation and experiment—History and tradition—Philosophic science—Theory of knowledge—Knowledge and the brain—Æstheta and phroneta—Seat of the soul, or organ of thought: phronema—Anatomy, physiology, ontogeny, and phylogeny of the phronema—Psychological metamorphoses—Evolution of consciousness—Monistic and dualistic theories of knowledge—Divergence of the two ways of attaining the truth.

What is truth? This great question has occupied the more thoughtful of men for thousands of years, and elicited myriads of attempts to answer it, myriads of truths and untruths. Every history of philosophy gives a longer or shorter account of these countless efforts of the advancing mind of man to attain a clear knowledge of the world and of itself. Nay, even "world-wisdom" itself, or philosophy in the proper sense of the word, is nothing but a connected effort to unite the general results of man's investigation, observation, reflection, and thought, and bring them to a common focus. Without prejudice and without fear, philosophy would tear the mantle from "the veiled statue of Sais," and attain a full vision of the truth. True philosophy, taken in this sense, may proudly and justly style itself "the queen of the sciences."

When philosophy, as a search for truth in the highest sense, thus unites our isolated discoveries and seeks to weld them into one unified system of the world, it comes at length to state certain fundamental problems, the answer to which varies according to the degree of culture and the point of view of the inquirer. These final and highest objects of scientific inquiry have been of late comprehended under the title of The Riddle of the Universe, and I gave this name to the work I published in 1899, which dealt with them, in order to make its aim perfectly clear. In the first chapter I dealt briefly with what have been called "the seven great cosmic problems," and in the twelfth chapter I endeavored to show that they may all be reduced to one final "problem of substance," or one great "riddle of the universe." The general formulation of this problem is effected by blending the two chief cosmic laws—the chemical law of the constancy of matter (Lavoisier, 1789), and the physical law of the constancy of force (Robert Mayer, 1842). This monistic association of the two fundamental laws, and establishment of the unified law of substance, has met with a good deal of agreement, but also with some opposition; but the most violent attacks were directed against my monistic theory of knowledge, or against the method I followed in seeking to solve the riddle of the universe. The only paths which I had recognized as profitable were those of experience and thought—or empirical knowledge and speculation. I had insisted that these two methods supplemented each other, and that they alone, under the direction of reason, lead to the attainment of truth. At the same time I had rejected as false two other much-frequented paths which purported to lead directly to a profounder knowledge, the ways of emotion and revelation; both of these are in opposition to reason, since they demand a belief in miracles.

"All natural science is philosophy, and all true philosophy is natural science. All true science is natural philosophy." I expressed in these words the general result of my monistic studies in 1866 (in the twenty-seventh chapter of my Generelle Morphologie). I then laid it down as the fundamental principle of the monistic system that the unity of nature and the unity of science follow absolutely from any connected study of modern philosophic science, and I expressed my conviction in these terms: "All human science is knowledge based on experience, or empirical philosophy; or, if the title be preferred, philosophic empiricism. Thoughtful experience, or thought based on experience, is the only way and method to be followed in the search for truth." I endeavored to establish these theses conclusively in the first book of the Generelle Morphologie, which contains (p. 108) a critical and methodological introduction to this science. Not only are those methods considered "which must necessarily supplement each other" (I. Empiricism and Philosophy; II. Analysis and Synthesis; III. Induction and Deduction), but also those "which necessarily exclude each other" (IV. Dogmatism and Criticism; V. Teleology and Causality, or Vitalism and Mechanicism; VI. Dualism and Monism). The monistic principles which I developed there thirty-eight years ago have only been confirmed by my subsequent labors, and so I may refer the interested reader to that work. The Riddle of the Universe is in the main an attempt to introduce to the general reader in a convenient form the chief points of the monistic system I established. However, the opposition which has been aroused by the general philosophic observations of the Riddle compels me to give a further explanation of the chief features of my theory of knowledge.

All true science that deserves the name is based on a collection of experiences, and consists of conclusions that have been reached by a rational connection of these experiences. "Only in experience is there truth," says Kant. The external world is the object that acts on man's organs of sense, and in the internal sense-centres of the cortex of the brain these impressions are subjectively transformed into presentations. The thought-centres, or association centres, of the cortex (whether or no one distinguishes them from the sense-centres) are the real organs of the mind that unite these presentations into conclusions. The two methods of forming these conclusions—induction and deduction, the formation of arguments and concepts, thought and consciousness—make up together the cerebral function we call reason. These long familiar and fundamental truths, the recognition of which I have described for thirty-eight years as the first condition for solving the riddle of life, are still far from being generally appreciated. On the contrary, we find them combated by the extreme representatives of both tendencies of science. On the one side, the empirical and descriptive school would reduce the whole task to experience, without calling in the aid of philosophy; while philosophic speculation, on the other side, would dispense with experience and endeavor to construct the world by pure thought.

Starting from the correct principle that all science originally has its source in experience, the representatives of "experimental science" affirm that their task consists solely in the exact observation of "facts" and the classification and description of them, and that philosophic speculation is nothing more than an idle play of ideas. Hence this one-sided sensualism, as Condillac and Hume especially maintained it, affirmed that the whole action of the mind consists in a manipulation of sense-impressions. This narrow empirical conception spread very widely during the nineteenth century, particularly in the second half, among the rapidly advancing sciences; it was favored by the specialism which grew up in the necessary division of labor. The majority of scientists are still of opinion that their task is confined to the exact observation and description of facts. All that goes beyond this, and especially all far-reaching philosophic conclusions from their accumulated observations, are regarded by them with suspicion. Rudolph Virchow strongly emphasized this narrow empirical tendency ten years ago. In his speech on the foundation of the Berlin University he explained the "transition from the philosophic to the scientific age"; he said that the sole aim of science is "the knowledge of facts, the objective investigation of natural phenomena in detail." The former politician seemed to forget that he had maintained a precisely opposite view forty years before (at Würtzburg), and that his own great achievement, the creation of cellular pathology, was a philosophic construction—the formation of a new and comprehensive theory of disease by the combination of countless observations and the conclusions deduced therefrom.

No science of any kind whatever consists solely in the description of observed facts. Hence we can only regard it as a pitiful contradiction in terms when we find biology classed in official documents to-day as a "descriptive science," and physics opposed to it as an "explanatory science." As if in both cases we had not, after describing the observed phenomena, to pass on to trace them to their causes—that is, to explain them—by means of rational inferences! But it is even more regrettable to find that one of the ablest scientists of Germany, Gustav Kirchhoff, has claimed that description is the final and the highest task of science. The famous discoverer of spectrum analysis says in his Lectures on Mathematical Physics and Mechanics (1877): "It is the work of science to describe the movements perceived in Nature, in the most complete and simplest fashion." There is no meaning in this statement unless we take the word "description" in a quite unusual sense—unless "complete description" is meant to include explanation. For thousands of years true science has been, not merely a simple description of individual facts, but an explanation of them by tracing them to their causes. It is true that our knowledge of them is always imperfect, or even hypothetical; but this is equally true of the description of facts. Kirchhoff's statement is in flagrant contradiction to his own great achievement, the founding of spectrum analysis; for the extraordinary significance of this does not lie in the discovery of the wonderful facts of spectroscopic optics and the "complete description" of individual spectra, but in the rational grouping and interpretation of them. The far-reaching conclusions that he has drawn from them have opened out entirely new paths to physics and chemistry. Hence Kirchhoff is in as sad a plight as Virchow when he formulates so precarious a principle. However, these statements of the two great scientists have done a great deal of harm, as they have widened still more the deep gulf between science and philosophy. It may be of some service if a few thousand of the thoughtless followers of "descriptive science" are persuaded to refrain from attempts at explanation of facts. But the master-builders of science cannot be content with the collection of dead material; they must press on to the knowledge of causes by a rational manipulation of their facts.

The accurate and discriminating observation of facts, supported by careful experiment, is certainly a great advantage that modern science has over all earlier efforts to attain the truth. The distinguished thinkers of classic antiquity were far superior to most modern scientists and philosophers in regard to judgment and reasoning, or all the subtler processes of thought; but they were superficial and unpractised observers, and were barely acquainted with experiment. In the Middle Ages scientific work degenerated in both its aspects, as the dominant creed demanded only faith and the recognition of its supernatural revelation, and depreciated observation. The great importance of this as a foundation of real knowledge was first appreciated by Bacon of Verulam, whose Novum Organon (1620) laid down the principles of scientific knowledge, in opposition to the current scholasticism derived from Aristotle and his Organon. Bacon became the founder of modern empirical investigation, not only by making careful and exact observation of phenomena the basis of all philosophy, but also in demanding the supplementing of this by experiment; by this experiment he understood the putting of a question to Nature, as it were, which she must herself answer—a kind of observation under definite and deliberate conditions.

This more rigorous method of "exact observation," which is hardly three hundred years old, was very strongly aided by the inventions which enable the human eye to penetrate into the farthest abysses of space and the profoundest depths of smaller bodies—the telescope and microscope. The great improvement in these instruments during the nineteenth century, and the support given by other recent inventions, have led to triumphs of observation in this "century of science" that surpassed all anticipation. However, this very refinement of the technique of observation has its drawbacks, and has led to many an error. The effort to obtain the utmost accuracy in objective observation has often led to a neglect of the part which is played by the subjective mental action of the observer; his judgment and reason have been depreciated in comparison with the acuteness and clearness of his vision. Frequently the means has been turned into the end of knowledge. In the reproduction of the thing observed the objective photograph, presenting all parts of the object with equal plainness, has been more valued than the subjective design that reproduces only what is essential and leaves out what is superfluous; yet the latter is in many cases (for instance, in histological observation) much more important and correct than the former. But the greatest fault has been that many of these "exact" observers have refrained altogether from reflection and judgment on the phenomena observed, and have neglected subjective criticism; hence it is that so often a number of observers of the same phenomenon contradict each other, while each one boasts of the "exactness" of his observations.

Like observation, experimentation has been wonderfully improved of late years. The experimental sciences which make most use of it—experimental physics, chemistry, physiology, pathology, etc.—have made astounding progress. But it is just as important in the case of experiment—or observation under artificial conditions—as of simple observation that it be undertaken and carried out with a sound and clear judgment. Nature can only give a correct and unambiguous answer to the question you put it when it is clearly and distinctly proposed. This is very often not the case, and the experimenter loses himself in meaningless efforts, with the foolish hope that "something may come of it." The modern province of experimental or mechanical embryology is especially marred by these useless and perverse experiments. Equally foolish is the conduct of those biologists who would transfer the experiment that is valuable in physiology to the field of anatomy, where it is rarely profitable. In the modern controversy about evolution the attempt is frequently made to prove or refute experimentally the origin of species. It is quite forgotten that the idea of species is only relative, and that no man of science can give an absolute definition of it. Nor is it less perverse to attempt to apply experimentation to historical problems where all the conditions for a successful application are lacking.

The knowledge which we obtain directly by observation and experiment is only sound when it refers to present events. We have to turn to other methods for the investigation of the past—to history and traditions; and these are less easily accessible. This branch of science has been investigated for thousands of years, as far as the history of man and civilization, of peoples and states, and their customs, laws, languages, and migrations, is concerned. In this, the oral and written tradition from generation to generation, the ancient monuments, and documents, and weapons, etc., furnish an abounding empirical material from which critical judgment can draw a host of conclusions. However, the door to error lies wide open here, as the documents are usually imperfect, and the subjective interpretation of them is often no clearer than their objective validity.

Natural history, properly so called, or the study of the origin and past history of the universe, the earth, and its organic population, is much more recent than the history of mankind. Immanuel Kant was the first to lay the foundations of a mechanical cosmogony in his remarkable Natural History of the Heavens (1755), and Laplace gave mathematical shape to his ideas in 1796. Geology, also, or the story of the evolution of the earth, was not founded until the beginning of the eighteenth century, and did not assume a definite shape until the time of Hoff and Lyell (1830). Later still (1866) were laid the foundations of the science of organic evolution, when Darwin provided a sound foundation, in his theory of selection, for the theory of descent which Lamarck had proposed fifty years before.

In sharp contrast to this purely empirical method, which is favored by most men of science in our day, we have the purely speculative tendency which is current among our academic philosophers. The great regard which the critical philosophy of Immanuel Kant obtained during the nineteenth century has recently been increased in the various schools of philosophy. As is known, Kant affirmed that only a part of our knowledge is empirical, or a posteriori—that is, derived from experience; and that the rest of our knowledge (as, for instance, mathematical axioms) is a priori—that is to say, reached by the deductions of pure reason, independently of experience. This error led to the further statement that the foundations of science are metaphysical, and that, though man can attain a certain knowledge of phenomena by the innate forms of space and time, he cannot grasp the "thing in itself" that lies behind them. The purely speculative metaphysics which was built up on Kant's apriorism, and which found its extreme representative in Hegel, came at length to reject the empirical method altogether, and insisted that all knowledge is obtained by pure reason, independently of experience.

Kant's chief error, which proved so injurious to the whole of subsequent philosophy, lay in the absence of any physiological and phylogenetic base to his theory of knowledge; this was only provided sixty years after his death by Darwin's reform of the science of evolution, and by the discoveries of cerebral physiologists. He regarded the human mind, with its innate quality of reason, as a completely formed entity from the first, and made no inquiry into its historical development. Hence, he defended its immortality as a practical postulate, incapable of proof; he had no suspicion of the evolution of man's soul from that of the nearest related mammals. The curious predisposition to a priori knowledge is really the effect of the inheritance of certain structures of the brain, which have been formed in man's vertebrate ancestors slowly and gradually, by adaptation to an association of experiences, and therefore of a posteriori knowledge. Even the absolutely certain truths of mathematics and physics, which Kant described as synthetic judgments a priori, were originally attained by the phyletic development of the judgment, and may be reduced to constantly repeated experiences and a priori conclusions derived therefrom. The "necessity" which Kant considered to be a special feature of these a priori propositions would be found in all other judgments if we were fully acquainted with the phenomena and their conditions.

Among the censures which the academic metaphysicians, especially in Germany, have passed on my Riddle of the Universe, the heaviest is perhaps the charge that I know nothing whatever about the theory of knowledge. The charge is correct to this extent, that I do not understand the current dualistic theory of knowledge which is based on Kant's metaphysics; I cannot understand how their introspective psychological methods—disdaining all physiological, histological, or phylogenetic foundations—can satisfy the demands of "pure reason." My monistic theory of knowledge is assuredly very different from this. It is firmly and thoroughly based on the splendid advances of modern physiology, histology, and phytogeny—on the remarkable results of these empirical sciences in the last forty years, which are entirely ignored by the prevailing system of metaphysics. It is on the ground of these experiences that I have adopted the views on the nature of the human mind which are expounded in the second part of The Riddle of the Universe (chapters vi.-xi.). The following are the chief points:

1. The soul of man is—objectively considered—essentially similar to that of all other vertebrates; it is the physiological action or function of the brain.

2. Like the functions of all other organs, those of the brain are effected by the cells, which make up the organ.

3. These brain-cells, which are also known as soul-cells, ganglionic cells, or neurona, are real nucleated cells of a very elaborate structure.

4. The arrangement and grouping of these psychic cells, the number of which runs into millions in the brain of man and the other mammals, is strictly regulated by law, and is distinguished within this highest class of the vertebrates by several characteristics, which can only be explained by the common origin of the mammals from one primitive mammal (or pro-mammal of the Triassic period).

5. Those groups of psychic cells which we must regard as the agents of the higher mental functions have their origin in 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 it as the most perfect piece of cell-machinery and the highest product of organic evolution. Millions of highly differentiated phronetal cells form the several stations of this telegraphic system, and thousands of millions of the finest nerve-fibrils represent the wires which connect the stations with one another and with the sense-centres on the one hand, and with the motor-centres on the other. Comparative anatomy, moreover, acquaints us with the long and gradual development which the phronema has undergone within the higher class of the vertebrates, from the amphibia and reptiles up to the birds and mammals, and, within the last class, from the monotremes and marsupials up to the apes and men. The human brain seems to us to-day to be the greatest marvel that plasm, or the "living substance," has produced in the course of millions of years.

The remarkable progress which has been made in the last few decades in the anatomic and histological investigation of the brain does not yet, it is true, enable us to make a clear delimitation of the region of the phronema and its relations to the neighboring sensory and motor spheres in the cortex. We must, in fact, assume that there is no sharp distinction in the lower stages of the vertebrate soul; in the older and phylogenetically more distant stages they were not yet differentiated. Even now there are still intermediaries between the æsthetal and phronetal cells. But we may expect with confidence that further progress in the comparative anatomy of the brain will, with the aid of embryology, throw more and more light on these complicated structures. In any case, the fundamental fact is now empirically established that the phronema (the real organ of the soul) forms a definite part of the cortex of the brain, and that without it there can be no reason, no mental life, no thought, and no knowledge.

Since we regard psychology as a branch of physiology, and examine the whole of the phenomena of mental life from the same monistic stand-point as all other vital functions, it follows that we can make no exception for knowledge and reason. In this we are diametrically opposed to the current systems of psychology, which regard psychology, not as a natural science, but as a mental science. In the next chapter we shall see that this position is wholly unjustified. Unfortunately, this dualistic attitude is shared by a number of distinguished modern physiologists, who otherwise adopt the monistic principles; they take the soul to be, in the Cartesian sense, a supernatural entity. Descartes—a pupil of the Jesuits—only applied his theory to man, and regarded animals as soulless automata. But the theory is quite absurd in modern physiologists, who know from innumerable observations and experiments that the brain, or psychic organ, in man behaves just as it does in the other mammals, and especially the primates. This paradoxical dualism of some of our modern physiologists may be partly explained by the perverse theory of knowledge which the great authority of Kant, Hegel, etc., has imposed on them; and partly by a concern for the current belief in immortality, and the dread of being decried as "materialists" if they abandon it. As I do not share this belief, I examine and appreciate the physiological work of the phroneta just as impartially as I deal with the organs of sense or the muscles. I find that the one is just as much subject as the other to the law of substance. Hence we must regard the chemical processes in the ganglionic cells of the cortex as the real factors of knowledge and all other psychic action. The chemistry of the neuroplasm determines the vital function of the phronema. The same must be said of its most perfect and enigmatic function, consciousness. Although this greatest wonder of life is only directly accessible by the introspective method, or by the mirroring of knowledge in knowledge, nevertheless the use of the comparative method in psychology leads us to believe confidently that the lofty self-consciousness of man differs only in degree, and not in kind, from that of the ape, dog, horse, and other higher mammals.

Our monistic conception of the nature and seat of the soul is strongly confirmed by psychiatry, or the science of mental disease. As an old medical maxim runs, Pathologia physiologiam illustrat—the science of disease throws light on the sound organism. This maxim is especially applicable to mental diseases, for they can all be traced to modifications of parts of the brain which discharge definite functions in the normal state. The localization of the disease in a definite part of the phronema diminishes or extinguishes the normal mental function which is discharged by this section. Thus disease of the speech-centre, in the third frontal convolution, destroys the power of speech; the destruction of the visual region (in the occipital convolutions) does away with the power of sight; the lesion of the temporal convolutions destroys hearing. Nature herself here conducts delicate experiments which the physiologist could only accomplish very imperfectly or not at all. And although we have in this way only succeeded as yet in showing the functional dependence of a certain part of the mental functions on the respective parts of the cerebrum, no unprejudiced physician doubts to-day that it is equally true of the other parts. Each special mental activity is determined by the normal constitution of the relevant part of the brain, a section of the phronema. Very striking examples of this are afforded in the case of idiots and microcephali, the unfortunate beings whose cerebrum is more or less stunted, and who have accordingly to remain throughout life at a low stage of mental capacity. These poor creatures would be in a very pitiable condition if they had a clear consciousness of it, but that is not the case. They are like vertebrates from which the cerebrum has been partly or wholly removed in the laboratory. These may live for a long time, be artificially fed, and execute automatic or reflex (and in part purposive) motions, without our perceiving a trace of consciousness, reason, or other mental function in them.

The embryology of the child-soul has been known in a general way for thousands of years, and has been an object of keen interest to all observant parents and teachers; but it was not until about twenty years ago that a strictly scientific study was made of this remarkable and important phenomenon. In 1884 Kussmaul published his Untersuchungen über das Seelenleben des neugeborenen Menschen, and in 1882 W. Preyer published his Mind of the Child [English translation; Dr. J. Sully has several works on the same subject]. From the careful manuals which these and other observers have published, it is clear that the new-born infant not only has no reason or consciousness, but is also deaf, and only gradually develops its sense and thought-centres. It is only by gradual contact with the outer world that these functions successively appear, such as speech, laughing, etc.; later still come the power of association, the forming of concepts and words, etc. Recent anatomic observations quite accord with these physiological facts. Taken together, they convince us that the phronema is undeveloped in the new-born infant; and so we can no more speak in this case of a "seat of the soul" than of a "human spirit" as a centre of thought, knowledge, and consciousness. Hence the destruction of abnormal new-born infants—as the Spartans practised it, for instance, in selecting the bravest—cannot rationally be classed as "murder," as is done in even modern legal works. We ought rather to look upon it as a practice of advantage both to the infants destroyed and to the community. As the whole course of embryology is, according to our biogenetic law, an abbreviated repetition of the history of the race, we must say the same of psychogenesis, or the development of the "soul" and its organ—the phronema.

Comparative psychology comes next in importance to embryology as a means of studying the ancestral history of the soul. Within the ranks of the vertebrates we find to-day a long series of evolutionary stages which reach up from the lowest acrania and cyclostoma to the fishes and dipneusta, from these to the amphibia, and from these again to the amniota. Among the latter, moreover, the various orders of reptiles and birds on the one hand, and of mammals on the other, show us how the higher psychic powers have been developed step by step from the lower. To this physiological scale corresponds exactly the morphological gradation revealed by the comparative anatomy of the brain. The most interesting and important part of this is that which relates to the highest developed class—the mammals; within this class we find the same ever-advancing gradation. At its summit are the primates (man, the apes, and the half-apes), then the carnivora, a part of the ungulates, and the other placentals. A wide interval seems to separate these intelligent mammals from the lower placentals, the marsupials and monotremes. We do not find in the latter the high quantitative and qualitative development of the phronema which we have in the former; yet we find every intermediate stage between the two. The gradual development of the cerebrum and its chief part—the phronema—took place during the Tertiary period, the duration of which is estimated by many recent geologists at from twelve to fifteen (at the least three to five) million years.

As I have gone somewhat fully, in chapters vi.-ix. of the Riddle, into the chief results of the modern study of the brain and its radical importance for psychology 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