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Psychology has a long past, yet its real history is short. For thousands of years it has existed and has been growing older; but in the earlier part of this period it cannot boast of any continuous progress toward a riper and richer development. In the fourth century before our era that giant thinker, Aristotle, built it up into an edifice comparing very favorably with any other science of that time. But this edifice stood without undergoing any noteworthy changes or extensions, well into the eighteenth or even the nineteenth century. Only in recent times do we find an advance, at first slow but later increasing in rapidity, in the development of psychology.

The general causes which checked the progress of this science and thus made it fall behind the others can readily be stated:—

“The boundaries of the Soul you cannot find, though you pace off all its streets, so deep a foundation has it,” runs a sentence of Heraclitus, and it hits the truth more fully than its author could ever have expected. The structures and functions of our mental life present the greatest difficulties to scientific investigation, greater even than those presented by the phenomena, in many respects similar, of the bodily life of the higher organisms. These structures and processes change so unceasingly, are so fleeting, so enormously complex, and dependent on so many factors hidden yet undoubtedly influential, that it is difficult even to seize upon them and describe their true substance, still more difficult to gain an insight into their causal connections and to understand their significance. We are just now beginning to recognize the full force of these difficulties. Wherever in recent years research in any of the many branches of psychology has made any considerable advance,—as in vision, audition, memory, judgment,—the first conclusion reached by all investigators has been, that matters are incomparably finer and richer and fuller of meaning than even a keen fancy would previously have been able to imagine.

There is, besides, a second obstacle. However difficult it may be to investigate the nature and causal connections of mental phenomena, everybody has a superficial knowledge of their external manifestations. Long before these phenomena were considered scientifically, it was necessary for practical human intercourse and for the understanding of human character, that language should give names to the most important mental complexes occurring in the various situations of daily life, such as judgment, attention, imagination, passion, conscience, and so forth; and we are constantly using these names as if everybody understood them perfectly. What is customary and commonplace comes to be self-evident to us and is quietly accepted; it arouses no wonder at its strangeness, no curiosity which might lead us to examine it more closely. Popular psychology remains unconscious of the fact that there are mysteries and problems in these complexes. It loses sight of the complications because of the simplicity of the names. When it has arranged the mental phenomena in any particular case under the familiar designations, and has perhaps said that some one has “paid attention,” or has “given free rein to his imagination,” it considers the whole matter explained and the subject closed.

Still a third condition has retarded the advance of psychology, and will probably long continue to do so. Toward some of its weightiest problems it is almost impossible for us to be open-minded; we take too much practical interest in arriving at one answer rather than the other. King Frederick William I was not the only person who could be persuaded of the danger of the doctrine that every mental condition is governed by fixed law, and that in consequence all of our actions are fully determined—a doctrine fundamental to serious psychological research. He believed that such a teaching undermined the foundations of order in state and army, and that according to it he would no longer be justified in punishing deserters from his tall grenadiers. There are even to-day numerous thinkers who brand such a doctrine dangerous. They believe that it destroys all possibility of punishment and reward, makes all education, admonition, and advice meaningless, paralyzes our action, and must because of all these consequences be rejected.

In a similar way the discussion of other fundamental questions, such as the real nature of mind, the relation of mind and body in life and death, becomes prejudiced and confused on account of their connection with the deepest-rooted sentiments and longings of the human race. In recent years this has been the case especially in connection with the question of the evolution of mental life from its lower forms in the animals to its higher in man. What ought to be taught and investigated on its own merits as pure scientific theory, as the probable meaning of experienced facts, comes to be a matter of belief and good character, or is considered a sign of courageous independence of spirit and superiority to superstition and traditional prejudice. All of this is quite comprehensible when we consider the enormous practical importance of the questions at issue. Yet such an attitude will scarcely be of much help in finding answers most correct from a purely objective standpoint; it rather discourages the advance of research along definite lines.

Nevertheless, as we have stated in the beginning, psychology has now entered upon a positive development. What favorable circumstances have made it possible to overcome, at least in part, the peculiar opposing difficulties?

There are many; but in the end they all lead back to one: the rise and progress of natural science since the sixteenth century. However, this has made itself felt in two quite different ways; the force of the first wave was increased to its full magnitude by a closely following second wave. First, natural science served—if we overlook the hasty identification of mind and matter which had its origin in natural science—as a shining and fruitful example to psychology. It suggested conceptions of mental life analogous to those conceptions which had been found to make material processes comprehensible. It led to attempts at employing methods similar to those which had proved valuable in natural science. This influence was especially active in the seventeenth and eighteenth centuries, and lasted into the nineteenth. Later a more direct influence began to make itself felt: an actual invasion by natural science of special provinces of psychology. Natural science, in the course of its further development, was led at many points into investigations which lay as well in the sphere of psychology as in its own prescribed paths. When it attacked them and worked out beautiful solutions for them, psychologists also received a strong impulse not to stand aside, but to take up those problems themselves and pursue them independently for their own quite different purposes. So it was in the nineteenth century, especially in its second half.

Let us discuss more in detail a few particular results of this twofold general influence.

As the first important fruit of that indirect advancement through analogy, may be instanced the idea of the absolute and inevitable subjection to law of all mental processes, which I have just said forms the foundation of all serious psychological work. This was a familiar idea as far back as the later period of ancient philosophy, but was afterwards repudiated by the theological representatives of philosophy and psychology in the Middle Ages. To be sure, they always felt more or less attracted toward this view on account of the doctrine of the omnipotence and omniscience of God. For if God is almighty, then there can be no event in the future, either in the outer world or in the heart of man, which does not depend entirely on him; and if he is also all-knowing, or if in the eternity of God the human differences of past and future altogether disappear, then the future must be already known to God, and in consequence be fixed unalterably. But in spite of this argument, these medieval thinkers felt bound to affirm a spiritual freedom (that is, a merely partial determination) under the pressure of popular psychological and ethical thought and in consequence of their contemplation of the holiness and justice of God. For how could God have willed the sinful deeds of man, or have caused them, even indirectly? Or how could he punish men for doing things which they were compelled to do by unalterable laws which he himself had made? Although, so it was argued, man had his origin in God, he was nevertheless not absolutely bound by the divine within him; he could turn away from it voluntarily, that is, causelessly.

The influence of the rising natural science led to the opposite answer to the question as to whether the basis of our responsibility is spiritual freedom or universal causation. Hobbes and Spinoza became the champions of universal causation, presenting their answer to the question with a clearness and incisiveness imposing even to-day. Leibniz too adopted it, but took care not to offend those holding to the other view. It has never been lost again from psychology. These men teach that the phenomena of the mental life are in one respect exactly like those of external nature, with which they are indeed closely connected: at any moment they are definitely fixed through their causes, and cannot be otherwise than as we actually find them. Freedom of action in the sense of causelessness is an empty concept. It follows from this that one can properly mean by freedom of action only that there is no compulsion from without, that the action of a thing or being is determined only by its own nature, its own indwelling properties. We say of water that it flows along freely if it is not checked by rocks or dams; or of a horse, that it runs about freely, if it is not tied up or locked in a stall. We can in this sense call the good deeds of a person or his living together with other people his own free action, if it springs from his own deliberations and desires and is not coerced by force or threats. Nevertheless all these manifestations, the flowing, the running about, and also the good actions, are alike the regular effects of definite causes.

What constantly prevents men from recognizing this causality and leads them to a belief in a misinterpreted freedom, is solely their ignorance. Out of the multitude of motives for their actions they see, in most cases, only a single one; and if the action which takes place does not correspond with it, they are convinced that the decision occurred without cause. “A top,” says Hobbes, “which is spun by boys and runs about, first towards one wall then towards another, would think, if it perceived its own motion, that it moved about by the exercise of its own will, unless it happened to know what was spinning it.” In the same way people apply for a job or try to make a bargain and think that they do this by their own wills; they do not see the whips by which their wills are driven. In order to understand correctly the thoughts and impulses of man, we must treat them just as we treat material bodies, or as we treat the lines and points of mathematics. The pretended dangers of such a conception of things disappear, as soon as we face them without prejudice and try to understand them. The conception may be misused, especially by people of immature mind, but “for whatever purpose truth may be used, true still remains true,” and the question is not, “what is fit to be preached, but what is true.”

Supported by this view of a universal determination of mental activity, there has arisen the idea of a special determination, likewise copied from natural science. The coming and going of our thoughts is ordinarily considered as an unregulated play, defying calculation. That order rules even here, that the train of thought is governed by similarity to the mental states just present, or by a previous connection with these mental states, was clearly recognized and expressed even in the times of Plato and Aristotle. Yet this had remained merely the knowledge of a curiosity; no theoretical use whatever was made of it. Now it was brought into connection with newly recognized physical facts. This determination of the trains of thought depends, according to Hobbes, on the fact that our ideas are connected with material movements within the nerves and other organs, and that these movements, when once started, cannot immediately cease, but must gradually be consumed by resistance. The laws of association are to him in the spiritual sphere, what the law of inertia is in the physical. To Hume, a hundred years later, they depend on a kind of attraction, an idea suggested by Newton’s law of gravitation. And since inertia and attraction had been recognized as the most important and fundamental causes of material processes, it was a natural thing to regard the laws of association, which had been compared with them, as the fundamental phenomena of mental life, and to derive from them as manifold and important consequences as had been done in the case of the physical world. So arose the English associational psychology. It attempted to explain the traditional faculties of the mind, such as memory, imagination, judgment, and also the results of their combined activity (for instance, the consciousness of self and of the outer world) as natural and, so to speak, mechanical effects of the laws of association governing the processes of mind. No doubt this attempt, appearing also in a somewhat different form in the sensationalism of France, represents, in spite of its one-sidedness, a very great advance over the psychology of the past.

Just as associationism corresponds to the explanatory natural science of Galileo and Newton, the empirical psychology of the German enlightenment corresponds to the descriptive science of Linnæus and Buffon. But aside from a few exceptions, such as Tetens, its work must be regarded as a failure. To be sure, its intention is also to explain mental phenomena, to comprehend them first by careful introspection, and then to find by analysis the simplest faculties from which they have sprung. But its actual accomplishment does not go beyond a mere description of the occurrences offering themselves to first observation. And the results reached teach impressively that description is an unfruitful task unless, as sometimes of late, it is made to include also explanation. The numerous different expressions of mind, already distinguished by popular psychology, are only arranged in certain groups beside and above each other, and the explanation consists in presenting each expression as the effect of a special faculty. Thus we obtain a great multitude of complicated mental performances, inwardly related to each other, which are made to stand on a footing of equality and perfect independence, for example, perception, judgment, reason, imagination, and also abstraction, wit, symbolism, and so on. Like mere little homunculi in the large homo, they act now in harmony, now in opposition. The poetic faculty, for example, “is a coöperation of imagination with judgment.” In connection with reason, imagination produces foresight. “Wit often does harm to judgment, and leads it to false verdicts.... Judgment must therefore be constantly on its guard against wit.” The advancement in this case did not result from a development of these views, but from their overthrow. But the opposition raised was turned also against associationism.

Of the defects of associationism this is the greatest: it gives no explanation of the phenomenon of attention. The peculiar fact that of a great number of conscious impressions or ideas simultaneously offered to the mind, only a few can ever be carried through and become effective, is not to be explained on the basis of the associative connection of ideas. The associationists pass over this important fact either with complete silence or with a very insufficient treatment, and thus put a weapon into the hands of their opponents. The mind seems, in fact, in the case of attention to mock at all attempts at explanation and to prove itself, quite in the sense of the popular conception, a reality separable from its own contents—standing face to face with them, and treating them capriciously now in one way, now in another.

It is the chief service of Herbart to have recognized a weak point here, and to have attempted to remedy it. “The regularity of the mental life,” he is convinced, “is fully equal to that of the movements of the stars.” Physical analogies guide him in his attempt at explanation. He regards ideas as mutually repellent structures, or, as it were, elastic bodies, assigned to a space of limited capacity, forced together and made smaller by mutual pressure, but never annihilating each other. If several ideas are simultaneously called forth, they become conflicting forces, on account of the unity of the mind, in which they are compelled to be together, and on account of the opposition which exists among them. In this struggle their clearness suffers and their influence on consciousness is impaired. However, they do not perish, but become, to the extent that they suffer, latent forces.

As soon as the opposing factors lose their strength these latent forces emerge again into full consciousness out of the obscurity in which they have been buried. After making some further simple assumptions as to the strength of these interferences, Herbart concludes that two ideas are sufficient to crowd a third completely out of consciousness. To his great satisfaction he thus gains from the consideration of a simple mechanism “a solution of the most general of all psychological problems.” By this problem he means the fact that of all the knowing, thinking, wishing, which at any moment might be brought about by the proper causes, only a very small part plays a significant rôle, while the rest is not really lost. That is, he means the fact of attention. But this principle of the mutual interference of ideas is not the only one he uses. The second principle upon which his theory is based is that of association. With these two weapons he takes up the fight against the faculty psychology, and carries it to a successful end. He believes that all those activities traditionally placed side by side, even feeling and desire, can be made comprehensible as results of the mechanics of ideas.

Yet Herbart seeks by still another means to “bring about a mental science similar to the natural science: ... by quantitative methods and the application of mathematics.” We find here and there before this time the idea of advancing psychology by such means. The brilliant results produced in natural science by measurement and calculation readily suggested the idea that something similar might be done for psychology. But the philosophical thinkers interested in psychology did not find the right tools; they justified their inability by asserting that such an undertaking was impossible. The most famous is the denial by Kant that mathematics can be applied to the inner mental life and its laws, because time, within which the mental phenomena would have to be represented as occurring, has but one dimension. To be sure Herbart is not actually the pioneer in this field: he never gave a single example of how a measurement of a mental process was to be taken. However, he at least recognized that the mental life is open to quantitative treatment, not only with regard to time, but also in other respects. And in attempting to solve problems quantitatively, through the statement of numerical assumptions and their logical development to their consequences, he so strongly emphasized a side of the matter which had previously been wholly neglected, that more correct ways of clearing it up were soon found.

A strong and enduring influence was exerted by Herbart, yet the further progress of psychology did not occur along the path marked out by him. Many of his general assumptions, particularly those upon which his calculations are based, were entirely too vague to appear probable merely because a few of their consequences agreed with experience. Besides, a strong opposition had arisen against the intellectualism supported by him and by the associationists,—against the almost exclusive regard for the thinking and knowing activities of the mind. If mental life is really nothing but a machinery of ideas, a coöperation and opposition of masses of ideas, what is such a thing as religion? Is it a small complex of true and rational ideas, to which is added a large complex of superstitious fables, invented, or at any rate cultivated, by priests and princes, in order to keep men under their authority? So low a valuation of religion is scarcely possible. Or, what is art? Are the lyric poems of Goethe or the symphonies of Beethoven really only institutions for the conveyance of knowledge through the senses, as the name esthetics indicates, or for the unsuspected instilling of ideas which make men more virtuous or more patriotic?

Certainly one thing which stands in the center of all mental life seems entirely incomprehensible as the result of a mere mechanics of ideas, that is, that unity of mind without which we could not speak of personality, of character, of individuality, without which we could not call one man haughty and another humble, one good and another bad, one noble and another base. Because of this weakness in the theory numerous great thinkers, Rousseau, Kant, Fichte, Schopenhauer, raised their voices to insist upon the significance of the life of feeling and will as well as of the life of ideas, even to give to the former the first place, as the expression of mind’s most real inner being. Thus intellectualism was opposed by what we now call voluntarism.

This transferring of the conceptions of natural science to psychological research, in spite of the mighty impulse it gave to psychology, was not without its disadvantage. The first brilliant advances in natural science were in the province of physics, especially of mechanics. It is no wonder, then, that psychologists, in their gropings after something similar, turned first to mechanical-physical processes. Inertia, attraction, and repulsion, as we have seen, aggregation and chemical combination, were the categories with which they worked. No wonder, either, that facts were often distorted and their comprehension made difficult. For if mind is a machine, it is certainly not such a machine as even the most ingeniously constructed clock or as a galvanic battery. It is bound up with the organic body, especially with the nervous system, and on the structure and functions of the nervous system its own existence and activity somehow depend. So, if one wishes to use material analogies and to make them fruitful for the comprehension of mental structures, they must be taken from organic life, from biology rather than from physics and chemistry. We may find phenomena comparable to individuality and character, to the mind’s feeling and willing, in the unitary existence of every plant and animal organism, in the peculiar determination of its instinct of life and in the many special branches into which this instinct ceaselessly unfolds. And indeed the specifically mechanical categories gradually disappeared from psychology during the nineteenth century, and made way for the biological categories—reflex, inhibition, practice, assimilation, adaptation, and so on. Especially that great acquisition of modern biology, the theory of evolution, was at once seized upon by psychologists, and was utilized for gaining an understanding of the processes as well in the mind of the individual as in human society.

But side by side with such advances, springing from analogy and adaptation, there arose in the nineteenth century another and more direct influence of natural science, as previously mentioned. In its natural progress scientific research came to touch upon psychological problems at several points, and since it laid hold of them and followed them out for its own ends, it immediately became a pioneer for psychology.

The first and at the same time the strongest of these impulses came from the advance of the physiology of the senses. In the fourth decade of the nineteenth century remarkably active and fruitful work in this field began. Physiologists and physicists vied with each other in accurate study of the structure and functions of sense organs. Naturally they were not able to stop at the material functions in which they were most directly interested. They could not forbear to draw into the circle of their investigations those mental functions mediated by the physiological functions and explainable on a physiological basis. The eye, especially, attracted scores of investigators, both because it is very richly endowed with dioptric and mechanical auxiliary apparatus and because it is particularly important on account of the delicacy and diversity of its functions. Yet cutaneous sensations and hearing were not neglected.

Johannes Müller, E. H. Weber, Brewster, and above all—especially versatile, far-seeing, and inventive—the somewhat younger Helmholtz, are only a few of the most noteworthy representatives of this class of research. They brought to psychology results such as it had never known before—results resting on well-conceived and original questions as to the nature of things, and on skillful attempts at arranging the circumstances for an answer, that is, on experiment and when possible on exact measurement of the effects and their causes. When Weber in 1828 had the seemingly petty curiosity to want to know at what distances apart two touches on the skin could be just perceived as two, and later, with what accuracy he could distinguish between two weights laid on the hand, or how he could distinguish between the perception received through the muscles in lifting the weights and the perception received through the skin, his curiosity resulted in more real progress in psychology than all the combined distinctions, definitions, and classifications of the time from Aristotle to Hobbes. The surprising discovery of hitherto unknown sense organs, the muscles and the semicircular canals, was made at that time, although not thoroughly verified until later. That discovery meant not only an increase of knowledge, but also a widening of the horizon, since the most conspicuous peculiarity of these organs is that they do not, like the others, bring to our consciousness external stimuli in the ordinary sense, but processes on the inside of the body.

One result in particular of these investigations in the physiology of the senses became the starting point of a strong new movement. The course of biology in the second quarter of the nineteenth century was toward methodical and exact study of empirical facts, and away from speculation in the philosophy of nature. But for some time this exact study and this speculation were often to be found combined in the same men. Fechner was one of these. On the one hand he was a speculative philosopher, a follower of Schelling’s philosophy of nature, a disciple of Herbart in his attempt at applying mathematics to psychology. So we find him speculating as to what might be the exact relations between body and soul, seeking for a mathematical formulation of the dependence of the corresponding mental and nervous processes. One October morning in 1850, while lying in bed, he conceived a formula which seemed to him plausible. In spite of this speculative tendency he was a physicist of scientific exactness, accustomed to demand a support of facts for such plausible formulas, ready to attack problems not only with his mind, but also with his hands. In following up his speculations he came across some of the results of the work of Weber. By the use of more exact methods and by long-continued series of experiments he carried Weber’s investigations farther, at the same time utilizing the observations of others to which no one had before paid any attention. He succeeded in formulating the first mathematical law of mental life, Weber’s law as he called it, according to which an increase of the external stimulus in geometrical progression corresponds to the increase of the mental process in arithmetical progression. (We shall discuss this law in § 4.) He classed together all of his speculations, investigations, formulations, and conclusions as a new branch of knowledge, Psychophysics, “the scientific doctrine of the relations obtaining between body and mind.”

Fechner’s work called forth numberless books and articles, confirming, opposing, discussing it, or carrying its conclusions still further. The chief question which they discussed, the question whether the law formulated by Fechner was correct or not, has gradually lost its importance, and made way for other problems. Quite aside from this question, which originally formed the center of interest, Fechner’s work has made itself felt in three different ways. Herbart’s mathematical fiction of the combat among ideas had made such an impression upon the thinkers of the time, that—incredible as it may seem—as late as 1852 Lotze confessed that he would prefer it to formulas found by experiment. For this fiction Fechner substituted a scientific law derived from actual measurement of physical forces. Further, he gave to these facts their proper place in a broad system, showed their significance for the deepest psychological problems, and thus compelled even those psychologists who had affiliated themselves with philosophy and had previously remained unaffected by the physiology of the senses, to take notice of the new movement in their science. And finally, he worked out a methodical procedure for all psychophysical investigations, which was far superior to the methods then employed by psychologists and which continues to be of great use for the study of sensation and perception.

At about the same time, in the sixties, psychology received a third kind of impulse. Although weaker than the two just mentioned, it contributed not a little toward increasing the number of psychological problems to which experimental methods could be applied.

In the year 1796 the Reverend Nevil Maskelyne, director of the Greenwich observatory, noticed that the transits recorded by his assistant, Kinnebrook, showed a gradually increasing difference from his own, finally amounting to almost a full second. He suspected his assistant of having deviated from the prescribed method of observation, the so-called eye and ear method, and of having substituted some unreliable method of his own. He admonished the young man to return to the correct method and do better in the future. But his admonition was in vain, and he found himself obliged to part with his otherwise satisfactory assistant. Kinnebrook lost his position on account of the deficient psychological knowledge of his time. It was not until two decades later that Bessel discovered that such differences between the results of observations by different individuals were quite general and normal, and that in Kinnebrook’s case they were only unusually great. They depend on the manner of giving attention to both the sound of the pendulum and the sight of the moving star, which naturally differs in different individuals.

At first this question of the so-called personal equation remained a purely practical astronomical problem. But a few decades later it gave rise to two classes of investigations of psychological importance, both of the experimental kind. The first was an investigation of a comparatively simple problem—the duration of the mental processes. Among such processes measured were the simple perception, the discrimination of several perceptions, the simple reaction to them, the reproduction of any suggested idea, the reproduction of a specific suggested idea, and so forth. Not only was the duration of these processes studied, but also their dependence on differences of stimulation, the accompanying circumstances, the individual differences, the subject’s trend of thought. The second class of investigations was concerned with the more complex mental processes of attending and willing. As examples may be mentioned inquiries into the attention of a person confronted by a multitude of impressions, a study of the order in which the several impressions are perceived, a determination of the largest number of impressions perceptible as a mental unit, and research into the causal relations between ideas and actions.