The first task of every science is the clear definition of the object it has to investigate. In no science, however, is this preliminary task so difficult as in psychology; and this circumstance is the more remarkable since logic, the science of defining, is itself a part of psychology. When we compare all that has been said by the most distinguished philosophers and scientists of all ages on the fundamental idea of psychology, we find ourselves in a perfect chaos of contradictory notions. What, really, is the “soul”? What is its relation to the “mind”? What is the inner meaning of “consciousness”? What is the difference between “sensation” and “sentiment”? What is “instinct”? What is the meaning of “free will”? What is “presentation”? What is the difference between “intellect” and “reason”? What is the true nature of “emotion”? What is the relation between all these “psychic phenomena” and the “body”? The answers to these and many other cognate questions are infinitely varied; not only are the views of the most eminent thinkers on these questions widely divergent, but even the same scientific authority has often completely changed his views in the course of his psychological development. Indeed, this “psychological metamorphosis” of so many thinkers has contributed not a little to the colossal confusion of ideas which prevails in psychology more than in any other branch of knowledge.
The most interesting example of such an entire change of objective and subjective psychological opinions is found in the case of the most influential leader of German philosophy, Immanuel Kant. The young, severely critical Kant came to the conclusion that the three great buttresses of mysticism—“God, freedom, and immortality”—were untenable in the light of “pure reason”; the older, dogmatic Kant found that these three great hallucinations were postulates of “practical reason,” and were, as such, indispensable. The more the distinguished modern school of “Neokantians” urges a “return to Kant” as the only possible salvation from the frightful jumble of modern metaphysics, the more clearly do we perceive the undeniable and fatal contradiction between the fundamental opinions of the young and the older Kant. We shall return to this point later on.
Other interesting examples of this change of views are found in two of the most famous living scientists, R. Virchow and E. du Bois-Reymond; the metamorphoses of their fundamental views on psychology cannot be overlooked, as both these Berlin biologists have played a most important part at Germany’s greatest university for more than forty years, and have, therefore, directly and indirectly, had a most profound influence on the modern mind. Rudolph Virchow, the eminent founder of cellular pathology, was a pure monist in the best days of his scientific activity, about the middle of the century; he passed at that time as one of the most distinguished representatives of the newly awakened materialism, which appeared in 1855, especially through two famous works, almost contemporaneous in appearance—Ludwig Büchner’s Matter and Force and Carl Vogt’s Superstition and Science. Virchow published his general biological views on the vital processes in man—which he takes to be purely mechanical natural phenomena—in a series of distinguished papers in the first volumes of the Archiv für pathologische Anatomie, which he founded. The most important of these articles, and the one in which he most clearly expresses his monistic views of that period, is that on “The Tendencies Towards Unity in Scientific Medicine” (1849). It was certainly not without careful thought, and a conviction of its philosophic value, that Virchow put this “medical confession of faith” at the head of his Collected Essays on Scientific Medicine in 1856. He defended in it, clearly and definitely, the fundamental principles of monism, which I am presenting here with a view to the solution of the world-problem; he vindicated the exclusive title of empirical science, of which the only reliable sources are sense and brain activity; he vigorously attacked anthropological dualism, the alleged “revelation,” and the transcendental philosophy, with their two methods—“faith and anthropomorphism.” Above all, he emphasized the monistic character of anthropology, the inseparable connection of spirit and body, of force and matter. “I am convinced,” he exclaims, at the end of his preface, “that I shall never find myself compelled to deny the thesis of the unity of human nature.” Unhappily, this “conviction” proved to be a grave error. Twenty-eight years afterwards Virchow represented the diametrically opposite view; it is to be found in the famous speech on “The Liberty of Science in Modern States,” which he delivered at the Scientific Congress at Munich in 1877, and which contains attacks that I have repelled in my Free Science and Free Teaching (1878).
In Emil du Bois-Reymond we find similar contradictions with regard to the most important and fundamental theses of philosophy. The more completely the distinguished orator of the Berlin Academy had defended the main principles of the monistic philosophy, the more he had contributed to the refutation of vitalism and the transcendental view of life, so much the louder was the triumphant cry of our opponents when in 1872, in his famous Ignorabimus-Speech, he spoke of consciousness as an insoluble problem, and opposed it to the other functions of the brain as a supernatural phenomenon. I return to the point in the [tenth chapter].
The peculiar character of many of the psychic phenomena, especially of consciousness, necessitates certain modifications of our ordinary scientific methods. We have, for instance, to associate with the customary objective, external observation, the introspective method, the subjective, internal observation which scrutinizes our own personality in the mirror of consciousness. The majority of psychologists have started from this “certainty of the ego”: “Cogito ergo sum,” as Descartes said—I think, therefore I am. Let us first cast a glance at this way of inquiry, and then deal with the second, complementary, method.
By far the greater part of the theories of the soul which have been put forward during the last two thousand years or more are based on introspective inquiry—that is, on “self-observation,” and on the conclusions which we draw from the association and criticism of these subjective experiences. Introspection is the only possible method of inquiry for an important section of psychology, especially for the study of consciousness. Hence this cerebral function occupies a special position, and has been a more prolific source of philosophic error than any of the others (cf. [chap. x].). It is, however, most unsatisfactory, and it leads to entirely false or incomplete notions, to take this self-observation of the mind to be the chief, or, especially, to be the only source of mental science, as has happened in the case of many and distinguished philosophers. A great number of the principal psychic phenomena, particularly the activity of the senses and speech, can only be studied in the same way as every other vital function of the organism—that is, firstly, by a thorough anatomical study of their organs, and, secondly, by an exact physiological analysis of the functions which depend on them. In order, however, to complete this external study of the mental life, and to supplement the results of internal observation, one needs a thorough knowledge of human anatomy, histology, ontogeny, and physiology. Most of our so-called “psychologists” have little or no knowledge of these indispensable foundations of anthropology; they are, therefore, incompetent to pronounce on the character even of their own “soul.” It must be remembered, too, that the distinguished personality of one of these psychologists usually offers a specimen of an educated mind of the highest civilized races; it is the last link of a long ancestral chain, and the innumerable older and inferior links are indispensable for its proper understanding. Hence it is that most of the psychological literature of the day is so much waste paper. The introspective method is certainly extremely valuable and indispensable; still it needs the constant co-operation and assistance of the other methods.
In proportion as the various branches of the human tree of knowledge have developed during the century, and the methods of the different sciences have been perfected, the desire has grown to make them exact; that is, to make the study of phenomena as purely empirical as possible, and to formulate the laws that result as clearly as the circumstances permit—if possible, mathematically. The latter is, however, only feasible in a small province of human knowledge, especially in those sciences in which there is question of measurable quantities; in mathematics, in the first place, and to a greater or less extent in astronomy, mechanics, and a great part of physics and chemistry. Hence these studies are called “exact sciences” in the narrower sense. It is, however, productive only of error to call all the physical sciences exact, and oppose them to the historical, mental, and moral sciences. The greater part of physical science can no more be treated as an exact science than history can; this is especially true of biology and of its subsidiary branch, psychology. As psychology is a part of physiology, it must, as a general rule, follow the chief methods of that science. It must establish the facts of psychic activity by empirical methods as much as possible, by observation and experiment, and it must then gather the laws of the mind by inductive and deductive inferences from its observations, and formulate them with the utmost distinctness. But, for obvious reasons, it is rarely possible to formulate them mathematically. Such a procedure is only profitable in one section of the physiology of the senses; it is not practicable in the greater part of cerebral physiology.
One small section of physiology, which seems amenable to the “exact” method of investigation, has been carefully studied for the last twenty years and raised to the position of a separate science under the title of psycho-physics. Its founders, the physiologists Theodor Fechner and Ernst Heinrich Weber, first of all closely investigated the dependence of sensations on the external stimuli that act on the organs of sense, and particularly the quantitative relation between the strength of the stimulus and the intensity of the sensation. They found that a certain minimum strength of stimulus is requisite for the excitement of a sensation, and that a given stimulus must be varied to a definite amount before there is any perceptible change in the sensation. For the highest sensations (of sight, hearing, and pressure) the law holds good that their variations are proportionate to the changes in the strength of the stimulus. From this empirical “law of Weber” Fechner inferred, by mathematical operations, his “fundamental law of psycho-physics,” according to which the intensity of a sensation increases in arithmetical progression, the strength of the stimulus in geometrical progression. However, Fechner’s law and other psycho-physical laws are frequently contested, and their “exactness” is called into question. In any case modern psycho-physics has fallen far short of the great hopes with which it was greeted twenty years ago; the field of its applicability is extremely limited. One important result of its work is that it has proved the application of physical laws in one, if only a small, branch of the life of the “soul”—an application which was long ago postulated on principle by the materialist psychology for the whole province of mental life. In this, as in many other branches of physiology, the “exact” method has proved inadequate and of little service. It is the ideal to aim at everywhere, but it is unattainable in most cases. Much more profitable are the comparative and genetic methods.
The striking resemblance of man’s psychic activity to that of the higher animals—especially our nearest relatives among the mammals—is a familiar fact. Most uncivilized races still make no material distinction between the two sets of mental processes, as the well-known animal fables, the old legends, and the idea of the transmigration of souls prove. Even most of the philosophers of classical antiquity shared the same conviction, and discovered no essential qualitative difference, but merely a quantitative one, between the soul of man and that of the brute. Plato himself, who was the first to draw a fundamental distinction between soul and body, made one and the same soul (or “idea”) pass through a number of animal and human bodies in his theory of metempsychosis. It was Christianity, intimately connecting faith in immortality with faith in God, that emphasized the essential difference of the immortal soul of man from the mortal soul of the brute. In the dualistic philosophy the idea prevailed principally through the influence of Descartes (1643); he contended that man alone had a true “soul,” and, consequently, sensation and free will, and that the animals were mere automata, or machines, without will or sensibility. Ever since the majority of psychologists—including even Kant—have entirely neglected the mental life of the brute, and restricted psychological research to man: human psychology, mainly introspective, dispensed with the fruitful comparative method, and so remained at that lower point of view which human morphology took before Cuvier raised it to the position of a “philosophic science” by the foundation of comparative anatomy.
Scientific interest in the psychic activity of the brute was revived in the second half of the last century, in connection with the advance of systematic zoology and physiology. A strong impulse was given to it by the work of Reimarus: “General observations on the instincts of animals” (Hamburg, 1760). At the same time a deeper scientific investigation had been facilitated by the thorough reform of physiology by Johannes Müller. This distinguished biologist, having a comprehensive knowledge of the whole field of organic nature, of morphology, and of physiology, introduced the “exact methods” of observation and experiment into the whole province of physiology, and, with consummate skill, combined them with the comparative methods. He applied them, not only to mental life in the broader sense (to speech, senses, and brain-action), but to all the other phenomena of life. The sixth book of his Manual of Human Physiology treats specially of the life of the soul, and contains eighty pages of important psychological observations.