1. Since psychology has for its object, not specific contents of experience, but general experience in its immediate character, it can make use of no methods except such as the empirical sciences in general employ for the determination, analysis, and causal synthesis of facts. The circumstance, that natural science abstracts from the subject, while psychology does not, can be no ground for modifications in the essential character of the methods employed in the two fields, though it does modify the way in which these methods are applied. [p. 19]

The natural sciences, which may serve as an example for psychology in this respect, since they were developed earlier, make use of two chief methods: experiment and observation. Experiment is observation connected with an intentional interference on the part of the observer, in the rise and course of the phenomena observed. Observation, in its proper sense, is the investigation of phenomena without such interference, just as they are naturally presented to the observer in the continuity of experience. Wherever experiment is possible, it is always used in the natural sciences; for under all circumstances, even when the phenomena in themselves present the conditions for sufficiently exact observation, it is an advantage to be able to control at will their rise and progress, or to isolate the various components of a composite phenomenon. Still, even in the natural sciences the two methods have been distinguished according to their spheres of application. It is held that the experimental methods are indispensable for certain problems, while in others the desired end may not infrequently be reached through mere observation. If we neglect a few exceptional cases due to special relations, these two classes of problems correspond to the general division of natural phenomena into processes and objects.

Experimental interference is required in the exact determination of the course, and in the analysis of the components, of any natural process such as, for example, light-waves or sound-waves, an electric discharge, the formation or disintegration of a chemical compound, and stimulation and metabolism in plants and animals. As a rule, such interference is desirable because exact observation is possible only when the observer can determine the moment at which the process shall commence. It is also indispensable in separating the various components of a complex phenomenon from one another. As a rule, this [p. 20] is possible only through the addition or subtraction of certain conditions, or a quantitative variation of them.

The case is different with objects of nature. They are relatively constant; they do not have to be produced at a particular moment, but are always at the observer's disposal and ready for examination. Here, then, experimental investigation is generally necessary only when the production and modification of the objects are to be inquired into. In such a case, they are regarded either as products or components of natural processes and come under the head of processes rather than objects. When, on the contrary, the only question is the actual nature of these objects, without reference to their origin or modification, mere observation is generally enough. Thus, mineralogy, botany, zoology, anatomy, and geography, are pure sciences of observation so long as they are kept free from the physical, chemical, and physiological problems that are, indeed, frequently brought into them, but have to do with processes of nature, not with the objects in themselves.

2. If we apply these considerations to psychology, it is obvious at once, from the very nature of its subject-matter, that exact observation is here possible only in the form of experimental observation; and that psychology can never be a pure science of observation. The contents of this science are exclusively processes, not permanent objects. In order to investigate with exactness the rise and progress of these processes, their composition out of various components, and the interrelations of these components, we must be able first of all to bring about their beginning at will, and purposely to vary the conditions of the same. This is possible here, as in all cases, only through experiment, not through pure introspection. Besides this general reason there is another, peculiar to psychology, that does not apply at all to natural [p. 21] phenomena. In the latter case we purposely abstract from the perceiving subject, and under circumstances, especially when favored by the regularity of the phenomena, as in astronomy, mere observation may succeed in determining with adequate certainty the objective components of the processes. Psychology, on the contrary, is debarred from this abstraction by its fundamental principles, and the conditions for chance observation can be suitable only when the same objective components of immediate experience are frequently repeated in connection with the same subjective states. It is hardly to be expected, in view of the great complexity of psychical processes, that this will ever be the case. The coincidence is especially improbable since the very intention to observe, which is a necessary condition of all observation, modifies essentially the rise and progress of psychical processes. Observation of nature is not disturbed by this intention on the part of the observer, because here we purposely abstract from the state of the subject. The chief problem of psychology, however, is the exact observation of the rise and progress of subjective processes, and it can be readily seen that under such circumstances the intention to observe either essentially modifies the facts to be observed, or completely suppresses them. On the other hand, psychology, by the very way in which psychical processes originate, is led, just as physics and physiology are, to employ the experimental mode of procedure. A sensation arises in us under the most favorable conditions for observation when it is caused by an external sense-stimulus, as, for example, a tone-sensation from an external tone-vibration, or a light-sensation from an external light-impression. The idea of an object is always caused originally by the more or less complicated cooperation of external sense-stimuli. If we wish to study the way in which an idea is formed, we can choose [p. 22] no other method than that of imitating this natural process. In doing this, we have at the same time the great advantage of being able to modify the idea itself by changing at will the combination of the impressions that cooperate to form it, and of thus learning what influence each single condition exercises on the product. Memory-images, it is true, cannot be directly aroused through external sense impressions, but follow them after a longer or shorter interval. Still, it is obvious that their attributes, and especially their relation to the primary ideas through direct impressions, can be most accurately be learned, not by waiting for their chance arrival, but by using such memory-ideas as may be aroused in a systematic, experimental way, through immediately preceding impressions. The same is true of feelings and volitions; they will be presented in the form best adapted to exact investigation when those impressions are purposely produced which experience has shown to be regularly connected with affective and volitional reactions. There is, then, no fundamental psychical process to which experimental methods can not be applied, and therefore none in whose investigation they are not logically required.

3. Pure observation, such as is possible in many departments of natural science, is, from the very character of psychic phenomena, impossible in individual psychology. Such a possibility would be conceivable only under the condition that there existed permanent psychical objects, independent of our attention, similar to the relatively permanent objects of nature, which remain unchanged by our observation of them. There are, indeed, certain facts at the disposal of psychology, which, although they are not real objects, still have the character of psychical objects inasmuch as they possess these attributes of relative permanence, and independence of the observer. Connected with these characteristics [p. 23] is the further fact that they are unapproachable by means of experiment in the common acceptance of the term. These facts are the mental products that have been developed in the course of history, such as language, mythological ideas, and customs. The origin and development of these products depend in every case on general psychical conditions which may be inferred from their objective attributes. Psychological analysis can, consequently, explain the psychical processes operative in their formation and development. All such mental products of a general character presuppose as a condition the existence of a mental community composed of many individuals, though, of course, their deepest sources are the psychical attributes of the individual. Because of this dependence on the community, in particular the social community, this whole department of psychological investigation is designated as social psychology, and distinguished from individual, or as it may be called because of its predominating method, experimental psychology. In the present stage of the science these two branches of psychology are generally taken up in different treatises; still, they are not so much different departments as different methods. So-called social psychology corresponds to the method of pure observation, the objects of observation in this case being the mental products. The necessary connection of these products with social communities, which has given to social psychology its name, is due to the fact that the mental products of the individual are of too variable a character to be the subjects of objective observation. The phenomena gain the necessary degree of constancy only when they become collective.

Thus psychology has, like natural science, two exact methods: the experimental method, serving for the analysis of simpler psychical processes, and the observation of general [p. 24] mental products, serving for the investigation of the higher psychical processes and developments.

3a. The introduction of the experimental method into psychology was originally due to the modes of procedure in physiology, especially in the physiology of the sense-organs and the nervous system. For this reason experimental psychology is also commonly called "physiological psychology"; and works treating it under this title regularly contain those supplementary facts from the physiology of the nervous system and the sense-organs, which require special discussion with a view to the interests of psychology, though in themselves they belong to physiology alone. "Physiological psychology" is, accordingly, an intermediate discipline which is, however, as the name indicates, primarily psychology, and is, apart from the supplementary physiological facts that it presents, just the same as "experimental psychology" in the sense above defined. The attempt sometimes made, to distinguish psychology proper from physiological psychology, by assigning to the first the psychological interpretation of inner experience, and to the second the derivation of this experience from physiological processes, is to be rejected as inadmissible. There is only one kind of causal explanation in psychology, and that is the derivation of more complex psychical processes from simpler ones. In this method of interpretation physiological elements can be used only as supplementary aids, because of the relation between natural science and psychology as above defined (§ 2, 4). Materialistic psychology denies the existence of psychical causality, and substitutes for this problem the other, of explaining psychical processes by brain-physiology. This tendency, which has been shown (§ 2, 10a) to be epistemologically and psychologically untenable, appears among the representatives of both "pure" and "physiological" psychology.