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 interpretation of facts. The fact 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 this fact does modify the way in which the methods are applied.

The natural sciences may serve, because they matured earlier, as an example for psychology in the matter of method. These sciences make use of two chief methods, namely experiment and observation. Experiment is observation under the condition of purposive control by the observer of the rise and course of the phenomena observed. Observation, in the narrower sense of the term, is the investigation of phenomena without such control, the occurrences being accepted just as they are naturally presented to the observer in the course 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 the rise and progress of these phenomena, or to isolate the various components of a composite phenomenon. Yet, 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 control is required in the exact determination of the course, and in the analysis of the components, of any natural process, such for example as light vibration, sound vibration, an electric discharge, or the contraction of a muscle. As a rule such control 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 is possible only through the addition or subtraction of certain conditions, or through a quantitative variation of them. The case is different with objects of nature. They are relatively constant and are always at the observer's disposal and ready for examination. In dealing with such constant objects, experimental investigation is really necessary only when the production and modification of the objects are the subjects to be investigated. When, on the contrary, the only question is the actual nature of these objects, 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 which are, indeed, frequently brought into them, but which 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. 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 first of all control their beginnings, and we must also vary their conditions at will. This is possible here, as in all cases, only through experiment. Besides this general reason there is another reason which is peculiar to psychology, and does not apply at all to natural phenomena. In the case of the natural sciences we purposely abstract from the perceiving subject, and under circumstances, especially when favored by the phenomena, as in astronomy, mere observation may succeed in determining with adequate certainty the objective contents of the processes. Psychology, on the contrary, is debarred from this abstraction by its fundamental principles; and proper conditions for chance observation can appear 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. Such chance coincidence is especially improbable since the very intention to observe, which is a necessary condition of all exact investigation, modifies essentially the rise and progress of psychical processes. The chief problem of psychology, however, is the exact investigation of the rise and progress of subjective processes, and it can readily be seen that in such investigations the intention to observe either essentially modifies the facts to be observed, or completely suppresses them, at least if the observation is of the ordinary introspective type unaided by experimental devices of any sort. If, on the other hand, we consider the experimental methods, we see that psychology is led, through the very nature of the origin of the processes with which it deals, to employ, just as do physics and physiology, the experimental mode of procedure. A sensation arises under the most favorable conditions for observation when it is aroused by an external sense stimulus. The idea of an object is always produced originally by the more or less complicated cooperation of sense stimuli. If we wish to study the way in which an idea is formed, we can choose no method other than that of imitating this natural way in which an idea arises. 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, can not be directly aroused through external sense impressions, but follow these impressions after a longer or shorter interval. Yet, it is obvious that the attributes even of memory images can be most accurately 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 the investigation of which such methods are not logically required.

3. Pure observation, such as is possible in many departments of natural science, is, from the very character of psychical phenomena, impossible in individual psychology. The possibility of pure observation 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. There are, however, certain facts at the disposal of psychology, which, although they are not real objects, nevertheless, have the character of psychical objects, inasmuch as they possess the attributes of relative permanence and independence of the observer, and are unapproachable by means of experiment in the common acceptance of the term. These facts are the mental products which have 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 the objective attributes of the products. All such mental products of a general character presuppose 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 on the social community, the whole department of psychological investigation here involved is designated as social psychology, and is distinguished from individual psychology, or experimental psychology as it may be called because of its predominating method. In the present stage of the science these two branches of psychology are generally taken up in different treatises, although 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 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 of the sense-organs, which require special discussion with a view to the interests of psychology, though in themselves these facts 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, essentially 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).
 
 

References. For a general discussion of the methodology of psychology, see chapter on "Logik der Psychologie" in the author's Logik, 2nd ed., 1895. On methods of experimentation see Phil. Stud., vol. I. Also, sanford, A Course in Experimental Psychology, 1897—1898. E. B. Titchener, Experimental Psychology, A Manual of Laboratory Practice, 4 vols., 1900—1905. Sommer, Lehrbuch der psychopatholog. Untersuchungsmethoden, 1899.