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 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 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.
