This view has influenced empirical psychology in two ways. First, it favored the opinion that psychology should employ empirical methods, but that these methods, like psychological experience, should be fundamentally different from those of natural science. Secondly, it gave rise to the necessity of showing some connection or other between these two kinds of experience, which were supposed to be different. In regard to the first demand, it was chiefly the psychology of the inner sense that developed the method of pure introspection (sec. 3, 2). In attempting to solve the second problem, this psychology was necessarily driven back to a metaphysical basis, because of its assumption of a difference between the physical and the psychical contents of experience. For, from the very nature of the case, it is impossible, to account for the relations of inner to outer experience, or the so­called "interaction between body and mind", from the position here taken, except through metaphysical presuppositions. These presuppositions must then, in turn, affect the psychological investigation itself in such a way as to result in the importation of metaphysical hypotheses into it.
4. Essentially distinct from the psychology of the inner sense is the form of psychology which defines itself as "the science of immediate experience".
Regarding, as it does, outer and inner experience, not as different parts of experience, but as different ways of looking at one and the same experience, this form of psychology can not admit any fundamental difference between the methods of psychology and those of natural science. It has, therefore, sought above all to cultivate experimental methods which shall lead to just such an exact analysis of psychical processes as that which the explanatory natural sciences undertake in the case of natural phenomena, the only differences being those which arise from the diverse points of view. It holds, also, that the special mental sciences which have to do with concrete mental processes and creations, stand on the same basis of a scientific consideration of the immediate contents of experience and of their relations to acting subjects. It follows, then, that psychological analysis of the most general mental products, such as language, mythological ideas, and laws of custom, is to be regarded as an aid to the understanding of all the more complicated psychical processes. In its methods, accordingly, this form of psychology stands in close relation to other sciences: as experimental psychology, to the natural sciences; as social psychology, to the special mental sciences.
Finally, from this point of view, the question of the relation between psychical and physical objects disappears entirely. They are not different objects at all, but one and the same content of experience, looked at in one case -- that of the natural sciences -- after abstracting from the subject, in the other -- that of psychology -- in their immediate character and complete relation to the subject. All metaphysical hypotheses as to the relation of psychical and physical objects are, when viewed from this position, attempts to solve a problem which never would have existed if the case had been correctly stated. Though psychology must then dispense with metaphysical supplementary hypotheses in regard to the interconnection of psychical processes, because these processes are the immediate contents of experience, still another method of procedure, however, is open since inner and outer experience are supplementary points of view. Wherever breaks appear in the interconnection of psychical processes, it is allowable to carry on the investigation according to the physical methods of considering these same processes, in order to discover whether the absent link can be thus supplied. The same holds for the reverse method of filling up the breaks in the continuity of our physiological knowledge, by means of elements derived from psychological investigation. Only on the basis of such a view, which sets the two forms of knowledge in their true relation, is it, possible for psycholo-gy to become in the fullest sense an empirical science.
Only in this way, too, can physiology become the true supplementary science of psychology, and psychology, on the other hand, the auxiliary of physiology.
5. Under the second principle of classification mentioned above (2), that is, according to the facts or concepts with which the investigation of psychical processes starts, there are two varieties of empirical psychology to be distinguished. They are, at the same time, successive stages in the development of psychological interpretation. The first corresponds to a descriptive, the second to an explanatory stage. The attempt to present a discriminating description of the different psychical processes, gave rise to the need of an appropriate classification .
Class­concepts were formed, under which the various processes were grouped; and the attempt was made to satisfy the need of an interpretation in each particular case, by subsuming the components of a given compound process under their proper class­concepts. Such concepts are, for example, sensation, knowledge, attention, memory, imagination, understanding, and will. They correspond to the general concepts of physics which are derived from the immediate perception of natural phenomena, such as weight, heat, sound, and light. Like those concepts of physics, these derived psychical concepts may serve for a first grouping of the facts, but they contribute nothing whatever to the explanation of these facts. Empirical psychology has, however, often been guilty of confounding this description with explanation.
Thus, the faculty-psychology considered these class­concepts as psychical forces or faculties, and referred psychical processes to their alternating or united activity.
