7. That the method of treating experience employed in natural science should have reached its maturity before that employed in psychology, is easily comprehensible in view of the practical interest connected with the discovery of regular [p. 316] natural phenomena thought of as independent of the subject; and it was almost unavoidable that this priority of the natural sciences should, for a long time, lead to a confusion of the two points of view. This did really occur as we see by the different psychological substance-concepts. It is for this reason that the reform in the fundamental position of psychology, which looks for the characteristics of this science and for its problems, not in the specifically distinct nature of its sphere, but in its method of considering all the contents presented to us in experience in their immediate reality, unmodified by any hypothetical supplementary concepts - this reform did not originate with psychology itself, but with the single mental sciences. The view of mental processes based upon the concept of actuality, was familiar in these sciences long before it was accepted in psychology. This inadmissible difference between the fundamental position of psychology and the mental sciences is what has kept psychology until the present time from fulfilling its mission of serving as a foundation for all the mental sciences.
8. When the concept of actuality is adopted, a question upon which metaphysical systems of psychology have been long divided is immediately disposed of.
This is the question of the relation of body and mind. So long as body and mind are both regarded as substances, this relation must remain an enigma, however the two concepts of substance may be defined. If they are like substances, then the different contents of experience as dealt with in the natural sciences and in psychology can no longer be understood, and there is no alternative but to deny the independence of one of these forms of knowledge. If they are unlike substances, their connection is a continual miracle. If we start with the theory of the actuality of mind, we recognize the immediate reality of the phenomena in psychological experience.
Our physiological [p. 317] concept of the bodily organism, on the other hand, is nothing but a part of this experience, which we gain, just as we do all the other empirical contents of the natural sciences, by assuming the existence of an object independent of the knowing subject. Certain components of mediate experience may correspond to certain components of immediate experience, without its being necessary, for this reason, to reduce the one to the other or to derive one from the other. In fact, such a derivation is absolutely impossible because of the totally different points of view adopted in the two cases. Still, the fact that we have here not different objects of experience, but different points of view in looking at a unitary experience, renders necessary the existence at every point of relations between the two. At the same time it must be remembered that there is an infinite number of objects that can be approached only immediately, through the method of the natural sciences: here belong all those phenomena that we are not obliged to regard as physiological substrata of psychical processes. On the other hand, there is just as large a number of important facts that are presented only immediately, or in psychological experience: these are all those contents of our subjective consciousness which do not have the character of ideational objects, that is, the character of contents which are directly referred to external objects.
9. As a result of this relation, it follows that there must be a necessary relation between all the facts that belong at the same time to both kinds of experience, to the mediate experience of the natural sciences and to the immediate experience of psychology, for they are nothing but components of a single experience which is merely regarded in the two cases from different points of view. Since these facts belong to both spheres, there must be an elementary process on the physical side, corresponding to every such process on the psychical [p. 318] side. This general principle is known as the principle of psycho-physical parallelism. It has an empirico-psychological significance and is thus totally different from certain metaphysical principles that have sometimes been designated by the same name, but in reality have an entirely different meaning. These metaphysical principles are all based on the hypothesis of a psychical substance. They all seek to solve the problem of the interrelation of body and mind, either by assuming two real substances with attributes which are different, but parallel in their changes, or by assuming one substance with two distinct attributes that correspond in their modifications.