In both these cases the metaphysical principle of parallelism is based on the assumption that every physical process has a corresponding psychical process and vice versa; or on the assumption that the mental world is a mirroring of the bodily world, or that the bodily world is an objective realization of the mental. This assumption is, however, entirely indemonstrable and arbitrary, and leads in its psychological application to in intellectualism contradictory to all experience. The psychological principle, on the other hand, as above formulated, starts with the assumption that there is only one experience, which, however, as soon as it becomes the subject of scientific analysis, is, in some of its components, open to two different kinds of scientific treatment: to a mediate form of treatment, which investigates ideated objects in their objective relations to one another, and to an immediate form, which investigates the same objects in their directly known character, and in their relations to all the other contents of the experience of the knowing subject. So far as there are objects to which both these forms of treatment are applicable, the psychological principle of parallelism requires, between the processes on the two sides, a relation at every point. This requirement is justified by the fact that both [p.
319] forms of analysis are in these two cases really analyses of one and the same content of experience, On the other hand, from the very nature of the case, the psychological principle of parallelism can not apply to those contents of experience which are objects of natural-scientific analysis alone, or to those which go to make up the specific character of psychological experience. Among the latter we must reckon the characteristic combinations and relations of psychical elements and compounds. To be sure, there are combinations of physical processes running parallel to these, in so far at least as a direct or indirect causal relation must exist between the physical processes whose regular coexistence or succession is indicated by a psychical interconnection, but the characteristic content of the psychical combination can, of course, in no way be a part of the causal relation between the physical processes. Thus, for example, the elements that enter into a spacial or temporal idea, stand in a regular relation of coexistence and succession in their physiological substrata also; or the ideational elements that make up the process of relating or comparing psychical contents, have corresponding combinations of physiological excitation of some kind or other, which are repeated whenever these psychical processes take place.
But the physiological processes can not contain anything of that which goes most of all to form the specific nature of spacial [sic] and temporal ideas, or of relating and comparing processes, because natural science purposely abstracts from all that is here concerned. Then, too, there are two concepts that result from the psychical combinations, which, together with their related affective elements, lie entirely outside the sphere of experience to which the principle of parallelism applies. There are the concepts of value and end. The forms of combination that we see in processes of fusion or in associative and apperceptive processes, as well [p. 320] as the values that they possess is the whole interconnection in of psychical development, can only be understood through psychological analysis, in the same way that objective phenomena, such as those of weight, sound, light, heat, etc., or the processes of the nervous system, can be approached only by physical and physiological analysis, that is, analysis that makes use of the supplementary substance-concepts of natural science.
10. Thus, the principle of psycho-physical parallelism in the incontrovertible empirico-psychological significance above attributed to it, leads necessarily to the recognition of an independent psychical causality, which is related at all points with physical causality and can they come into contradiction with it, but is just as different from it's physical causality as the point of view adopted in psychology, or that of immediate, subjective experience, is different from the point of view taken in the natural sciences, or that of mediate, objective experience due to abstraction.
And just as the nature of physical causality can be revealed to us only in the fundamental laws of nature, so the only way that we have of accounting for the characteristics of psychical causality is to abstract certain fundamental laws of psychical phenomena from the totality of psychical processes. We may distinguish two classes of such laws.
The laws of one class show themselves primarily in the processes which condition the rise and immediate interaction of the psychical compounds; we call these the psychological laws of relation. Those of the second class are derived laws. They consist in the complex effects that are produced by combinations of the laws of relation within more extensive series of psychical facts; these we call the psychological laws of development.