Now, if we adhere firmly to the above conception of organisms as physical systems, it is really impossible for us to include any mental or psychical terms whatsoever in their supposed operations. However, it would be equally absurd to deny the existence of these psychical factors, since their reality is far more evident than that of the physical terms. The physical ideas are the outcome of an elaborate and frequently hazardous process of reasoning, whereas the psychological phenomena are always with us, no matter what our opportunities or capabilities may be in the domain of scientific thought. Consequently, it appears that the physical and the psychological data must be organized into logically distinct scientific systems, and that the problem of their interrelations must be considered as a third question, separate from that of the internal relationships of the terms within the two respective accounts. This method of treating the facts is given the name, psychophysical parallelism.
The parallelistic view, first proposed by Leibnitz, has been the working scheme of the majority of scientific psychologists since his time. However, it has aroused severe intellectual pains in philosophic minds, thus stimulating many a violent reaction against it. It is fundamentally opposed to the natural desire for a monistic explanation, and to the demand for continuity between processes which go on in synchronism with each other, as do those of body and mind. Behaviorists and exclusive physiologists turn their thoughts away from the mental side of the parallelism, in an endeavor to avoid recognition of its obnoxious features. Idealists and exclusive introspectionists find unity in a shut-in mentalism. Interactionists, and blurry thinkers in general, scoff at the scheme as an academic creation, and mix physical and psychological notions at random. But if we envisage all of the facts in their empirically demonstrable relationships, and in the light of the most firmly established modern theories, we cannot escape the parallelistic method. Other views lead to confusion or neglect. Nevertheless, the mystery which is generated is far from being insoluble, without entailing either of these undesirable features. Concerning such a solution, we shall have more to say later.