§ 4. Metaphysical Explanation of PsychoPhysical Parallelism.—If the doctrine of psychophysical parallelism is true, the reason of the connexion between conscious process and the correlated nervous process is not to be found in the nervous and conscious processes themselves. Both must be regarded as belonging to a more comprehensive system of conditions; and it is within this system as a whole that the reason of their connexion is to be sought. In particular, the individual consciousness, as we know it, must be regarded as a fragment of a wider whole, by which its origin and its changes are determined. As the brain forms only a fragmentary portion of the total system of material phenomena, so we must assume the stream of individual consciousness to be in like manner part of an immaterial system. We must further assume that this immaterial system in its totality is related to the material world in its totality as the individual consciousness is related to nervous processes taking place in the cortex of the brain. Within the immaterial system the individual consciousness is a determining factor: within this system it acts and is acted on. But this interaction is virtually interaction between conscious process and the material world: for the total immaterial system to which the individual consciousness belongs is correlated with material phenomena in general, as the individual consciousness is correlated with nervous occurrences in the cortex. When a volition sets the finger moving, the volition acts within its own sphere of influence, and the corresponding cortical process acts within its own sphere of influence.
We have yet to consider the relation of the immaterial system as a whole to the material system as a whole. If this relation be regarded as one of mere parallelism or concomitance, the fundamental difficulty, so far from being removed, is aggravated. To obtain light on this ultimate question, we must take an entirely new point of departure. We must consider the problem of the ultimate nature of matter. To do so here at length is of course impossible; but we may say that the explanation of psychophysical parallelism is ultimately based on an idealistic view of material phenomena. The sensible qualities of matter exist only for minds which have certain experiences in the way of sensation. The extension, configuration, and other qualities of material bodies all presuppose the existence of certain modes of conscious experience. In like manner, the ultimate constituents of matter as they are recognised by scientific theories are abstract constructions of the human mind. In general, all that makes matter material presupposes some consciousness which takes cognisance of it. Matter, as perceived and conceived by common sense and science, is essentially a phenomenon; and phenomenon simply means appearance or presentation. There can be no appearance or presentation apart from a subject to which an object appears or is prosented. Hence the nature of matter as known is constituted by its being known, or at least knowable. On the other hand, it is equally certain that the existence of what is known to us as matter does not depend on our knowledge of it. We do not make matter. Only its appearance as material phenomenon is dependent on us. Hence it follows that, so far as it exists independently of its presentation to a cognitive subject, it cannot have material properties, such as extension, hardness, colour, weight, and the like. It is an agency which is an essential condition of material phenomena, but is not itself a material phenomenon. Thus we are led by a quite different line of investigation to the same conclusion which was suggested by the relation of conscious process to nervous process. The world of material phenomena presupposes a system of immaterial agency. In this immaterial system the individual consciousness originates. To it, in some way, the sensational experiences are due which form the basis of our knowledge of the material world. It is on it the individual consciousness acts when it produces changes in the material world. All this is possible because the system of immaterial agency is identical with what we know as matter, in so far as matter exists independently of its possible presentation to a perceiving subject. This theory has been purposely stated in a vague form. There are varying views as to the nature of the system of immaterial agency. Some say that it is will, others that it is absolute thought, others that it is unknowable; in any case, the student should guard against the assumption that the immaterial system is a sort of repetition of the material system, involving the same sort of interactions, and similar distinctions and relations of its parts. One thing seems clear,—that we are nearer the truth in speaking of it as consciousness, than in speaking of it as matter.
§ 5. Conclusion.—We have discussed three theories of the immediate connexion between conscious and nervous process. Of these, what we have described as materialism must be rejected. The other two, interaction and parallelism, have each advocates among the best psychologists and metaphysicians of the present day. The student is recommended to avoid hastily deciding between them. The hypothesis of parallelism is that to which we are ourselves inclined. It certainly covers the known facts, and forms the most convenient working hypothesis. It escapes the difficulties which attach to che theory of interaction. But it must be admitted that it does so only by somewhat bold speculation.
For psychological purposes the doctrine of psychophysical parallelism is, as we have said, a sufficiently good working hypothesis, if we take it merely as a mode of formulating facts. We shall accordingly assume its validity in this work. In indicating the theoretical explanation of psychophysical parallelism we have passed beyond the limits of psychology proper, and entered upon ontological speculation. It has been thought advisable to adopt this course for two reasons: first, because the intelligent student always feels a keen interest in the relation between body and mind, and cannot, as a rule, rest satisfied with the statement of the simple concomitance of nervous and mental processes. In the second place, theories on the subject are in the air and are put forward in a more or less dogmatic fashion by popular writers. Hence a mere attempt to give a formula for the facts is apt to be interpreted as a decision in favour of one or other of these theories. To avoid being misunderstood it is necessary to be explicit. But the reader should take note that we do not pretend to have given more than a general indication of the main lines of thought on this profoundly important topic.