If Psychology is to be of any use in supplying Ethics and History with the subject-matter of their appreciations, it is manifest that it must make the assumption that desire and choice are operative in determining the course of human action, and thus must—at certain points at least—explicitly employ the categories of teleology. These categories, again, cannot possibly be translated into the rigidly non-teleological symbolism of a physical science, based upon the mechanical postulates, as every science of “general laws” must be. It follows that “exact parallelism without mutual interference” cannot, consistently with the purpose which Psychology subserves, be employed, even as a working hypothesis throughout the whole field of psychological investigation itself. When the attempt to extend its employment to the whole sphere of psychical processes is seriously made, it leads inevitably to the crude fatalism of the doctrine that there is no such thing as choice or action (free or otherwise) in the universe. In actual practice, the supporters of Parallelism, who reject this doctrine when it is explicitly avowed under the name of Epiphenomenalism, only succeed in doing so because they do not really insist on carrying out the parallelistic hypothesis in their Psychology. They commonly make their hypothesis prominent, while they are dealing with the comparatively passive and routine-like aspects of mental life, association, habituation, etc., but allow themselves to lose sight of it as soon as they come to treat of such explicitly teleological concepts as attention and choice. Their procedure is also rendered easier for them by the liberal use which evolutionary biologists, even while professing with their lips fidelity to the mechanical postulates, allow themselves to make of teleological categories which are really purely psychological.
It would be an easy task, if space permitted, to show in detail how the fundamentally different principles underlying the construction of the mechanical and the teleological series involve the presence, in the individual members of each series, of characters to which nothing corresponds in those of the other. Thus we might ask, with Dr. Ward, what corresponds in the psychical scheme to the composition of the units of the physiological scheme out of their various chemical components, and of these, again, out of more elementary physical “prime atoms”?[[182]] or, from the opposite side, we might ask, what is the cerebral equivalent, in terms of a rigidly mechanical Physiology, of the psychological character of “meaning” or “significance”? But the multiplication of these problems becomes superfluous if the reader has once grasped our principle, that exact correspondence is only possible between series which are either both mechanical or both—and both in the same degree—teleological. Between a genuinely teleological and an honestly mechanical series such correspondence is logically impossible, because of the fundamental difference between their types of construction.
§ 7. For the reasons just produced, it is, I think, necessary to hold that the oldest and simplest hypothesis of the connection between body and mind, that of Interaction, is after all the most satisfactory. According to this view, the two series cannot be thought of as presenting an exact correspondence, and must be thought of as causally influencing each other at different points, precisely as any two sets of physical events do. If we adopt it we shall recognise in sensation a psychical state which has physical processes among its immediate antecedents, and in motor reaction similarly a physical process with psychical antecedents. It is scarcely to be denied that this conception of body and mind, as two things which stand in causal relation, is the hypothesis which most naturally presents itself, when once we have artificially broken up the unity of immediate experience into a physical and a psychical side, and so created the problem of psychophysical connection. So natural is it, that even psychologists who accept one of the other hypotheses are to be found constantly speaking of voluntary movement in terms which, if they mean anything, imply causal determination of bodily by mental process, while no psychologist of any school has ever succeeded in expressing the relation of sensation to stimulus in any other phraseology than that of Interaction. Probably the hypothesis would never have been exposed to hostile criticism at all, but for the metaphysical objections, already dismissed by us as fallacious, founded upon the notion that the mechanical postulates with which Interaction conflicts are ascertained truths about the actual structure of the reality with which we are in touch in immediate experience.
It is clear that, from the nature of the problem to be solved, we cannot be called upon to prove the actual occurrence of psychophysical interaction. As a working hypothesis for the interrelation of two sets of scientific abstractions, the theory is in principle incapable of direct establishment by the “appeal to facts.” All that is requisite for its justification is to show that it is (a) not in principle at variance with any fundamental axiom of scientific procedure, and (b) enables us to co-ordinate our scientific results in the manner most suitable for the uses to which we propose to put them. Both these conditions are fulfilled by the hypothesis of Interaction, if our foregoing arguments are sound. We have seen the fallacious nature of the objections brought against it on a priori grounds of logical method, and have also seen that it is positively demanded if we are at once to be faithful to the mechanical postulates upon which physical science depends for its successes, and to recognise in our psychological constructions that teleological character of human action which is all-essential for History and Ethics. In substance this is the whole case for the Interaction hypothesis, and no further accession of strength would result from its elaboration in detail.
It may be added that it is one great recommendation of the hypothesis of Interaction, that it is quite consistent with the full recognition of the relative usefulness of the alternative theories, though they, as we have seen, are unable to do justice to those aspects of fact which can only be expressed in terms of Interaction. Thus the hypothesis of Interaction can readily afford to admit that, for certain purposes and up to a certain point, it is possible to treat physical or psychical processes as if they were determined solely by physical or psychical conditions respectively, and even to treat some physical processes as if the presence of their psychical concomitants made no difference at all to their occurrence. The reason of this is, that whereas a mechanical hypothesis can give no intelligible account of a purposive process at all, a teleological hypothesis can quite easily account for the apparently mechanical character of some of the processes which fall under it. As we have seen (Book III. chap. 3, § 6), a purposive reaction, once established, approximates to mechanical uniformity in the regularity with which it continues to be repeated, while the conditions are unchanged, and the end of the reaction is therefore still secured by its repetition.
Thus we can readily see that, even if we contented ourselves with the attempt to translate into the language of psychological science the processes which make up the life of an individual subject, many of them would appear to be going on with routine uniformity. And when we deliberately set ourselves to obtain uniformities by taking an average result, derived from comparison of a multitude of subjects, our results are, of course, always mechanical in appearance, because the element of individual purpose and initiative has been excluded by ourselves from our data in the very process of taking the average. Hence we can understand how, on the hypothesis of Interaction itself, all those mental processes which consist in the repetition of an already established type of reaction should come to appear mechanical, and thus to suggest that mechanical conception of psychical processes which is common to the epiphenomenalist and the parallelist view. Interaction, and Interaction alone, is thus a hypothesis capable of being applied to the whole field of psychological investigation.
I will conclude this chapter with some considerations on the bearing of our result upon the special problems of Metaphysics. We have explicitly defended Interaction as being no statement of actual experienced fact, but a working hypothesis for the convenient correlation of two scientific constructions, neither of which directly corresponds to the actualities of experience. This means, of course, that Interaction cannot possibly be the final truth for Metaphysics. It cannot ultimately be the “fact” that “mind” and “body” are things which react upon each other, because, as we have seen, neither “mind” nor “body” is an actual datum of experience; for direct experience and its social relations, the duality subsequently created by the construction of a physical order simply has no existence. Nor can it be maintained that this duality, though not directly given as a datum, is a concept which has to be assumed in order to make experience consistent with itself, and is therefore the truth. For the concept of Interaction manifestly reposes upon the logically prior conception of the physical as a rigidly mechanical system. It is because we have first constructed the notion of the “body” on rigidly mechanical lines that we have subsequently to devise the concept of “mind” or “soul” as a means of recognising and symbolising in our science the non-mechanical character of actual human life. And since we have already seen that the mechanical, as such, cannot be real, this whole scheme of a mechanical and a non-mechanical system in causal relation with one another can only be an imperfect substitute for the Reality it is intended to symbolise. In fact, we might have drawn the same conclusion from the very fact that the psychophysical hypothesis we have adopted is couched in terms of Transeunt Causality, since we have already satisfied ourselves that all forms of the causal postulate are more or less defective appearance.
The proposition that the psychophysical theory of the “connection” of “body” and “mind” is an artificial transformation, due to the needs of empirical science, of the actual teleological unity of human experience, is sometimes expressed by the statement that mind and body are really one and the same thing. In its insistence upon the absence of the psychophysical duality from actual experience, this saying is correct enough, but it perhaps fails to express the truth with sufficient precision. For, as it stands, the saying conveys no hint of the very different levels on which the two concepts stand in respect to the degree of truth with which they reproduce the purposive teleological character of real human experience. It would perhaps be nearer the mark to say that, while the physiologist’s object, the “body,” and the psychologist’s object, the “mind,” are alike conceptual symbols, substituted, from special causes, for the single subject of actual life, and may both be therefore said to “mean” or “stand for” the same thing, their actual content is different. For what in the language of physiology I call my “body” includes only those processes of actual life which approximate to the mechanical ideal sufficiently closely to be capable of being successfully treated as merely mechanical, and therefore brought under a scheme of general “laws” of nature. Whereas what, as a psychologist, I call my “mind” or “soul,” though it includes processes of an approximately mechanical type, includes them only as subordinate to the initiation of fresh individual reactions against environment which can only be adequately expressed by teleological categories. Thus, though “mind” and “body” in a sense mean the same actual thing, the one stands for a fuller and clearer view of its true nature than the other. In Dr. Stout’s terminology their intent may be the same, but their content is different.[[183]]
Consult further:—R. Avenarius, Der Menschliche Weltbegriff; B. Bosanquet, Psychology of the Moral Self, lect. 10; F. H. Bradley, Appearance and Reality, chap. 23; Shadworth Hodgson, Metaphysic of Experience, vol. ii. pp. 276-403; William James, Principles of Psychology, vol. i. chaps. 5 and 6; H. Lotze, Metaphysic, bk. iii. chaps. 1 and 5 (Eng. trans., vol. ii. pp. 163-198, 283-517); H. Münsterberg, Grundzüge der Psychologie, i. chaps. 11. (pp. 402-436), 15 (pp. 525-562); G. F. Stout, Manual of Psychology,3 Introduction, chap. 3; James Ward, Naturalism and Agnosticism, vol. ii. lects. 11 and 12 (art. “Psychology” in Supplement to Encyclopædia Britannica, p. 66 ff.).