As we saw in our discussion of Causality, mathematical Physics only succeeds in its constructions on the condition of excluding all qualitative change, as “subjective,” from its purview. But we also saw there that the origination of the qualitatively new is an essential part of the idea of Causality, and that in reducing all change in the physical world to quantitative transformation, mathematical Physics really does away with the causal concept. We are, in fact, in precisely the same logical position if we speak of physiological changes as causing sensation, as when we speak of a quantitative change in the proportions of a chemical compound as the cause of alteration in its qualities. The objection that the psychical effect cannot be connected by an equation with its alleged cause, would hold equally in any case of the production of the qualitatively new, i.e. in every case where we use the category of causality at all. And for that very reason it has no force when urged as an objection to psychophysical causality in particular.[[178]]
(2) The argument from the Conservation of Energy may be more briefly dismissed, as its fallacious character has been fully recognised by the ablest recent exponents of the parallelistic view, such as Dr. Stout and Professor Münsterberg. As Dr. Stout points out, the argument involves a formal petitio principii. The principle of Conservation of Energy has only been established for what are technically known as conservative material systems, and no absolute proof has been given, or seems likely to be given, that the human organism is such a conservative system. Further, as has been urged by many critics, and notably by Professor Ward, the principle of conservation, taken by itself, is simply a law of exchanges. It asserts that the quantity of the energy of a conservative system remains constant under all the transformations through which it passes, but, apart from the rest of the postulates of mechanical science, it affords no means of deciding what transformations of energy shall occur in the system, or when they shall occur. Hence there would be no breach with the special principle of Conservation of Energy if we were to assume that psychical conditions can determine the moment at which energy in the organism is transformed e.g., from the kinetic to the potential state, without affecting its quantity.
(3) It is, however, true that it is inconsistent with the postulates of mechanical Physics, taken as a whole, to admit the determination of physical sequences by non-physical conditions. To admit such determination would be to stultify the whole procedure of the mechanical sciences. For, as we have seen in our Third Book, the primary object of mechanical science is to reduce the course of events to rigid laws of uniform sequence, and thus to facilitate the formulation of practical rules for our own interference with it. It is therefore a legitimate postulate of mechanical science that—for its special object—desire and will shall be excluded from our conception of the conditions which determine events, and the whole course of nature treated as if conditioned only by physical antecedents. If there is any department of experienced reality which cannot be successfully dealt with according to these postulates, then the formulation of rigid laws of uniform sequence is, in principle, impossible for that department, and it must be excluded from the “world” which mechanical science investigates.
But the fact that mechanical science can only attain its end by treating all physical events as independent of non-physical conditions, does not afford the slightest presumption that they must be treated in the same way for all purposes and by every branch of inquiry. Whether Psychology, in particular, is under the logical necessity of conforming to the mechanical postulates, will depend upon our view as to whether the object subserved by Psychology is the same as that of the mechanical sciences, or different. If our purpose in psychological investigation is not identical with the purposes of mechanical science, there is no sense in demanding that we shall hamper our procedure as psychologists by adherence to postulates based upon the special nature of the interests to which mechanical science has to minister.
Now, we have already contended that the aims of Psychology only partially and temporarily coincide with those of the mechanical sciences. If we were right in holding that the principal object of Psychology is to provide a general terminology of which History and Ethics can avail themselves in their appreciations of life, it follows at once that Psychology imperatively needs the recognition of that very teleological aspect of human action which is excluded on principle, and rightly so for the special purpose of mechanical Physics, by the fundamental mechanical postulates. Thus the argument that the parallelistic hypothesis must be the most suitable for the psychologist, because it conforms to the mechanical postulates of sciences which deal with experience from a different standpoint and in a different interest, loses all its cogency.[[179]]
Now that we have, as I trust, sufficiently disposed of the a priori arguments for the parallelistic view, we are in a position to estimate it, as a psychological hypothesis, purely on its merits as evinced by its actual success. But first we must point out once more that the whole question is not one as to actualities, but purely as to the most satisfactory way of bringing two sets of abstractions, originally devised for divergent purposes, into touch with one another; and further, that if the hypothesis were put forward as a final metaphysical truth about the constitution of the real world, it would be manifestly self-contradictory.
In the first place, Parallelism, taken for anything more than a convenient working hypothesis, would involve a flagrant breach of logic. It is obvious that, as Mr. Bradley has urged, you cannot infer from the premisses that one total state, containing both a physical and a psychical element, causes another complex state of the same kind, the conclusion that the physical aspect of the first, by itself, has caused the physical, and the psychical the psychical aspect of the second. To get this conclusion you need a “negative instance,” in which either the physical or the psychical state is found apart from its correlate, but followed by the same consequent as before, and Parallelism itself denies the possibility of such an instance. From the premisses that a α is always followed by b β, it attempts to infer, without any “dissection of nature,” that a by itself was the necessary and sufficient condition of b, and α of β. And this is, of course, logically fallacious. Dr. Ward expresses the same point differently when he urges that unvarying and precise concomitance without causal connection is a logical absurdity.
That the supporters of the hypothesis themselves are conscious of the difficulty, is shown by their unanimous assertion that the psychical and physical series are ultimately manifestations of one and the same reality. What they do not explain is how, if this is so, the two series can be phenomenally so utterly disparate as to exclude mutual influence on one another. The difficulty becomes insuperable when we reflect that on the parallelistic view the physical series must be rigidly mechanical, as otherwise we shall have a breach with those mechanical postulates which are supposed to require the exclusion of psychical states from the determining conditions of physical occurrences. Thus, if teleology is to be recognised anywhere in our scientific constructions, it must be in our conception of the psychical series. And on the whole the supporters of Parallelism admit this in practice by the free use of teleological categories in their Psychology. But it ought by now to be clear to us that the nature of the identical reality cannot be expressed with equal adequacy in a teleological series, and in one which is, by the principles of its construction, purely mechanical. Here, again, most of the parallelists are really in agreement with us, for they usually in the end call themselves “Idealists,” and assert that the “mental” series is a more faithful representation of Reality than the physical. But if the two series are not on the same level in respect of their nearness to Reality, it is hard to see how there can be exact correspondence between them. This is a point to which we shall immediately have to return.[[180]]
§ 6. When we ask, however, whether Parallelism, apart from these questions of ultimate philosophy, is legitimate as a working hypothesis in Psychology, the answer must be that, in certain departments of psychological investigation, it certainly is so. In practice, the doctrine of the parallel but independent series amounts, for the most part, to little more than a methodological device for the division of labour between the physiologist and the psychologist, the physiologist restricting himself to the formulation of such uniformities as can be established between nervous processes, considered as if independent of external influence, and the psychologist doing the same for their psychical accompaniments. As a principle of methodical procedure, therefore, in those parts of Psychology which deal with the more passive and, as we may say, routine-like aspects of mental life, Parallelism is a useful and therefore a legitimate working hypothesis.
The question by which its claim to be the best hypothesis must be decided is, to my mind, that of its applicability to the case of the fresh initiation of new purposive adaptations to changes in the organism’s environment.[[181]] For it is just in dealing with these cases that Psychology, if it is to fulfil the purpose we have ascribed to it, must most obviously discard mechanical for teleological categories. Hence it is here, if anywhere, that a difficulty of principle must make itself felt when we attempt to treat the psychical and the physical series as exactly parallel and corresponding. It seems to follow necessarily from the conception of physical science as based upon the mechanical postulate, that a teleological and a mechanical series cannot possibly run “parallel” in all their details in the fashion presupposed by the hypothesis under consideration.