But in the exposition of this hypothesis these two meanings of parallelism are frequently confused or interchanged. The same term “ body " is applied both to an aggregate of matter and to the living organism. Now life must be regarded as either inherent in matter, or as the result simply of a particular material configuration, or as physically inexplicable. But, for the present at all events, it cannot be explained physically; nor are we even within measurable distance of such an explanation: so much is beyond cavil. Yet the h pothesis of psycho physical parallelism confines us to one or other of, the former alternatives: at the same time its unwarrantable identification with psycho neural parallelism-where we find a real correspondence between mind and organism-tends to conceal the ravity of such assumptions. The standpoint of physiology, therefore, must be described not as identical with that of physics, but as intermediate between it and the standpoint of psychology. If the fact of life could be reduced to physical terms, physiology then, no doubt, would have to fall into line with physics, much as chemistry, for example, may have had to do. On the other hand, till a physical explanation of life is forthcoming, physiology belongs, with psychology, to the biological grou of sciences, and cannot divest itself completely of the teleological)concepts essential to them, not a vestige of which belongs to bare physics. It is just because of this community in their concepts that there actually is a certain “ point to point" correspondence or parallelism between the psychical and the neural: as an organ a neuron is a unit; physically regarded, it ceases to be one. Yet this illicit identification of organism and material body is thought to be legitimate, inasmuch as physiological processes are found to rest invariably on a physical basis: and inasmuch as, thou h methodological parallelism forbids the physiologist to identify psychosis with neurosis, no limits can be imposed on his efforts to ascertain the mechanism of the neurosis itself. But if this be granted, is not psycho physical parallelism justified, in principle at all events? By no means: as little, for example, as an explanation of the mechanism of a locomotive would justify us in ascribing its origin, its maintenance or its guidance to the machine itself. When life and mind are explained by their mechanism the physicist may summon the biologist, as Mephistopheles did Faust, “ Her zu mir "z then, but not before. A favourite mode of stating psycho physical parallelism is that known as the Double Aspect Theory. In this, besides —Danby, the unjustified identification of the first and third ASPH-'¢” meanings, we find also an equally unjustified inter"'°°'7 predation of parallelism in the second sense. All that methodology prescribes is that psychologists and neurologists-and, we may add, that physicists too-shall severally, as “ specialists, ” mind their own business. Again, all that the first two jointly ascertain is simply the fact of correspondence: the explanation of it is still to seek. Two propositions are now advanced which are held to meet this need. First—and negatively-the connexion, it is said, is not causal: mind does not act on body, nor body on mind: the changes on each side form two independent series, each “ going along by itself.” In other words, the series themselves are said to exemplify what methodology enjoins on the sciences that investigate them-they mind their own business and never intrude into each other's domains. Nevertheless their interaction is not prima facie contradictory or absurd, and ordinary thought, as we have seen, assumes that it exists. What evidence, then, is there for denying it absolutely? Empirical evidence for such a universal negative there can hardly be; it must be established therefore-if established at all-on a priori grounds. Meanwhile two facts, already noticed, make seriously against it. On the psychical side sensations point to an intrusion of some sort, and are not psychically explicable (cf. § 16), and the like-for the present at all events-must be said of the fact of life on the physical side. Apart from all this, it seems plain that methodological parallelism, so far from justifying the denial of interaction, simply precludes its discussion on the dualistic level to which that parallelism is confined. The gulf implied is indeed not absolute—of so much, parallelism in the first sense assures us-but those who are forced to keep to their own side of it obviously are not the people to settle how it is crossed. We are aware that the dualism is not absolute, it is replied: it is only phenomenal, and the two series of phenomena are conditioned by an underlying unity of substance. Such is the second, and positive, proposition of the theory. Again asking for evidence, we are told that this underlying unity is unknown in fact, unknowable. This unknowable substance is assumed, then, simply becausrfthedmpossibility of causal comzexion being taken as established-no other alternative remains. The negative proposition is thus the foundation of the theory, and without it this agnostic monism becomes entirely arbitrary. We have, therefore, to continue our search for the grounds on which the possibility of interaction is denied. But it will be worth while irst to examine certain ambiguities besetting the positive statement.
Diderence of aspect may result solely from difference of standpoint, or it may be due to difference in the reality itself. The circle, seen as concave from within and as convex from without is an ancient instance of the first still in great favour; the pillar that was cloud and darkness to the Egyptians, but light to the children of Israel, may serve to exemplify the second. The former we may call the phenomenal, and the latter the ontal, meaning of “ aspect.” With these two very different meanings our theory plays fast and loose, as suits its own convenience. To do this is easy-in so far as the reality is unknown and unknowable; and necessary-since in the end, the reality, however unknowable, must somehow include both the phenomenal aspects and all that pertains to them, and so far therefore be known. In dealing with “ aspect” in the first sense, the one question to be raised concerns the nature and relation of the respective standpoints. To one belongs what we know as individual experience, and this is essentially concrete, immediate, and qualitatively diverse; to the other belongs an abstract, conceptual scheme, wholly quantitative, familiarly known as the mechanical theory. Between these there is plainly no such co-ordination as the inept comparison with the inside and the outside of a circle implies.1 Neither is there, on the other hand, the same complete opposition; for the entire mechanical theory is based upon individual experience as enlarged and developed by inter-subjective intercourse. Both the sense knowledge of the one and the thought-knowledge of the other relate to the one objective factor involved in both. So far, then, there is fundamentally only one standpoint—that of the subjective factor to the objective factor, which is immediately perceived in the one and mediately conceived in the other. The question here raised is thus primarily epistemological, but it is a question, as we have seen, in which psychology is intimately concerned. “ Aspect ” in the second sense is independent of standpoints. We have here to deal with attributes of the one reality, more or less in Spinoza's sense: this reality itself, as possessed of disparate attributes, is so far dual, and the question of causal connexion between these attributes is not escaped. For to know that a thing has invariably two distinct attributes does not enable us to determine straightway how the changes or “ modes ” of the one are connected with those of the other. (1) The same attribute might be always the initiating or independent variant, and then would come the question of finding out which of the two it was; or (2) it might be that now one, now the other, took the lead, the grounds of this alternation being then the topic for inquiry; or, finally, (3) it might be, as our theory assumes, that there was but a single series of double changes. The questions here raised are philosophical questions, but again they are questions in which psychology is intimately concerned. Our examination thus yields two results: first, there is fundamentally only a single standpoint-~that of experience, now at the perceptual, now at the conceptual, level; and secondly, the distinction of aspects is not merely phenomenal, but pertains “ somehow ” to reality. The question is how; and this leads us to resume our inquiry into the grounds on which interaction is denied.
These grounds neither pertain to psychology nor to physiology. In spite of the outstanding difficulties connected with sensation and life, which these sciences severally raise, such denial is upheld 1 In fact, if there were, since it is only as we contemplate finite portions of the circle that the distinction of concave and convex is present, the nearer we ap roximated to its elements the more this difference of aspect wouhf disappear. If on the physical side we called these elements atoms, there would be an answering element of “ mind-stuff ” on the psychical; and there would be no more unity and no other diversity in a iven man's mind than in his brain regarded as a complex of primordial atoms. Wild as all this seems yet views of the kind have been seriously put forward more than once as the logical outcome of psycho physical parallelism. 7