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A doctrine which states that the relation between mental processes, on the one hand, and physical, physiological, or cerebral processes on the other, is one merely of invariable concomitance: each psychical change or psychical state, each psychosis , involves a corresponding neural change or neural state, neurosis , and vice versa. It denies the possibility of interaction between body and mind. At most there can be a certain point-for-point correlation such that, given any process in the nervous system, a definite mental process is its invariable accompaniment; and, given any particular process in consciousness, a corresponding brain-state or neurosis will invariably be present.

The fundamental principles of Psycho-physical Parallelism are based;

  • (1) upon the fact that all psychical processes presuppose as their condition sine qua non processes of a physical character in the nervous organism;
  • (2) upon the principle of the conservation of energy ; and
  • (3) upon the assumption that mind and matter are so utterly unlike and so utterly opposed in character that interaction between them is impossible.

The psychological data upon which the theory rests we may in general grant. The modern science of psychophysics (q.v.) aided by cerebral anatomy, cerebral physiology, and pathology, proves fairly conclusively that:

  • (1) sensation and perception are conditioned by nervous processes in the brain and in the peripheral end-organs of sense, depending in part at least upon external stimuli;
  • (2) that memory and imagination likewise presuppose, and are conditioned by, cerebral connexions and cerebral activity; and
  • (3) that this is also to some extent the case with regard to intellectual operations and rational volition.

We have so far little more than an experimental verification of two Scholastic principles:

  • (1) that sensation is an act of the composite organism, and
  • (2) that intellectual activity is conditioned by phantasmata, and indirectly by nervous processes.

In truth the data scarcely warrant us in going further than this. But the parallelist goes further. He asserts that intellectual operations have an exact physiological counterpart, which is more than he can prove. An image has doubtless its counterpart, physiologically in the brain and physically in the outside world. The association of ideas is conditioned by, and in a sense is the psychical parallel of, the simultaneous or successive activity of different parts of the brain, between which there is a physical and functional connexion; and without such association of ideas intellectual operations are impossible — so long, that is, as soul and body are united in one being. But that intellectual operations proper — judgment, logical inference, general concepts, vast and far-reaching as they are in their significance, should have an exact counterpart in the activity of brain-cells and their neuronic connexions, is a hypothesis which the known facts of psycho-physics fail to bear out, and which is also inconceivable. How, for instance, can a general concept, referring as it does to objective reality and embracing schematically in a single act many diverse notes, bear any resemblance to the disturbance of nervous equilibrium that accompanies it, a disturbance which has no unity at all except that it occurs in different parts of the same brain more or less simultaneously? Or, how can cerebral processes of a peculiarly unstable and almost haphazard type be, as they are alleged to be, the physiological counterpart of processes of reasoning, rigid, exact, logical, necessary ?

The assertion that all psychical processes have a physiological "parallel" is unwarranted, and scarcely less unwarranted is the assertion that all physiological processes have a psychical "parallel". This latter point can be established only by appeal to the fiction of "subliminal" or "subconscious" consciousness. The existence of a "threshold of consciousness ", or, in other words, of a limit of intensity which must be exceeded by the stimulus, as also by the nervous impulse which results, before the latter can affect our consciousness, has been experimentally proved, and this fact cannot be accounted for by the parallelist except on the assumption that there are states of consciousness of which we are wholly unconscious.

The second line of argument advanced in favour of Parallelism is as follows: The principle of the conservation of energy supposes, we are told, that the universe is a closed mechanical system in which events, whether past or future, are calculable with the utmost precision, given the knowledge of any one stage in the development of that universe and the laws according to which that development takes place. Such a system will brook no interference whatsoever from without. Hence interaction between mind and matter is impossible, and parallelism is the only other alternative.

This conclusion is quite illegitimate. Energy, as understood in the law which states that its sum is invariable, is strictly a non-directed quantity. Hence, even though this law is applicable to the lower phenomena of animal life, as the experiments of Atwater and Hübner show, it by no means disproves the influence of consciousness and will, for mind could still direct material energy and the law remain intact. This is admitted by Fechner, Mach, Boltzmann, Höfler, and von Hartmann, the latter being a determinist. (Cf. ENERGY, THE LAW OF THE CONSERVATION OF.)

Moreover, were the absolute independence of the physical world indeed a fact, the existence of consciousness would become an insoluble mystery, and the existence of a parallelism between it and the physical world a manifest contradiction. If there be no interaction between mind and matter, consciousness ceases to be an instrument whereby we modify our physical environment to suit our needs. Purposive striving, deliberation, choice, volition, are thus rendered wholly unnecessary and irrelevant, and the belief that we can really do something to change things in the outside world and so promote both our comfort and that of our neighbour is a hopeless delusion. The practical utility of physical science also becomes illusory, for our bodies, which alone can give it effect, are declared to be merely automata with the working of which consciousness has nothing to do. Parallelism is useless here, if interaction be abolished; nay, more, is incompatible with that very independence on account of which its existence is affirmed. Absolute independence and universal concomitance are contradictory. If there is concomitance, directly or indirectly, as Mill said, there must be causal connexion.

That such a causal connexion between mind and matter really exists the consciousness of activity, purpose, will, and responsibility, directly testifies; and in the face of this testimony to hark back to the Cartesian doctrine of radical opposition between body and soul, extension and thought, is futile and contrary to experience.

Variations and developments of parallelism may in general be classed under two heads; conscious automatism — the theory of Huxley that the human body is a mere machine of which consciousness is the "collateral product", a shadow or epiphenomenon which symbolically indicates, though it in no wise influences, the mechanical processes which underlie it; and the "Dual-aspect Theory" which maintains that psychical and physical phenomena between which there is a point-for-point correspondence all along the line, are but different aspects or expressions of the same common substance. Huxley's view emphasizes the material at the expense of mental, curiously oblivious of the fact that all we know of the physical universe and all the theories that we are able to formulate about it, originate in, and belong to, consciousness. The dual-aspect view improves upon this, by giving to consciousness a value at any rate equal to that of mechanical movement. It is in fact a form of Monism akin to that of Spinoza and involves most of the difficulties to which that system leads. But from our point of view its chief error lies in its assertion that parallelism is the only relation which holds between the physical and the psychical, a relation which can be proved to hold so far as sensation and perception are concerned, but which, if further generalized to the exclusion of interaction, inevitably leads to contradiction.

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