Elements of metaphysics

by A. E. Taylor

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Usually it is further added that the ultimate metaphysical explanation of this parallelism without mutual dependence must be found in the (Spinozistic) doctrine of Identity, i.e. the doctrine that the physical and psychical series are two different “sides” or “aspects” of a single reality. Some supporters of Parallelism (e.g., Ebbinghaus) conceive this single reality as a tertium quid, equally adequately expressed by both the series, others (e.g., Stout) hold that its real nature is more adequately revealed in the mental than in the physical series.

The grounds commonly adduced in favour of the parallelistic view as the most satisfactory psychophysical theory, are of two kinds. As a positive argument it is urged that cerebral anatomy has already to some extent confirmed the doctrine of correspondence between definite physical and psychical processes by its successful “localisation” of specific sensory and motor processes in various cortical “centres,” and may reasonably be expected to accomplish further such “localisations” in the future. Stress is also laid upon the formal analogy between the psychological laws of retentiveness, association, and habit, and the physiological theories of the formation of “conduction-paths” in the brain. These positive contentions do not, however, take us far. The correspondences upon which they rest, so far as they are ascertained experimentally and are not mere deductions from the principle of Parallelism itself, would be equally natural on a theory of Interaction, or of one-sided dependence of either series on the other. The real strength of the case for Parallelism rests upon certain negative assumptions which are widely believed to exclude the hypothesis of causal dependence of either series on the other. These negative assumptions appear to be in the main three.

(1) It is said that, while we can without difficulty conceive how the later stages of a continuous physical or psychical process can be connected by causal law with its earlier stages, we are entirely unable to conceive how psychical events can arise from physical antecedents, or vice versâ, because of the utter disparateness of the physical and the psychical. The physical process, it is urged, is continuous, and so, on the other side, is the psychical, but when we attempt to think of a cerebral change conditioning a mental change, or vice versâ, there is a complete solution of continuity which we cannot bridge by any causal formula.

(2) The doctrine of Conservation of Energy is sometimes supposed to be incompatible with the admission of psychical states among the antecedents or consequents of physical states. It is said that if psychical states can influence the course of nervous change, there will be “work” done in the organism without the expenditure of energy, and if the total effect of nervous change is not exclusively physical there will be loss of energy without “work” being done by the organism, and in either case the principle of Conservation will be contravened.

(3) Finally, it is maintained that it is a fundamental postulate of the physical sciences, that every change of configuration in a material system such as the living organism is assumed to be, is due to exclusively physical antecedents, and that this postulate must therefore be respected in Psychophysics. These are, so far as I can gather them from the works of the psychologists who adopt the parallelist view, the principal arguments by which their case is supported.

It is clear that if all—or any—of these contentions are valid, it must follow that Parallelism is not only a legitimate but the only legitimate hypothesis for the co-ordination of physical and psychical science. I believe, however, that every one of them is fallacious, and that for the following reasons:—

(1) The argument from the inconceivability of causal relation between the physical and the psychical is perhaps the most effective of the alleged grounds for denying interaction between the psychical and the physical. Yet its force is not really so great as it might appear. It is not denied that we can, in simple cases, assign the conditions under which a mental state follows on a physical state (e.g., we can assign the physical conditions of the emergence of a given sensation). But, it is argued, we cannot show why those conditions (e.g., the stimulation of the retina, and indirectly of the “optical centres” in the brain by light of a given wave length) should be followed by this particular sensation (e.g., green, and not some other colour). This means that we cannot construct a mathematical equation connecting the character of the sensation with that of the stimulus, as we can to connect the earlier with the later stages of a purely physical process. This is, of course, obvious enough. It is only by making complete abstraction from the appearance of new qualities in the course of a process, and by treating it as a purely geometrical and quantitative transformation, that we can render it amenable to our equations.