as a shoemaker with a given quantity of leather may be said to produce a certain number of pairs of shoes. But we cannot properly say that a physical cause produces a mental effect, or vice versa, because no such quantitative relations connect the cause with the effect as in the former case. We cannot speak of production, but only of invariable succession.
It is entirely credible that a nervous impulse coming inwards along a sensory fibre should have a side-effect in the shape of a sensation; but why the physical energy embodied in the nerve-impulse should on that account be debarred from producing all the physical effects which it would have produced had there been no sensation, and why these in turn should not go on to produce all their natural effects in conformity with the laws of physics, it is difficult to understand. In the same way, it is entirely credible that the event in the cerebral cortex which causes a motor discharge should have been immediately preceded by a volition; but how the presence of the volition absolves us from the duty of explaining this cortical event by means of previous cortical events, and supposing the energy contained in it to have been derived from them in a manner strictly consistent with the law of the conservation of energy, it is difficult to understand. If we were obliged to choose between explaining the motor discharge by means of previous cortical events or by means of the antecedent volition, we should have good reason to hesitate. But this is not the alternative; the motor discharge cannot be explained by means of the volition, which can at best be only its invariable antecedent; and the question simply is, whether we will explain the motor discharge in the usual way, by means of previous physical events, or will decline to explain it at all. If we take the latter course, we not only reject the only explanation of which the case admits, but load ourselves with the gratuitous hypothesis that a quantity of physical energy suddenly came into existence.
Whether advocates of the common-sense theory believe they have immediate experience of the mind's power to inaugurate physical changes, or are influenced by ethical and æsthetical considerations, the idea of the physical efficacy of mind involves precisely that fallacious notion of an invisible causal nexus which Hume, Kant, and Lotze strove to overthrow. Once eliminate this imaginary bond between mental cause and physical effect, and we are left with a bare empirical succession, for which it is evident that the parallelism theory makes as ample provision as the theory of common sense.
It is unnecessary here to make more than a passing reference to the wealth of physiological facts, such, for example, as the effects upon the mental functions of injuries, poisons, intoxicants, hard labor, insufficient food, etc., which find a more natural explanation on the theory of parallelism than on that of common sense.
