The same difficulty is felt from a practical as well as from a theoretical point of view. The physiologist, in his endeavour to make organic processes intelligible, by connecting them with the general order of physical nature, cannot but regard the presence of a factor which does not enter into this order as a most serious stumblingblock, which may upset all his calculations. A favourite way of putting the objection from this point of view, is to say that the intervention of conscious process in physiological process would contradict the law of conservation of energy. This is not strictly true, because the conservation of energy is a law framed expressly for a material system; when a factor is introduced which is not material, though the law may not be applicable, it is not violated. Apart from interfering" conditions stones will fall to the earth; this law is not violated when I lift a stone in my hand. Similarly no change in the material world, as such, produces loss or gain of power to do work; the power being merely transferred from one portion of matter to another. Nevertheless, it is quite conceivable that loss or gain of energy might ensue from the operation of a factor which does not belong to the material world at all. But, though no contradiction is involved in such a supposition, it is clear that the fresh creation of material energy by conscious process would introduce an incalculable and disturbing factor, seriously interfering with the work of scientific discovery and explanation. Nor is this objection limited to the law of conservation of energy; it applies to all the ultimate principles on which our knowledge of the physical world is based. So far as the conservation of energy is concerned, it might be supposed that there is a transfer of energy from material process to conscious process. Physical energy might be transformed into intensity and complexity of consciousness, and vice versa. But there is no sufficient evidence of this, and all that we know points in the contrary direction. Intensive quantity is not measurable and calculable in such a way as to make it comparable with other forms of energy. The hypothesis of interaction, it is clear, labours under very serious difficulties, and though it cannot be pronounced impossible, yet it will be well to avoid it, if we can find some alternative which is on the whole more tolerable.
To the second alternative, onesided action, either of matter on mind, or of mind on matter, the theoretical objections which have been brought against interaction apply with equal force. It also involves the additional difficulty that all other action with, which we are acquainted, is interaction. Onesided action would therefore be contrary to our general experience of the order of nature. Yet the hypothesis that matter causally determines consciousness, without being itself determined by consciousness, is one which has so much currency that it requires special criticism. This doctrine of materialism, as it is called, seems incapable of any precise statement; whatever plausibility it possesses, arises from the use, or rather from the misuse, of the word function. Digestion is a function of the alimentary canal; breathing is a function of the lungs; why cannot we simply affirm that consciousness is a function of the brain? The objection is, that we do not make two things the same by applying the same word to them, when in their own nature they are radically and essentially different. When we say that digestion is a function of the stomach, we mean that digestion is the stomach engaged in digesting. When we say that breathing is a function of the lungs, we mean that breathing is the lungs at work. In describing the process of digestion, we, ipso facto, describe the stomach itself as engaged in the process. In describing the process of breathing, we, ipso facto, describe the lungs as filling themselves with air by a certain movement, and expelling it by an alternate movement. But if we describe the brain at work, there is no need to mention consciousness at all; and in naming and describing conscious processes, there is no need to mention the brain. The function of the brain as a physiological organ is to move the body; the contraction of muscles is the result of neural impulses; and in describing it we have to mention the nervous system, including the cortex, as engaged in it. But the process of consciousness cannot be analysed or resolved into such processes as chemical and physical changes in nervecells. If consciousness is supposed to be produced by the nervous process, the production is simply creation out of nothing. An objection of an equally serious kind is that the materialistic theory destroys all possibility of agency on the part of conscious beings. According to it, the appearance of causal connexion within the process of consciousness itself is an illusion ; no judgment was ever due to a train of reasoning ; no volition was ever due to motives. The sole cause in every case was a certain modification of the nervous system. Similarly conscious process can, on this view, never determine external action. No man ever lifted a finger because he willed to do so. No tears were ever the consequence of emotion. This question is sometimes confused by the supposition that materialism would only interfere with what is called freewill ; in truth, it makes impossible any real operation of consciousness of any kind whatever. The logical consequence is not only that man as a conscious being never does anything freely, but that no man ever does anything at all.
We now come to the third hypothesis. This differs both from the theory of interaction and from materialism, inasmuch as it separates the statement of facts from theoretical explanation. Its first problem is to state the facts without implying direct interaction between nervous and conscious change, and without implying that the one creates the other. The formula which it uses for this purpose is that of psychophysical parallelism, which simply states that modifications of consciousness emerge contemporaneously with corresponding modifications of nervous process. The nervous changes are supposed to be parts of the total continuous process of the physical universe, so that science will require none but material conditions to explain them. On the other hand, there is causal connexion within the process of consciousness itself, as such. This psychical causation runs parallel with the material, but is not itself material. When a bodily action, such as moving a finger, follows upon volition, it is the cortical process concomitant with the volition which sets the muscles in contraction and so produces the movement. When an external impression is followed by a sensation, what the external impression produces is a cortical process, which is concomitant with but does not cause the sensation. The external impression maybe regarded as if it were a cause of the sensation, inasmuch as it is a cause of the cortical process correlated with the sensation. Similarly, the volition may be regarded as if it were a cause of the movement, in as much as it is correlated with the cortical process which sets the appropriate muscles in contraction. This account of the matter covers the facts as they are known to us; but it is merely a way of formulating these facts; it is not an explanatory theory. On the contrary, if it is a true formulation of the facts it is evident that these facts do not contain their own explanation. If the concomitance of cortical and conscious process is regarded as an ultimate principle, it is simply a miracle. That the cortical process which sets in motion the muscles moving the finger should happen to be accompanied by the conscious volition to move the finger without causal connection between them, is in itself utterly unintelligible. If we are to find an explanation, we must frame some hypothesis to account for psychophysical parallelism, and in so doing we are compelled to plunge into ontology.