The difficulty of conceiving how mind can act on matter and matter can act on mind seems to be empirically insuperable. The physiological series and the psychological series are incommensurate. Empirically we must confess our complete ignorance of the nature of the ultimate relation of the one to the other. But, none the less, in many cases observed facts show that they are in some way related. Empirically, therefore, our only course seems to be to leave on one side the ultimate relation of the one to the other, and deal with the concomitance as an unexplained fact of observation; or, if an assumption must be made, the safest assumption is that what from a physical and physiological point of view is a complex molecular disturbance is at the same time from a psychological point of view a state of consciousness. The two are different aspects of one natural occurrence. Why such an occurrence should have two so different aspects we have not the faintest idea; but here we are not one whit worse off than we were before. It does, however, enable the physiologist and the psychologist to deal independently, each with that group of facts which may be requisite for his ideal construction. What does this imply ? It certainly does not imply that either series would remain just what it is if the other series were non-existent. As well might two Irishmen assert that because each saw but one side of their boundary wall the other side did not "count for anything at all, at all." A Scotchman would point out that each side counted, but that you could not see both at once. The inevitable incommensurability of the physiological and the mental series implies that we can only pass from one to the other in the same argument by changing the point of view. But if the state of consciousness actually is the very same something which the physiologist calls, in the language of physics, a molecular disturbance, we may change the point of view as often as we like with perfect freedom. In fact we do so, in effect, every day of our lives whenever we say that certain occurrences in the external world give rise to a train of thought as the outcome of which we act in this or that particular manner. If, however, we adopt this course, and if we allow ourselves to say that the occurrences cause, or are the antecedents of, the train of thought, we have every bit as much right to affirm on precisely the same grounds that the train of thought causes the acts which follow. In saying that consciousness influences behaviour one who accepts the double-aspect theory of concomitance is merely avoiding a cumbrous form of circumlocution. He puts it in this way instead of saying that the nerve-changes in the cerebral cortex, which from a psychological point of view are a conscious situation or a train of thought, influence and determine the course of behaviour. But from this point of view it is absurd to say that consciousness is merely an adjunct, a by-product, an epi-phenomenon absurd to say that were there no conscious situation the neural situation would remain unchanged. They are the very same thing regarded from different points of view; and to say that there is no influential conscious guidance and control is simply equivalent to saying that there is not the determining condition of the cerebral cortex, which is its organic concomitant. For, as I said before, on the assumption of concomitance, "cerebral control system" and "mind" are interchangeable terms.
As a matter of fact, in large sections of human life and conduct we know a great deal more about the mental aspect, with its antecedences and sequences, with its doctrine of values for consciousness, than we do about its physiological aspect. When a man receives a letter offering him an appointment under certain conditions, we can at least in some degree interpret in psychological terms what intervenes between the receipt of the offer and the despatch of a reply. We can only in the vaguest and most general terms interpret what goes on in his cerebral hemispheres. If we admit that there are cortical concomitants, we must also admit that we know very little about them. That is why we are forced to change our point of view in the midst of an interpretation of what naturalism—physical and psychological—regards as a strictly determinate series. But to say that these conscious deliberations are merely a by-product of the physiological processes, of which we know so little; to say that they are only collateral effects of brain mechanics; is tantamount to saying, as Professor Herbert put it, "that the actions, words, and gestures of every individual of the human race would have been exactly what they have been in the absence of mind; had mind been wanting, the same empires would have risen and fallen, the same battles would have been fought and won, the same literature, the same masterpieces of painting and music would have been produced, the same religious rites would have been performed, and the same indications of friendship and affection given." That is what the steam-whistle theory of consciousness involves, and that is a view which, I venture to think, cannot possibly be accepted.
In place of this we have, still however on the naturalistic interpretation, a continuous biological series of events completely explicable in terms of antecedence and sequence; and a discontinuous mental series, sections of which may be explained in terms of subjective or ejective sequence, while intervening sections have to be explained by referring us to organic conditions. By the doctrine of concomitance the ideal construction of psychology is brought into relation with the ideal construction of physiology. But according to naturalism the ideal construction of physiology may itself be brought into relation with the ideal construction of physics, and the phenomena may be interpreted in terms of physical configurations. Hence, in naturalistic analysis and synthesis concomitance links together physical configurations, and what we may term psychological configurations or dispositions. But there is no necessary reference to any underlying cause through the agency of which the configurations or dispositions change in this way or in that.