cells, and that these manifest themselves in consciousness in the form of a sensation; and that the motor nerve-impulses are caused by other commotions in the cortical nerve-cells, which last manifest themselves in consciousness in the form of a volition. We must suppose, in short, that physical processes constitute a locked system, every subsequent state of which is fully accounted for by the immediately preceding state; that the nervous system is a highly complex mechanism for adapting the movements of an animal to environing conditions in a manner useful to the animal; and that consciousness is a parallel manifestation, concomitant to certain processes in the highest nerve-centres.
We thus have two series of events running parallel to each other — a continuous series of physical events, and side by side with it an interrupted series of mental events, corresponding to certain portions of the physical series. The doctrine thus far developed is that of a psycho-physical parallelism. Now physics in its sphere can never explain the mental series, any more than psychology in its sphere can explain the physical series. Yet the two sustain the most intimate relations to each other, for no mental events ever happen, so far as our experience goes, except in connection with physical events. We therefore need an hypothesis to account for this connection.
Such an hypothesis must either conceive the two series as co-ordinate, or regard one of them as subordinate to the other. They might, in the first place, be conceived to be co-ordinate, yet in other respects independent, each series taking place according to laws of its own. But this hypothesis would afford no explanation of their remarkable correspondence — no explanation of the fact that every bodily disturbance of sufficient intensity is accompanied by a sensation, and every emotion and volition by bodily changes. Our knowledge of the spatial world and our ability to act in it depend upon this fact, which cannot therefore be an accident. The hypothesis in question is thus as good as none at all.
A second hypothesis, first proposed by Leibniz, assumes the two series to be co-ordinate, but supposes them to have been created and set running by a cosmic Intellect, like two clocks which always correspond, yet do not mutually influence each other. This hypothesis — that of pre-established harmony — only repeats the problem in another form; for the relations between such a cosmic Intellect and the world of matter constitute a far more difficult psychophysical problem than the one we are seeking to solve.
Turning therefore to the hypothesis which regards one series as subordinate to the other, we assume in the first place, with Aristotle, Spinoza, Hegel, von Hartmann, and Wundt, that the mental series is the condition of the physical.
