natural events. If it were only an induction drawn from the observation of non-organic processes, we might well anticipate an exception to its universality where matter borders upon mind. That it does not express an observation, but a postulate which we bring with us to the explanation of nature, appears when we consider what is meant by explanation.
To explain means to resolve the complicated phenomena we meet with in experience into simpler, more perspicuous ones. We understand a physical event when we have analyzed it into the elementary processes that make it up. If we wish to explain these, we resolve them into others still more elementary, till we reach the simplest processes conceivable, namely such as are continuously imaginable. These last cannot be analyzed into others still simpler, for what is imaginable is incapable of further analysis. But the causal impulse does not require that they should be further analyzed; for we think we discover in them a certain intrinsic necessity which makes further explanation superfluous. These simplest processes are described in the physical axioms. When an event is shown to be a consequence of the physical axioms, we regard it as explained.
A complicated process is thus explained by being resolved into simple processes which are continuously imaginable and necessary. The entire cosmic process would be completely explained, on its physical side, if we could resolve it without remainder into an infinity of simple processes, each continuously imaginable and necessary. We should then be able, if our minds were only vast enough, to picture to ourselves the entire course of nature in all its details as one continuous panorama, in which every transition would appear to us to be a necessary one.
Since the explanation of natural processes presupposes their continuous imaginability, it is evident that any psychophysical hypothesis which assumes the presence of lacunæ or discontinuities in nature violates the fundamental postulate upon which physical science depends. That postulate, commonly known as the law of the conservation of energy, forbids us to suppose that a motion can arise out of nothing or pass away into nothing. "That motion arises out of motion is in itself no more intelligible than that motion arises out of nothing; but only the former is continuously imaginable, only the former therefore explicable."
The conservation of energy therefore forbids us to suppose that any physical energy is lost when sensory nerve-impulses give rise to a sensation, or that any energy comes into existence when a volition is followed by a nervous discharge into the muscles. We must rather suppose that the sensory nerve-impulses produce commotions in the cortical nerve-
