Philosophy

averages as to what happens to matter; but they do not tell us when an individual atom will absorb or radiate energy. Whether there are laws, other than those of statistics, governing the behaviour of an individual atom in this respect, we do not know. It should be observed that there are causal laws of a different sort from those of pure physics; such are the laws that light-waves “cause” visual sensations and sound-waves “cause” auditory sensations. All the empirical evidence for physics rests upon such laws, therefore nothing in physics can have a higher degree of certainty than such laws have. Let us stop a moment to ask what we mean by “cause” in this connection. The connection of light-waves and visual sensations looks a little different according as we start with physics or with psychology, though, of course, ultimately the result must be the same. Let us first start with physics. I say, then, that when a light-wave travels outwards from a body there are successive events at successive places, and that the corresponding event in a brain behind a normal eye is a visual sensation. This is the only event in the whole series about which I can say anything not purely abstract and mathematical. Now let us start from the sensation. I say, then, that this sensation is one of a vast series of connected events, travelling out from a centre according to certain mathematical laws, in virtue of which the sensation enables me to know a good deal about events elsewhere. That is why the sensation is a source of physical knowledge. It will be seen that, according to the view I have been advocating, there is no difficulty about interaction between mind and body. A sensation is merely one link in a chain of physical causation; when we regard the sensation as the end of such a chain, we have what would be regarded as an effect of matter on mind; when as the beginning, an effect of mind on matter. But mind is merely a cross-section in a stream of physical causation, and there is nothing odd about its being both an effect and a cause in the physical world. Thus physical causal laws are those that are fundamental. There seems no reason to regard causation as a priori, though this question is not simple. Given certain very general assumptions as to the structure of space-time, there are bound to be what we have called causal laws. These general assumptions must really replace causality as our basic principles. But, general as they are, they cannot be taken as a priori; they are the generalisation and abstract epitome of the fact that there are causal laws, and this must remain merely an empirical fact, which is rendered probable, though not certain, by inductive arguments.