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. CHAPTER XV THE NATURE OF OUR KNOWLEDGE OF PHYSICS In this chapter, we shall seek an answer to two questions: First, _how_ do we know about the world dealt with in physics? Secondly, _what_ do we know about it, assuming the truth of modern physics?