But now consider what happens when the wave of light which started from our hydrogen atom comes in contact with matter. Various things may happen. The matter may absorb all or some of the energy of the light-ray; this is the interesting case from our point of view. The absorption may take the form of causing the electrons to move in larger orbits, in which case, later, when they return to their previous orbits, we get the phenomenon of fluorescence. Or the body may become heated; or it may visibly move, like a radiometer. The effects upon bodies depend upon the bodies as well as the light. Some of them can be individually predicted, others can only be calculated in statistical averages; this depends upon whether quantum considerations come in or not. Where they do, we can enumerate possibilities, and state the relative frequencies with which they will be realised, but we cannot tell which will be realised in any given case. So far, we have considered the radiation of energy from matter into empty space, its propagation in empty space and its impact on matter from empty space. We have not considered the history of a given piece of matter, or the distinction between matter and empty space. The essence of matter appears to be this: We can distinguish series of events in space-time which have a certain kind of close resemblance to each other, such that common sense regards them as manifestations of one “thing”. But when we look closely at the question, it turns out that what physics offers is something more abstract than this. Take, _e. g._ the continued existence of a certain electron. This means to say that events in a certain neighbourhood will be such as can be calculated on the assumption that there is an electric charge of a certain standard magnitude in the middle of that neighbourhood; and that the neighbourhoods of which this is true form a tube in space-time. So long as we stick to the standpoint of pure physics there is a certain air of taking in each other’s washing about the whole business. Events in empty space are only known as regards their abstract mathematical characteristics; matter is only an abstract mathematical characteristic of events in empty space. This seems rather a cold world. But as a matter of fact we know some things that are a little more concrete. We know, _e.g._ what it feels like when we see things. From the point of view of physics, when our light-wave starts out through empty space, if it presently reaches our eye we know one link in the causal chain, namely the visual sensation, otherwise than as a term in an abstract mathematical formula. And it is this one term which forms the basis for our belief in all the rest. Seeing is believing.