Imagine a hydrogen atom, in which the electron is revolving not in the minimum orbit, but in the next, which has four times the minimum radius. So long as this state continues, the atom has no external effects, apart from its infinitesimal gravitational action; we cannot, therefore, obtain any evidence of its existence except when it changes its state. In fact, our knowledge of atoms is like that which a ticket collector has of the population of his town: he knows nothing of those who stay quietly at home. Now at some moment, according to laws of which we have only statistical knowledge, the electron in our atom jumps to a smaller orbit, and the energy lost to the atom travels outward in a light-wave. We know no causal law as to when the electron will jump, though we know how far it will jump and exactly what will happen in the neighbourhood when it does. At least, when I say we know exactly what will happen, I ought to say that we know exactly the mathematical laws of what will happen. A series of events, having quantitative characteristics which obey certain equations, will travel outward in all directions from the electron, and will proceed quite regularly, like ripples on a pool, until other matter is encountered. We have here one important and apparently fundamental kind of causal law, the kind regulating the propagation of light _in vacuo_. This is summed up in Maxwell’s equations, which enable us to calculate the diffusion of an electro-magnetic disturbance starting from a source. So long as two such disturbances do not meet, the matter is exceedingly simple; but the equations also tell us what happens when they do meet. We then have, as always in traditional physics, two separate tendencies, which have a resultant compounded according to mathematical laws, of which the parallelogram law is the oldest and simplest. That is to say, each previous circumstance in the space-time neighbourhood contributes a tendency, and the resulting event is obtained by compounding these tendencies according to a mathematical law. So far, we have been considering only electro-magnetic phenomena in empty space. We have another set of facts about empty space, namely those upon which gravitation depends. These have to do with the structure of space-time, and show that this structure has singularities in the regions where there is matter, which spread with diminishing intensity as we get away from these regions. You may conceive the structure of space-time on the analogy of a pond with a fountain playing in it, so that wherever a spray falls from the fountain there is a little hill of water which flattens quickly as you get away from the spot where the spray falls. Here again the same sort of thing applies: to infer the structure in a small region of space-time from that in the neighbourhood, it will be necessary to superpose a number of tendencies according to mathematical rules. Thus philosophically this introduces no novelty.