The notion of a “place” is also quite vague. Is London a “place”? But the earth is rotating. Is the earth a “place”? But it is going round the sun. Is the sun a “place”? But it is moving relatively to the stars. At best you could talk of a place at a given time; but then it is ambiguous what is a given time, unless you confine yourself to one place. So the notion of “place” evaporates. We naturally think of the universe as being in one state at one time and in another at another. This is a mistake. There is no cosmic time, and so we cannot speak of the state of the universe at a given time. And similarly we cannot speak unambiguously of the distance between two bodies at a given time. If we take the time appropriate to one of the two bodies, we shall get one estimate; if the time of the other, another. This makes the Newtonian law of gravitation ambiguous, and shows that it needs restatement, independently of empirical evidence. Geometry also goes wrong. A straight line, for example, is supposed to be a certain track in space whose parts all exist simultaneously. We shall now find that what is a straight line for one observer is not a straight line for another. Therefore geometry ceases to be separable from physics. The “observer” need not be a mind, but may be a photographic plate. The peculiarities of the “observer” in this region belong to physics, not to psychology. So long as we continue to think in terms of bodies moving, and try to adjust this way of thinking to the new ideas by successive corrections, we shall only get more and more confused. The only way to get clear is to make a fresh start, with _events_ instead of bodies. In physics, an “event” is anything which, according to the old notions, would be said to have both a date and a place. An explosion, a flash of lightning, the starting of a light-wave from an atom, the arrival of the light-wave at some other body, any of these would be an “event”. Some strings of events make up what we regard as the history of one body; some make up the course of one light-wave; and so on. The unity of a body is a unity of history--it is like the unity of a tune, which takes time to play, and does not exist whole in any one moment. What exists at any one moment is only what we call an “event”. It may be that the word “event”, as used in physics, cannot be quite identified with the same word as used in psychology; for the present we are concerned with “events” as the constituents of physical processes, and need not trouble ourselves about “events” in psychology.