The events in the physical world have relations to each other which are of the sort that have led to the notions of space and time. They have relations of order, so that we can say that one event is nearer to a second than to a third. In this way we can arrive at the notion of the “neighbourhood” of an event: it will consist roughly speaking of all the events that are very near the given event. When we say that neighbouring events have a certain relation, we shall mean that the nearer two events are to each other, the more nearly they have this relation, and that they approximate to having it without limit as they are taken nearer and nearer together. Two neighbouring events have a measurable quantitative relation called “interval”, which is sometimes analogous to distance in space, sometimes to lapse of time. In the former case it is called space-like, in the latter time-like. The interval between two events is time-like when one body might be present at both--for example, when both are parts of the history of your body. The interval is space-like in the contrary case. In the marginal case between the two, the interval is zero; this happens when both are parts of one light-ray. The interval between two neighbouring events is something objective, in the sense that any two careful observers will arrive at the same estimate of it. They will not arrive at the same estimate for the distance in space or the lapse of time between the two events, but the interval is a genuine physical fact, the same for all. If a body can travel freely from one event to the other, the interval between the two events will be the same as the time between them as measured by a clock travelling with the body. If such a journey is physically impossible, the interval will be the same as the distance as estimated by an observer to whom the two events are simultaneous. But the interval is only definite when the two events are very near together; otherwise the interval depends upon the route chosen for travelling from the one event to the other. Four numbers are needed to fix the position of an event in the world; these correspond to the time and the three dimensions of space in the old reckoning. These four numbers are called the co-ordinates of the event. They may be assigned on any principle which gives neighbouring co-ordinates to neighbouring events; subject to this condition, they are merely conventional. For example, suppose an aeroplane has had an accident. You can fix the position of the accident by four numbers: latitude, longitude, altitude above sea-level, and Greenwich Mean Time. But you cannot fix the position of the explosion in space-time by means of less than four numbers.