Everything in the world is composed of “events”; that, at least, is the thesis I wish to maintain. An “event”, as I understand it, is something having a small finite duration and a small finite extension in space; or rather, in view of the theory of relativity, it is something occupying a small finite amount of space-time. If it has parts, these parts, I say, are again events, never something occupying a mere point of instant, whether in space, in time, or in space-time. The fact that an event occupies a finite amount of space-time does not prove that it has parts. Events are not impenetrable, as matter is supposed to be; on the contrary, every event in space-time is overlapped by other events. There is no reason to suppose that any of the events with which we are familiar are infinitely complex; on the contrary, everything known about the world is compatible with the view that every complex event has a finite number of parts. We do not know that this is the case, but it is an hypothesis which cannot be refuted and is simpler than any other possible hypothesis. I shall therefore adopt it as a working hypothesis in what follows. When I speak of an “event” I do not mean anything out of the way. Seeing a flash of lightning is an event; so is hearing a tire burst, or smelling a rotten egg, or feeling the coldness of a frog. These are events that are “data” in the sense of Chapter XXV; but, on the principles explained in that chapter, we infer that there are events which are not data and happen at a distance from our own body. Some of these are data to other people, others are data to no one. In the case of the flash of lightning, there is an electro-magnetic disturbance consisting of events travelling outward from the place where the flash takes place, and then when this disturbance reaches the eye of a person or animal that can see, there is a percept, which is causally continuous with the events between the place of the lightning and the body of the percipient. Percepts afford the logical premisses for all inferences to events that are not precepts, wherever such inferences are logically justifiable. Particular colours and sounds and so on are events; their causal antecedents in the inanimate world are also events. If we assume, as I propose to do, that every event has only a finite number of parts, then every event is composed of a finite number of events that have no parts. Such events I shall call “minimal events.” It will simplify our discussion to assume them, but by a little circumlocution this assumption could be eliminated. The reader must not therefore regard it as an essential part of what follows.