Common sense thinks of the physical world as composed of “things” which persist through a certain period of time and move in space. Philosophy and physics developed the notion of “thing” into that of “material substance”, and thought of material substance as consisting of particles, each very small, and each persisting throughout all time. Einstein substituted events for particles; each event had to each other a relation called “interval”, which could be analyzed in various ways into a time-element and space-element. The choice between these various ways was arbitrary and no one of them was theoretically preferable to any other. Given two events A and B, in different regions, it might happen that according to one convention they were simultaneous, according to another A was earlier than B, and according to yet another B was earlier than A. No physical facts correspond to these different conventions.
From all this it seems to follow that events, not particles, must be the “stuff” of physics. What has been thought of as a particle will have to be thought of as a series of events. The series of events that replaces a particle has certain important physical properties, and therefore demands our attention; but it has no more substantiality than any other series of events that we might arbitrarily single out. Thus “matter” is not part of the ultimate material of the world, but merely a convenient way of collecting events into bundles.
Quantum theory reinforces this conclusion, but its chief philosophical importance is that it regards physical phenomena as possibly discontinuous. It suggests that, in an atom (interpreted as above), a certain state of affairs persists for a certain time, and then suddenly is replaced by a finitely different state of affairs.