A piece of matter, like a space-time point, is to be constructed out of events, but the construction is considerably more complicated, and in the end is only an approximation to what the physicist supposes to be really taking place. There are, at the moment, two somewhat different views of matter, one appropriate to the study of atomic structure, the other to the general theory of relativity as affording an explanation of gravitation. The view appropriate to atomic structure has itself two forms, one derived from Heisenberg, the other from De Broglie and Schrödinger. These two forms, it is true, are mathematically equivalent, but in words they are very different. Heisenberg regards a piece of matter as a centre from which radiations travel outward; the radiations are supposed really to occur, but the matter at their centre is reduced to a mere mathematical fiction. The radiations are, for example, such as constitute light; they are all avowedly systems of events, not changes in the conditions or relations of “substances.” In the De Broglie-Schrödinger system, matter consists of wave motions. It is not necessary to the theory to postulate anything about these wave-motions except their mathematical characteristics, but, obviously, since they are to explain matters they cannot serve their purpose if they consist of motions of matter. In this system also, therefore, we are led to construct matter out of systems of events, which just happen, and do not happen “to” matter or “to” anything else. Gravitation, as explained by the general theory of relativity, is reduced to “crinkles” in space-time. Space-time being, as we have already seen, a system constructed out of events, the “crinkles” in it are also derived from events. There is no reason to suppose that there is a “thing” at the place where the “crinkle” is most crinkly. Thus in this part of physics, also, matter has ceased to be a “thing” and has become merely a mathematical characteristic of the relations between complicated logical structures composed of events.