Another department of theoretical physics, the theory of relativity, has philosophical consequences which are, if possible, even more important. The substitution of space-time for space and time has made the category of substance less applicable than formerly, since the essence of substance was persistence through time, and there is now no one cosmic time. The result of this is to turn the physical world into a four-dimensional continuum of events, instead of a series of three-dimensional states of a world composed of persistent bits of matter. A second important feature of relativity-theory is the abolition of force, particularly gravitational force, and the substitution of differential causal laws having to do only with the neighbourhood of an event, not with an influence exerted from a distance, such as gravitation formerly seemed to be. The modern study of the atom has had two consequences which have considerably changed the philosophical hearing of physics. On the one hand, it appears that there are discontinuous changes in nature, occasions when there is a sudden jump from one state to another without passing through the intermediate states. (Schrödinger, it is true, questions the need for assuming discontinuity; but so far his opinion has not prevailed.) On the other hand, the course of nature is not so definitely determined by the physical laws at present known as it was formerly thought to be. We cannot predict when a discontinuous change will take place in a given atom, though we can predict statistical averages. It can no longer be said that, given the laws of physics and the relevant facts about the environment, the future history of an atom can theoretically be calculated from its present condition. It may be that this is merely due to the insufficiency of our knowledge, but we cannot be sure that this is the case. As things stand at present, the physical world is not so rigidly deterministic as it has been believed to be during the last 250 years. And in various directions what formerly appeared as laws governing each separate atom are now found to be only averages attributable in part to the laws of chance.