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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 environ ment, 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.

From these questions concerning the physical world in itself, we were led to others concerning the causation of our perceptions, which are the data upon which our scientific knowledge of physics is based. We saw that a long causal chain always intervenes between an external event and the event in us which we regard as perception of the external event. We cannot therefore suppose that the external event is exactly what we see or hear; it can, at best, resemble the percept only in certain structural respects. This fact has caused considerable confusion in philosophy, partly because philosophers tried to think better of perception than it deserves, partly because they failed to have clear ideas on the subject of space. It is customary to treat space as a characteristic of matter as opposed to mind, but this is only true of physical space. There is also perceptual space, which is that in which what we know immediately through the senses is situated. This space cannot be identified with that of physics. From the standpoint of physical space, all our percepts are in our heads; but in perceptual space our percept of our hand is outside our