What do we mean by a “piece of matter” in this statement? We do not mean something that preserves a simple identity throughout its history, nor do we mean something hard and solid, nor even a hypothetical thing-in-itself known only through its effects. We mean the “effects” themselves, only that we no longer invoke an unknowable cause for them. We find that energy in various forms spreads outwards from various centres; we find also that such centres have a certain degree of persistence, though this persistence is not absolute--the modern physicist faces cheerfully the possibility that an electron and a proton may mutually annihilate each other, and even suggests that this may be the main source of the radiant energy of the stars, because when it happens it makes an explosion. What is asserted may be put as follows: When energy radiates from a centre, we can describe the laws of its radiation conveniently by imagining something in the centre, which we will call an electron or a proton according to circumstances, and for certain purposes it is convenient to regard this centre as persisting, _i.e._ as not a single point in space-time but a series of such points, separated from each other by time-like intervals. All this, however, is only a convenient way of describing what happens elsewhere, namely the radiation of energy away from the centre. As to what goes on in the centre itself, if anything, physics is silent. What Dr. Whitehead calls the “pushiness” of matter disappears altogether on this view. “Matter” is a convenient formula for describing what happens where it isn’t. I am talking physics, not metaphysics; when we come to metaphysics, we may be able, tentatively, to add something to this statement, but science alone can hardly add to it. Materialism as a philosophy becomes hardly tenable in view of this evaporation of matter. But those who would formerly have been materialists can still adopt a philosophy which comes to much the same thing in many respects. They can say that the type of causation dealt with in physics is fundamental, and that all events are subject to physical laws. I do not wish, as yet, to consider how far such a view should be adopted; I am only suggesting that it must replace materialism as a view to be seriously examined. PART III MAN FROM WITHIN CHAPTER XVI SELF-OBSERVATION