It was traditionally a property of substance to be permanent, and to a considerable extent matter has retained this property in spite of its loss of substantiality. But its permanence now is only approximate, not absolute. It is thought that an electron and a proton can meet and annihilate each other; in the stars this is supposed to be happening on a large scale.[14] And even while an electron or a proton lasts, it has a different kind of persistence from that formerly attributed to matter. A wave in the sea persists for a longer or shorter time: the waves that I see dashing themselves to pieces on the Cornish coast may have come all the way from Brazil, but that does not mean that a “thing” has travelled across the Atlantic; it means only that a certain process of change has travelled. And just as a wave in the sea comes to grief at last on the rocks, so an electron or a proton may come to grief when it meets some unusual state of affairs. [14] See _The Analysis of Matter_, by the present author, chap. xxviii. Thus “matter” has very definitely come down in the world as a result of recent physics. It used to be the cause of our sensations: Dr. Johnson “disproved” Berkeley’s denial of matter by kicking a stone. If he had known that his foot never touched the stone, and that both were only complicated systems of wave-motions, he might have been less satisfied with his refutation. We cannot say that “matter” is the cause of our sensations. We can say that the events which cause our sensations usually belong to the sort of group that physicists regard as material; but that is a very different thing. Impenetrability used to be a noble property of matter, a kind of Declaration of Independence; now it is a merely tautological result of the way in which matter is defined. The events which are the real stuff of the world are not impenetrable, since they can overlap in space-time. In a word, “matter” has become no more than a convenient shorthand for stating certain causal laws concerning events.