3. Nature is not made up of electrons, but of waves. The atom must be considered as a system of electric waves spread over its whole volume. ‘Electrons’ are merely an inaccurate way of describing some of the properties of these waves. The wave picture of the atom is, however, to be considered only as a temporary expedient to be used until some better description of the atom can be invented, in which both the wave and the corpuscular properties of atoms will appear as aspects of some more profound physical property. (Schrödinger.) The first alternative is a mere cry of despair, since it does not propose any line of advance. But the other two suggestions may be combined thus: 4. The view of the atom as a structure of Newtonian particles is wrong since it gives rise to discontinuities, and provides more quantities than we at present need. A new formulation of atomic processes must be found using fewer quantities which will explain why we find wave properties, and why sometimes the electron does behave like a small billiard ball though really it is some different sort of thing. Now since the Newtonian mathematics of moving particles is inadequate for describing the changes that go on in the atom--just as it is for describing organic processes--there must be some assumption implicit in Newton’s laws which is valid neither for atom nor for organism. Such an assumption can be found very easily, though physics has never given it much attention. It is that the elementary processes in nature are _reversible_, or would be if they could be isolated. By reversible is here meant that the laws governing the process remain unchanged when the direction of time is reversed, i.e. when -t is substituted for +t. If the law is changed by this substitution so that the reversed process never occurs or is recognizably different, then the process is called irreversible. An irreversible process can therefore be used to yield an objective criterion of past and future, when these terms have been once defined. To take an example. If I am standing behind a hedge and take a cinematograph film of a stone which suddenly rises in the air and disappears from sight, I could not tell from an examination of the film which way to wind it. Thus if it is wound one way the stone appears to rise, and if wound the other way to fall from the sky. To tell which was the right way I should have to use my subjective sense of the direction of time, i.e. remember the fact that I saw the stone low in the air before I saw it high up. This case, like every gravitational process, is reversible, and motions of this kind have provided the basis for modern physical conceptions.