The point is not that the facts do not fit with the hypothesis of the atom as a planetary system. There are, it is true, certain difficulties which afford empirical grounds for the newer theory which has superseded Bohr’s, and which we shall shortly consider. But even if no such grounds existed, it would be obvious that Bohr’s theory states more than we have a right to infer from what we can observe. Of theories that state so much, there must be an infinite number that are compatible with what is known, and it is only what all of these have in common that we are really entitled to assert. Suppose your knowledge of Great Britain were entirely confined to observing the people and goods that enter and leave the ports; you could, in that case, invent many theories as to the interior of Great Britain, all of which would agree with all known facts. This is an exact analogy. If you delimit in the physical universe any region, large or small, not containing a scientific observer, all scientific observers will have exactly the same experiences whatever happens inside this region, provided it does not affect the flow of energy across the boundary of the region. And so, if the region contains one atom, any two theories which give the same results as to the energy that the atom radiates or absorbs are empirically indistinguishable, and there can be no reason except simplicity for preferring one of them to the other. On this ground, even if on no other, prudence compels us to seek a more abstract theory of the atom than that which we owe to Rutherford and Bohr. The newer theory has been put forward mainly by two physicists already mentioned, Heisenberg and Schrödinger, in forms which look different, but are in fact mathematically equivalent. It is as yet an impossible task to describe this theory in simple language, but something can be said to show its philosophical bearing. Broadly speaking, it describes the atom by means of the radiations that come out of it. In Bohr’s theory, the planetary electrons are supposed to describe orbits over and over again while the atom is not radiating; in the newer theory, we say nothing at all as to what happens at these times. The aim is to confine the theory to what is empirically verifiable, namely radiations; as to what there is where the radiations come from, we cannot tell, and it is scientifically unnecessary to speculate. The theory requires modifications in our conception of space, of a sort not yet quite clear. It also has the consequence that we cannot identify an electron at one time with an electron at another, if in the interval, the atom has radiated energy. The electron ceases altogether to have the properties of a “thing” as conceived by common sense; it is merely a region from which energy may radiate.