From the agnostic, the sceptical, point of view this is a fine and strong attitude. But in the course of this volume we have so much admired Einstein’s powerful theoretical synthesis and the surprising verifications to which it led that we are now entitled to make some reserves. It is legitimate to call into question even the denials of doubters, because, after all, they are really themselves affirmations. We believe that in face of this philosophic attitude of the Einsteinians—in face of what I should like to call their absolute relativism—we are justified in rebelling a little and saying something like this: “Yes, everything is possible; or, rather, many things are possible, but all things are not. Yes, if I go into a strange house, the drawing-room clock may be round, square, or octagonal. But once I have entered the house and seen that the clock is square, I have a right to say: ‘The clock is square. It has the privilege of being square. It is a fact that it is neither round nor octagonal.’ “It is the same in nature. The physical continuum which contains, like a vase, all the phenomena of the universe, might have, relatively to me—and as long as I have not observed it—any forms or movements whatever. But as a matter of fact, it is what it is. It cannot be different things at the same time. The drawing-room clock cannot at one and the same time be composed entirely of gold and entirely of silver. “There is therefore one privileged possibility amongst the various possibilities which we imagine in the external world. It is that which has been effectively realised: that which exists.” The complete relativism of the Einsteinians amounts to making the universe external to us to such an extent that we have no means of distinguishing between what is real and what is possible in it, as far as space and time are concerned. The Newtonians, on the other hand, say that we can recognise real space and real time by special signs. We will analyse these signs later. In a word, the pure Relativists have tried to escape the necessity of supposing that reality is inaccessible. It is a point of view that is at once more modest and much more presumptuous than that of the Newtonians, the Absolutists. It is more modest because according to the Einsteinian we cannot know certain things which the Absolutist regards as accessible: real time and space. It is more presumptuous because the Relativist says that there is no reality except that which comes under observation. For him the unknowable and non-existent are the same thing. That is why Henri Poincaré, who was the most profound of Relativists before the days of Einstein, used to repeat constantly that questions about absolute space and time have “no meaning.” One might sum it up by saying that the Einsteinians have taken as their motto the words of Auguste Comte: “Everything is relative, and that is the only absolute.”