must not think of the waves as being necessarily “in” the æther or “in” anything else; they are to be thought of merely as progressive periodic processes, whose laws are more or less known, but whose intrinsic character is not known and never can be. The theory of relativity has arisen from the study of what goes on in the regions where there are no electrons and protons. While the study of the atom has led us to discontinuities, relativity has produced a completely continuous theory of the intervening medium--far more continuous than any theory formerly imagined. At the moment, these two points of view stand more or less opposed to each other, but no doubt before long they will be reconciled. There is not, even now, any logical contradiction between them; there is only a fairly complete lack of connection. For philosophy, far the most important thing about the theory of relativity is the abolition of the one cosmic time and the one persistent space, and the substitution of space-time in place of both. This is a change of quite enormous importance, because it alters fundamentally our notion of the structure of the physical world, and has, I think, repercussions in psychology. It would be useless, in our day, to talk about philosophy without explaining this matter. Therefore I shall make the attempt, in spite of some difficulty. Common-sense and pre-relativity physicists believed that, if two events happen in different places, there must always be a definite answer, in theory, to the question whether they were simultaneous. This is found to be a mistake. Let us suppose two persons A and B a long way apart, each provided with a mirror and means of sending out light-signals. The events that happen to A still have a perfectly definite time-order, and so have those that happen to B; the difficulty comes in connecting A’s time with B’s. Suppose A sends a flash to B, B’s mirror reflects it, and it returns to A after a certain time. If A is on the earth and B on the sun, the time will be about sixteen minutes. We shall naturally say that the time when B received the light-signal is half way between the times when A sent it out and received it back. But this definition turns out to be not unambiguous; it will depend upon how A and B are moving relatively to each other. The more this difficulty is examined, the more insuperable it is seen to be. Anything that happens to A after he sends out the flash and before he gets it back is neither definitely before nor definitely after nor definitely simultaneous with the arrival of the flash at B. To this extent, there is no unambiguous way of correlating times in different places.