X. TIME1

It seems that the one all-embracing time is a con- struction, like the one all-embracing space. Physics itself has become conscious of this fact through the dis- cussions connected with relativity.
Between two perspectives which both belong to one person's experience, there will be a direct time-relation of before and after. This suggests a way of dividing history in the same sort of way as it is divided by different experiences, but without introducing experience or any thing mental : we may define a " biography " as every- thing that is (directly) earlier or later than, or simul- taneous with, a given " sensibile." This will give a series of perspectives, which might all form parts of one person's experience, though it is not necessary that all or any of them should actually do so. By this means, the history of the world is divided into a number of mutually exclusive biographies.
1 On this subject, compare A Theory of Time and Space, by Mr. A. A. Robb (Camb. Univ. Press), which first suggested to me the views advocated here, though I have, for present purposes, omitted what is most interesting and novel in his theory. Mr. Robb has given a sketch of his theory in a pamphlet with the same title (Heffer and Sons. Cambridge. 1913).
168 MYSTICISM AND LOGIC
We have now to correlate the times in the different biographies. The natural thing would be to say that the appearances of a given (momentary) thing in two different perspectives belonging to different biographies are to be taken as simultaneous ; but this is not convenient. Suppose A shouts to B, and B replies as soon as he hears A's shout. Then between A's hearing of his own shout and his hearing of B's there is an interval ; thus if we made A's and B's hearing of the same shout exactly simultaneous with each other, we should have events exactly simultaneous with a given event but not with each other. To obviate this, we assume a ' velocity of sound." That is, we assume that the time when B hears A 's shout is half-way between the time when A hears his own shout and the time when he hears B's. In this way the correlation is effected.
What has been said about sound applies of course equally to light. The general principle is that the appearances, in different perspectives, which are to be grouped together as constituting what a certain thing is at a certain moment, are not to be all regarded as being at that moment. On the contrary they spread outward from the thing with various velocities according to the nature of the appearances. Since no direct means exist of correlating the time in one biography with the time in another, this temporal grouping of the appearances belonging to a given thing at a given moment is in part conventional. Its motive is partly to secure the verifica- tion of such maxims as that events which are exactly simultaneous with the same event are exactly simul- taneous with one another, partly to secure convenience in the formulation of causal laws.
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