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. 



