308 INTRODUCTION TO PHILOSOPHY OF SCIENCE 



would then be parallel. But suppose that the whole system 

 A-B is moving toward A or toward B ; in either of these cases 

 the time required to go from A to B would presumably not 

 be one-half of the total, since in one case B comes to meet 

 the signal and in the other case B must be overtaken by it. 

 Further difficulties arise if the system is considered to be 

 moving perpendicularly to the line from A to B. A situa- 

 tion of this kind determined the set-up for the Michelson- 

 Morley experiment, whose results are too well known to 

 require examination here. The point of this discussion is 

 simply to show that the determination of coexistence is a 

 highly complicated operation involving the use of messages. 

 But the introduction of messages involves the introduction 

 of motion, which itself brings space into the problem. The 

 result is that measurement of time is seen to be impossible 

 unless reference is made to space. Analogous considerations 

 show that the measurement of space is impossible unless 

 reference is made to time. Hence time and space are inter- 

 woven in a manner which defies separation. 



This constitutes the second sense in which space-time 

 may be said to be relative. Space measurements are relative 

 to time measurements, and vice versa. But it is well to note 

 here also that something absolute is implied in the concep- 

 tion. The abstract structure by which space and time are 

 united into a four-dimensional scheme is not relative but 

 absolute. 'According to the Galilean system of reference x 

 we may adopt, we divide space-time into space and time in 

 one way or in another. Hence the resolution of the drawings 

 of phenomena into space and time components is effected 

 in various ways. We thus obtain variations in the spatio- 

 temporal appearances of phenomena. Notwithstanding 

 these variations, dependent on our relative motion, the 

 absolute space-time drawings themselves, with their inter- 

 sections, remain unchanged, and a direct study of the abso- 

 lute world would consist in a study of these absolute draw- 



1 A system of reference is Galilean if it is in uniform rather than accelerated or 

 rotational motion. 



