SPACE, TIME 305 



One, to be considered immediately, is the relativity of space- 

 time to events. The other, to be considered under the dis- 

 cussion of the serial properties, is the relativity of space and 

 time to one another. But in both of these cases there is some- 

 thing absolute. In the one case it is the law according to 

 which measuring devices vary with changing systems of 

 reference; in the other it is the abstract structure by which 

 space and time unite into a four-dimensional whole. Hence 

 there is a strict sense in which the theory of relativity in- 

 volves the assumption of an absolute structure. But this 

 absolute structure is different in important ways from the 

 absolute structure of the classical physics, and reference will 

 be made in what follows primarily to these differences. 



The outstanding sense in which space-time is relative is 

 in its dependence on events. But whereas at the empirical 

 level the insistence is on events of any kind, at the level of 

 the relativity theory the insistence is on measuring devices. 

 Time and space are identified with measurement of time 

 and space. This involves an inevitable reference to clocks 

 and to scales. The result is that time and space both become 

 relative to the techniques by which they are measured. 

 Not only is time meaningless apart from the readings of 

 clocks, but the setting up and reading of clocks is all that 

 can be meant by time. Not only is length meaningless apart 

 from the readings of scales, but the application and reading 

 of scales is all that can be meant by length. This leads 

 immediately to a consideration of the second aspect of space- 

 time. 



Plurality. If space and time are nothing apart from 

 measurement techniques, they will vary in character accord- 

 ing to the variation in the methods of applying and reading 

 recording devices. The most important feature of measure- 

 ment is that someone always measures; reference is therefore 

 necessary in every case to an observer. However, this fact 

 does not make space and time relative to the individual in 

 the same sense that empirical space and time are; it does not 

 make them private and relative to any subjective experience. 



