SUMMARY 61 



magnetic equations of the electron it was thought that 

 it depended on the fact that an electron is an electrical 

 structure; and the agreement with observation was 

 believed to confirm the hypothesis that an electron is 

 pure electricity and nothing else. Our treatment above 

 makes no reference to any electrical properties of the 

 electron, the phenomenon having been found to arise 

 solely from the relativity of mass. Hence, although 

 there may be other good reasons for believing that an 

 electron consists solely of negative electricity, the in- 

 crease of mass with velocity is no evidence one way or 

 the other. 



In this chapter the idea of a multiplicity of frames of 

 space has been extended to a multiplicity of frames of 

 space and time. The system of location in space, called 

 a frame of space, is only a part of a fuller system of 

 location of events in space and time. Nature provides 

 no indication that one of these frames is to be preferred 

 to the others. The particular frame in which we are 

 relatively at rest has a symmetry with respect to us 

 which other frames do not possess, and for this reason 

 we have drifted into the common assumption that it is 

 the only reasonable and proper frame; but this egocen- 

 tric outlook should now be abandoned, and all frames 

 treated as on the same footing. By considering time 

 and space together we have been able to understand 

 how the multiplicity of frames arises. They correspond 

 to different directions of section of the four-dimensional 

 world of events, the sections being the "world-wide 

 instants". Simultaneity (Now) is seen to be relative. 

 The denial of absolute simultaneity is intimately con- 

 nected with the denial of absolute velocity; knowledge 

 of absolute velocity would enable us to assert that 



