Common Sense of Relativity . 507 



have to do now is to solve the problem under consideration 

 for the limiting case o£ infinitesimal velocity, and then effect 

 a mere algebraical transformation. The only objection that 

 seems likely to be raised is that the Principle proves too 

 much, that it appears impossible that such far-reaching 

 conclusions can be drawn from such simple assumptions : the 

 only difficulty, in fact, is that the thing is too easy. 



5. That the arguments by which the conclusion is attained 

 are valid can, of coarse, only be proved by examining them, 

 but I think a few remarks of a general nature may remove 

 one cause of uneasiness. It is felt that the universal im- 

 portance attributed to the velocity of light is strange, when 

 it is proposed to apply the principle to laws which have 

 nothing to do with optics. Why, it may be questioned, do we 

 drag in the velocity of light rather than that of sound or of 

 the trains on the twopenny tube? Some part of this un- 

 easiness may arise from the unfortunate way in which Einstein 

 introduces the Second Postulate in his paper : he seems 

 almost to try to deduce it from the First Postulate. In 

 describing the First Postulate he says : — "In particular the 

 same number must be found for the velocity of light in vacuo 

 for both reference systems." It is very pertinent to ask 

 here why, then, the velocity of light rather than that of 

 sound. 



Of course the Second Postulate cannot really be deduced 

 from the First. What the First Postulate asserts is that all 

 laws must be the same for all quiet systems having no relative 

 acceleration : in particular, for all such systems, the velocity 

 of light or the velocity of sound determined from a source 

 which forms part of the system to a receiver which forms 

 part of the system must be the same. But the proposition 

 which is necessary for the argument is quite different from 

 this. It is that the velocity of light from some source 

 common to two systems will be found to be the same by 

 observers on both systems, even if those systems are in 

 relative unaccelerated motion. Since the source is common 

 to the tw T o systems in relative motion, it is clear that both 

 systems, if they both include the source, cannot be quiet, and, 

 therefore, that the First Postulate, which refers only to quiet 

 systems, can have nothing to do with the matter. 



The Second Postulate is really made up of three distinct 

 propositions *. The first is that there is some velocity which 



* The complexity of the Second Postulate appears very clearly in 

 Minkowski's ' Raum mid Zeit.' Minkowski's treatment is somewhat 

 different from that of Einstein and involves an entire rejection of the 

 conceptions of space and time. 



2 L2 



