174 THE LIMITATIONS OF SCIENCE 



The first is; we have no ability in us to determine 

 absolute rest or motion. We can say only that one 

 body is at rest or changes position with respect to 

 others. Phenomena are conditioned entirely by the 

 relative positions of bodies and we can gain nothing 

 toward an explanation by introducing the idea of abso- 

 lute position. The second postulate is; the velocity 

 of light, V, is a universal and absolute constant. 



Apparently no objection has been made to his state- 

 ment that these two postulates are sufficient from 

 which to derive all the conclusions mentioned. But it 

 seems to me, at least two more independent postulates 

 are advanced in the memoir on relativity, which must 

 also be granted. I shall propose as a third, Professor 

 Einstein's definition of time, and as a fourth, the as- 

 sumption of the atomic nature of electricity. The 

 fourth postulate carries with it as corollaries, that the 

 amount of electricity per atom, or the electron as it is 

 called, is a universal constant, and that the mass of the 

 electron is variable. My excuse for offering them is, 

 they cannot be derived from his postulates, and his con- 

 clusions require them. 



The first postulate can be granted at once and unre- 

 servedly for all purely mechanical motions of bodies. 

 These involve merely changes of position and do not 

 affect the nature of bodies nor their phenomena. But 

 it must be accepted with limitations when in addition 

 to mechanical motion, and by mechanical motion I 



