i8o THE LIMITATIONS OF SCIENCE 



When the ether was a substance which periodically 

 varied electromagnetically, we could still say that V 

 had a possible meaning. But the latest definition, that 

 the ether is absolutely quiescent space to be distin- 

 guished from vacuous space only by the fact that it is 

 the seat of an entity, called electro-magnetic energy, 

 and contains a light vector, makes the word velocity 

 when applied to V absolutely without meaning in any 

 ordinary sense of the term. To speak of a motion in 

 an absolutely quiescent space is a rather startling state- 

 ment. The ether also becomes a local affair, shifting 

 back and forth in vacuous space according as light is 

 present or not. The feeble light of a candle, appa- 

 rently, changes nothing into something. 



But aside from these considerations, which lie in the 

 debatable land and which will be decided largely by the 

 temperament of the individual, there seems to be an 

 absolute contradiction between the first two postulates 

 as I understand them. If V be the value of the velocity 

 of light in an absolutely quiescent ether and itself a 

 constant, then the velocity of light in a material me- 

 dium, such as air, is an absolute velocity or motion 

 when referred to V. Now the first postulate declares 

 that we can have no knowledge of absolute motion 

 and, in addition, that the explanation of phenomena is 

 not conditioned by absolute motion. 



The third postulate, which I have proposed as neces- 

 sary for the system proposed by Professor Einstein, 



