THE VALVE OF SCIENCE 319 



the measurement of time; no matter whether a chronometer is used, 

 or whether account must be taken of a velocity of transmission, as 

 that of light, because such a velocity could not be measured without 

 measuring a time. 



XIII 



To conclude : We have not a direct intuition of simultaneity, nor of 

 the equality of two durations. If we think we have this intuition, this 

 is an illusion. We replace it by the aid of certain rules which we 

 apply almost always without taking count of them. 



But what is the nature of these rules? No general rule, no 

 rigorous rule; a multitude of little rules applicable to each particular 

 case. 



These rules are not imposed upon us and we might amuse ourselves 

 in inventing others; but they could not be cast aside without greatly 

 complicating the enunciation of the laws of physics, mechanics and 

 astronomy. 



We therefore choose these rules, not because they are true, but be- 

 cause they are the most convenient, and we may recapitulate them as 

 follows : " The simultaneity of two events, or the order of their succes- 

 sion, the equality of two durations, are to be so defined that the enun- 

 ciation of the natural laws may be as simple as possible. In other 

 words, all these rules, all these definitions are only the fruit of an 

 unconscious opportunism." 



(To be continued.) 



A 



