3 i4 POPULAR SCIENCE MONTHLY 



language; it would evidently be much less simple. So that the defini- 

 tion implicitly adopted by the astronomers may be summed up thus: 

 Time should be so defined that the equations of mechanics may be as 

 simple as possible. In other words, there is not one way of measuring 

 time more true than another; that which is generally adopted is only 

 more convenient. Of two watches, we have no right to say that the one 

 goes true, the other wrong; we can only say that it is advantageous to 

 conform to the indications of the first. 



The difficulty which has just occupied us has been, as I have said, 

 often pointed out; among the most recent works in which it is con- 

 sidered, I may mention, besides M. Calinon's little book, the treatise 

 on mechanics of M. Andrade. 



VI. 



The second difficulty has up to the present attracted much less atten- 

 tion ; yet it is altogether analogous to the preceding ; and even, logically, 

 I should have spoken of it first. 



Two psychological phenomena happen in two different conscious- 

 nesses; when I say they are simultaneous, what do I mean? When I 

 say that a physical phenomenon, which happens outside of every 

 consciousness, is before or after a psychological phenomenon, what do 

 I mean? 



In 1572, Tycho Brahe noticed in the heavens a new star. An im- 

 mense conflagration had happened in some far distant heavenly body; 

 but it had happened long before; at least two hundred years were 

 necessary for the light from that star to reach our earth. This con- 

 flagration therefore happened before the discovery of America. Well, 

 when considering this gigantic phenomenon, which perhaps had no 

 witness, since the satellites of that star were perhaps uninhabited, 

 I say this phenomenon is anterior to the formation of the visual image 

 of the isle of Espanola in the consciousness of Christopher Columbus, 

 what do I mean? 



A little reflection is sufficient to understand that all these affirma- 

 tions have by themselves no meaning. They can have one only as the 

 outcome of a convention. 



VII. 



We should first ask ourselves how one could have had the idea of 

 putting into the same frame so many worlds impenetrable to each 

 other. We should like to represent to ourselves the external universe, 

 and only by so doing could we feel that we understood it. We know 

 we never can attain this representation: our weakness is too great. 

 But at least we desire the ability to conceive an infinite intelligence 

 for which this representation would be possible, a sort of great con- 



