THE VALUE OF SCIENCE 317 



XI. 



Another difficulty; have we really the right to speak of the cause 

 of a phenomenon? If all the parts of the universe are interchained in 

 a certain measure, any one phenomenon will not be the effect of a 

 single cause, but the resultant of causes infinitely numerous; it is, one 

 often says, the consequence of the state of the universe a moment 

 before. How enunciate rules applicable to circumstances so complex? 

 And yet it is only thus that these rules can be general and rigorous. 



Not to lose ourselves in this infinite complexity let us make a 

 simpler hypothesis. Consider three stars, for example, the sun, Jupiter 

 and Saturn; but, for greater simplicity, regard them as reduced to 

 material points and isolated from the rest of the world. The positions 

 and the velocities of three bodies at a given instant suffice to determine 

 their positions and velocities at the following instant, and consequently 

 at any instant. Their positions at the instant t determine their posi- 

 tions at the instant t -f- h as well as their positions at the instant 

 t — li. 



Even more; the position of Jupiter at the instant t, together with 

 that of Saturn at the instant t -f- a, determines the position of Jupiter 

 at any instant and that of Saturn at any instant. 



The aggregate of positions occupied by Jupiter at the instant t -\- e 

 and Saturn at the instant t -f- a -j- e is bound to the aggregate of posi- 

 tions occupied by Jupiter at the instant t and Saturn at the instant 

 t + &> by laws as precise as that of Newton, though more complicated. 

 Then why not regard one of these aggregates as the cause of the other, 

 which would lead to considering as simultaneous the instant t of 

 Jupiter and the instant t -f- a of Saturn ? 



In answer there can only be reasons, very strong, it is true, of 

 convenience and simplicity. 



XII. 



But let us pass to examples less artificial ; to understand the defini- 

 tion implicitly supposed by the savants, let us watch them at work and 

 look for the rules by which they investigate simultaneity. 



I will take two simple examples, the measurement of the velocity 

 of light and the determination of longitude. 



When an astronomer tells me that some stellar phenomenon, which 

 his telescope reveals to him at this moment, happened nevertheless 

 fifty years ago, I seek his meaning, and to that end I shall ask him 

 first how he knows it, that is, how he has measured the velocity of 

 light. 



He has begun by supposing that light has a constant velocity, and in 

 particular that its velocity is the same in all directions. That is a 

 postulate without which no measurement of this velocity could be 

 attempted. This postulate could never be verified directly by experi- 



