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 positions at the instant _t_ + _h_ as well as their positions at the instant _t_ - _h_. Even more; the position of Jupiter at the instant _t_, together with that of Saturn at the instant _t_ + _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_ + _a_ + _e_ is bound to the aggregate of positions occupied by Jupiter at the instant _t_ and Saturn at the instant _t_ + _a_, 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_ + _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 definition 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 experiment; it might be contradicted by it if the results of different measurements were not concordant. We should think ourselves fortunate that this contradiction has not happened and that the slight discordances which may happen can be readily explained.