14 INDUCTION. 



of the distance, merely because that ratio would enable him 

 to account for terrestrial gravity : for it would have been im- 

 possible for him to prove that the observed law of the fall of 

 heavy bodies to the earth could not result from any force, save 

 one extending to the moon, and proportional to the inverse 

 square. 



It appears, then, to be a condition of a genuinely scien 

 tific hypothesis, that it be not destined always to remain an 

 hypothesis, but be of such a nature as to be either proved or 

 disproved by comparison with observed facts. This condition 

 is fulfilled when the effect is already known to depend on the 

 very cause supposed, and the hypothesis relates only to the 

 precise mode of dependence ; the law of the variation of the 

 effect according to the variations in the quantity or in the 

 relations of the cause. With these may be classed the hypo- 

 theses which do not make any supposition with regard to 

 causation, but only with regard to the law of correspondence 

 between facts which accompany each other in their variations, 

 though there may be no relation of cause and effect between 

 them. Such were the different false hypotheses which Kepler 

 made respecting the law of the refraction of light. It was 

 known that the direction of the line of refraction varied with 

 every variation in the direction of the line of incidence, but it was 

 not known how; that is, what changes of the one corresponded 

 to the different changes of the other. In this case any law, 

 different from the true one, must have led to false results. 

 And, lastly, we must add to these, all hypothetical modes of 

 merely representing, or describing, phenomena ; such as the 

 hypothesis of the ancient astronomers that the heavenly bodies 

 moved in circles ; the various hypotheses of excentrics, 

 deferents, and epicycles, which were added to that original 

 hypothesis ; the nineteen false hypotheses which Kepler made 

 and abandoned respecting the form of the planetary orbits ; 

 and even the doctrine in which he finally rested, that those 

 orbits are ellipses, which was but an hypothesis like the rest 

 until verified by facts. 



In all these cases, verification is proof; if the supposition 

 accords with the phenomena there needs no other evidence 



