1 () INDUCTION 



very cause supposed, and the hypothesis must relate 

 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 hypotheses which do not 

 make any supposition with regard to causation, hut 

 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 are the different false hypo- 

 theses 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 corre- 

 sponded 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 describing 

 phenomena ; such as the hypothesis of the ancient 

 astronomers that the heavenly bodies moved in cir- 

 cles ; 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 true 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 of it. But in order that this may 

 be the case, it is (as we have seen) necessary, when 

 the hypothesis relates to causation, that the supposed 

 cause should not only be a real phenomenon, some- 



