HYPOTHESES. 293 



Central force of the solar system, by the same hypothetical method. 

 When the law of tlie moon's attraction had been proved from the data 

 of the moon itself, then on finding the same law to accord with the phe- 

 nomena of teiTOstrial gravity, he was wan-anted in adopting it as the 

 law of those phenomena likewise : but it would not have bcim allow- 

 ai)le for him, without any lunar data, to assume that the moon was 

 attracted towards -the earth with a force as the inverse square of the 

 distance, merely because that ratio would enable him to account for 

 gravity by a similar attraction : for it would have been impossible 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 scientific hy- 

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

 but be certain to be either proved or disproved by that comparison 

 with observed tacts which is tenned Verification. In hypotheses of 

 this character, if they relate to causation at all, the effect must be al- 

 ready known to depend upon the very cause supposed, and the hypo- 

 thesis 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 hypo- 

 theses which do not make any supposition with regard to causation, 

 but only with regard to the law of coiTespondence between facts which 

 accompany each other in their variations, though there may be no rela- 

 tion of cause and effect between them. Such are 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 refi-action varied 

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

 not known hoAV ; 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 descrihhig phenomena; such as 

 the hypothesis of the ancient' astronomers that the heavenly bodies 

 moved in circles ; the various hypotheses of eccentrics, 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 hypo- 

 thesis 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 tWs may be the case, it is (as we have seen) necessary, 

 when the hypothesis relates to causation, that the sup])osed cause 

 should not only be a real phenomenon, something actually existing in 

 nature, but should be already known to have some influence upon the 

 supposed effect ; the precise degree and manner of the influence being 

 the only point undetermined. In any other case, it is no evidence of 

 the truth of the hypothesis that we are able to deduce the real phe- 

 nomena from it. - f 



Is it, then, never allowable, in a scientific hypothesis, to assume a 

 cause ; but only to ascribe an assumed law to a known cause ? I do 

 not assert this. I only say, that in the latter case alone can the hypo- 

 thesis be received as true merely because it explains the phenomena : 



