HYPOTHESES. 



325 



the law of the moon's attraction had 

 been proved from the data of the 

 moon itself, then on finding the same 

 law to accord with the phenomena of 

 terrestrial gravity, he was warranted 

 in adopting it as the law of those 

 phenomena likewise ; but it would 

 not have been allowable 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 ac- 

 count for terrestrial gravity : 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 propor- 

 tional to the inverse square. 



It appears, then, to be a condition 

 of the most genuinely scientific hypo- 

 thesis, 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 ob- 

 served facts. This condition is ful- 

 filled 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 accord- 

 ing 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, 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 dif- 

 ferent false hypotheses which Kepler 

 made respecting the law of the refrac- 

 tion 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, pheno- 

 mena ; 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 nine- 

 teen 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 of it. But in order that this 

 may be the case, I conceive it to be 

 necessary, when the hjrpothesis relates 

 to causation, that the supposed cause 

 should not only be a real phenomenon, 

 something actually existing in nature, 

 but should be already known to ex- 

 ercise, or at least to be capable of ex- 

 ercising, an influence of some sort 

 over the effect. In any other case, 

 it is no sufficient evidence of the 

 truth of the hypothesis that we are 

 able to deduce the real phenomena 

 from it. 



Is it, then, never allowable, in a 

 scientific hypothesis, to assume a 

 cause ; but only to ascribe an as- 

 sumed 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. 

 In the former case it may be very 

 useful by suggesting a line of in- 

 vestigation which may possibly ter- 

 minate in obtaining real proof. But, 

 for this purpose, as is justly remarked 

 by M. Comte, it is indispensable that 

 the cause suggested by the hypothesis 

 should be in its own nature suscep- 

 tible of being proved by other evi- 

 dence. This seems to be the philoso- 

 phical import of Newton's maxim, (so 

 often cited with approbation by sub- 

 sequent writers,) that the cause as- 

 signed for any phenomenon must not 



