HYPOTHESES. 15 



diminished, and diminished as it increased ; Newton's 

 argument would not have proved his conclusion. 

 These facts, however, being already certain, the 

 range of admissible suppositions was limited to the 

 various possible directions of a line, and the various 

 possible numerical relations between the variations of 

 the distance and the variations of the attractive 

 force : now among these it was easily shown that 

 different suppositions could not lead to identical 

 consequences. 



Accordingly, Newton could not have performed 

 his second great philosophical operation, that of iden- 

 tifying terrestrial gravity with the central force of the 

 solar system, by the same hypothetical method. 

 When 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 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 hypothesis, that it be not destined always to 

 remain an hypothesis, but be certain to be either 

 proved or disproved by that comparison with observed 

 facts which is termed Verification. In hypotheses of 

 this character, if they relate to causation at all, the 

 effect must be already known to depend upon the 



