HYPOTHESES. 13 



instance B C, b c ; by showing that no antecedent, except the 

 one assumed in the hypothesis, would in conjunction with B 

 produce a. 



Now it appears to me that this assurance cannot be 

 obtained, when the cause assumed in the hypothesis is an 

 unknown cause, imagined solely to account for a. When we 

 are only seeking to determine the precise law of a cause 

 already ascertained, or to distinguish the particular agent 

 which is in fact the cause, among several agents of the same 

 kind, one or other of which it is already known to be, we may 

 then obtain the negative instance. An inquiry, which of the 

 bodies of the solar system causes by its attraction some par- 

 ticular irregularity in the orbit or periodic time of some 

 satellite or comet, would be a case of the second description. 

 Newton's was a case of the first. If it had not been previously 

 known that the planets were hindered from moving in 

 straight lines by some force tending towards the interior 

 of their orbit, though the exact direction was doubtful ; or if 

 it had not been known that the force increased in some pro- 

 portion or other as the distance diminished, and diminished as it 

 increased; Newton's argument would not have proved his con- 

 clusion. 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 rela- 

 tions between the variations of the distance, and the varia- 

 tions 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 scientific operation, that of identifying terrestrial gravity 

 with the central force of the solar system, by the same hypo- 

 thetical 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 



