OF NATURAL PHILOSOPHY. 167 



small deviations in those of the planets, and some 

 very considerable ones in that of the moon and other 

 satellites, still unaccounted for; residual phenomena, 

 which still remain to be traced up to causes. By 

 further examining these, their causes have at length 

 been ascertained, and found to consist in the mutual 

 actions of the planets on each other, and the dis- 

 turbing influence of the sun on the motions of the 

 satellites. * 



(176.) But a law of nature has not that degree of 

 generality which fits it for a stepping-stone to 

 greater inductions, unless it be universal in its ap- 

 plication. We cannot rely on its enabling us to 

 extend our views beyond the circle of instances 

 from which it was obtained, unless we have already 

 had experience of its power to do so ; unless it 

 actually has enabled us before trial to say what will 

 take place in cases analogous to those originally 

 contemplated ; unless, in short, we have studiously 

 placed ourselves in the situation of its antagonists, 

 and even perversely endeavoured to find exceptions 

 to it without success. It is in the precise pro- 

 portion that a law once obtained endures this 

 extreme severity of trial, that its value and im- 

 portance are to be estimated ; and our next step in 

 the verification of an induction must therefore con- 

 sist in extending its application to cases not origin- 

 ally contemplated ; in studiously varying the cir- 

 cumstances under which our causes act, with a view 

 to ascertain whether their effect is general ; and in 

 pushing the application of our laws to extreme 

 cases. 



(177.) For example, a fair induction from a 

 M 4 



