508 INDUCTION. 



mon except A. The moment, however, that we let 

 in the possibility of a plurality of causes, the conclu- 

 sion fails. For it involves a tacit supposition, that a 

 must have been produced in both instances by the 

 same cause. If there can possibly have been two 

 causes, those two may, for example, be C and E : the 

 one may have been the cause of a in the former of the 

 instances, the other in the latter, A having no influ- 

 ence in either case. 



Suppose, for example, that two great artists, or great 

 philosophers, that two extremely selfish, or extremely 

 generous characters, were compared together as to the 

 circumstances of their education and history, and the 

 two cases were found to agree only in one circumstance : 

 would it follow that this one circumstance was the 

 cause of the quality which characterized both those 

 individuals ? Not at all ; for the causes at work to 

 produce any given type of character are innumerable ; 

 and the two persons might equally have agreed in 

 their character, although there had been no manner 

 of resemblance in their previous history. 



This, therefore, is a characteristic imperfection of 

 the Method of Agreement; from which imperfection 

 the Method of Difference is free. For if we have two 

 instances, ABC and B C, of which B C gives b c, 

 and A being added converts it into a b c, it is certain 

 that in this instance at least A was either the cause 

 of a, or an indispensable portion of its cause, even 

 though the cause which produces it in other instances 

 may be altogether different. Plurality of Causes, 

 therefore, not only does not diminish the reliance due 

 to the Method of Difference, but does not even render 

 a greater number of observations or experiments 

 necessary: two instances, the one positive and the 

 other negative, are still sufficient for the most com- 



