484 



INDUCTION. 



sophers, 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 which 

 may produce any type of character are innumerable ; and the 

 two persons might equally have agreed in their character, 

 though there had been no manner of resemblance in their pre 

 vious 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 ob 

 servations or experiments necessary : two instances, the one 

 positive and the other negative, are still sufficient for the most 

 complete and rigorous induction. Not so, however, with the 

 Method of Agreement. The conclusions which that yields, 

 when the number of instances compared is small, are of no 

 real value, except as, in the character of suggestions, they may 

 lead either to experiments bringing them to the test of the 

 Method of Difference, or to reasonings which may explain and 

 verify them deductively. 



It is only when the instances, being indefinitely multiplied 

 and varied, continue to suggest the same result, that this re 

 sult acquires any high degree of independent value. If there 

 are but two instances, ABC and A D E, though these in 

 stances have no antecedent in common except A, yet as the 

 effect may possibly have been produced in the two cases by 

 different causes, the result is at most only a slight probability 

 in favour of A ; there may be causation, but it is almost 

 equally probable that there was only a coincidence. But the 



