312 INDUCTION. 



tain. To illustrate that method, we supposed two instances, ABC follow- 

 ed by a h e, and A D E followed by a d e. From these instances it might 

 apparently be concluded that A is an invariable antecedent of a, and even 

 that it is the unconditional invariable antecedent, or cause, if we could be 

 sure that there is no other antecedent common to the two cases. That this 

 difficulty may not stand in the way, let us suppose the two cases positive- 

 ly ascertained to have no antecedent in common except A. The moment, 

 iujwever, that we let in the possibihty of a plurality of causes, the conclu- 

 sion fails. For it involves a tacit supposition, that a must have been pro- 

 duced in both instances by the same cause. If there can possibly have 

 been two causes, those two may, for example, be 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 influence 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 which may produce 

 any type of character are very numerous; and the two persons might 

 equally have agreed in their charactei', though there had been no manner 

 of resemblance in their previous history. 



This, therefore, is a characteristic imperfection of the Method of Agree- 

 ment, 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 ab c,\t 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 complete and rigor- 

 ous 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 de- 

 ductively. 



It is only when the instances, being indefinitely multiplied and varied, 

 continue to suggest the same result, that this result acquires any high de- 

 gree of independent value. If there are but two instances, ABC and 

 A DE, though these instances have no antecedent in common except A, yet 

 as the effect may possibly have been produced in the two cases by differ- 

 ent causes, the result is at most only a slight probability in favor of A ; 

 there may be' causation, but it is almost equally probable that there was 

 only a coincidence. But the oftener we repeat the observation, varying 

 the circumstances, the more we advance toward a solution of this doubt. 

 For if we try A F G, A H K, etc., all unlike one another except in contain- 

 ing the circumstance A, and if we find the effect a entering into the re- 

 sult in all these cases, we must suppose one of two things, either that it is 

 caused by A, or that it has as many different causes as there are instances. 

 With each addition, therefore, to the number of instances, the presump- 

 tion is strengthened in favor of A. The inquirer, of course, will not neg- 



