PLURALITY OF CAUSES, 251 



duce some kinds of sensation : many causos may produce death. A 

 given effect may really be produced by a certain cause, and yet be 

 perfectly capable of being produced without it. 



§ 2. One of the principal consequences of this fact of Plurality of 

 Causes is, to render the tirst of our inductive riiethods, that of Agree- 

 ment, uncertain. To illustrate that method, we supposed two instances. 

 A B C followoil by a h r, and A J) 1^] followed by (? d c. From these in- 

 stances it miirht be concluded that A is an invariable antecedent of o; 

 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 staml in the way, let us suppose 

 the two cases positively ascertained to have no antecedent in common 

 except A. The moment, however, that we let in tin* possibility of a plu- 

 rality of causes, the conclusion fails. For it involves a tacit suppo- 

 sition 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 ha\'ing no influence 

 in either case. 



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

 that two extremely selfish, or extremely generous charactei-s, were 

 compared together as to the circumstances of ;their education and his- 

 tory, and the two cases were found to agree only in one circumstance": 

 would it follow that this one circuriistance 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 innumer- 

 able ; and the two persons might equally have agreed in their char- 

 acter, although there had been no manner of reserabhmce in their 

 previous history. 



This, therefore, is a characteristic imperfection of the INIethod of 

 Agreement ; from which imperfeciion the Method of Difference is fre^. 

 For if we have two instances, ABC and BC, of which BC gives be, 

 and A being added converts it into a he, it is certain that in this instance 

 at least A was either the cause of a, or an indispensable portion of its 

 cause, even thoui^h the cause which produces it in other instances may 

 be altogether different. Plurality of Causes, therefore, not only does 

 nf»t diminish the reliance due to the Method of Difference, but does not 

 even render a greater number of obstjrvations or experiments necessary : 

 two instances, the one positive and the other negative, are still suffi- 

 cient 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 suggestione, 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 result accpiires any high 

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

 and ADE, although the-se instances 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 proba- 

 bility in favor of A ,• there may be causation, but it is almost equally 



