2S6 



INDUCTION. 



It is not true, then, that one effect 

 must be connected with only one 

 cause, or assemblage of conditions ; 

 that each phenomenon can be pro- 

 duced only in one way. There are 

 often several independent modes in 

 which the same phenomenon could 

 have originated. One fact may be 

 the consequent in several invariable 

 seqiiences ; it may follow, with equal 

 uniformity, any one of several ante- 

 cedents, or collections of antecedents. 

 Many causes may produce mechanical 

 motion : many causes may produce 

 some kinds of sensation : many causes 

 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 conse- 

 quences of this fact of Plurality of 

 Causes is, to render the first of the 

 inductive methods, that of Agreement, 

 uncertain. To illustrate that method 

 we supposed two instances, ABC 

 followed by a b c, and A D E followed 

 hy 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 diflficulty may not stand 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 the 

 possibility of a plurality of causes, 

 the conclusion fails. For it involves 

 a tacit supposition that a must have 

 been produced in both instances by 

 the same cause. If there can pos- 

 sibly 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 influence 

 in either case. 



Suppose, for example, that two 

 great artists or great philosophers, 

 that two extremely selfish or ex- 

 tremely generous characters, were 



compared together as t© the circum. 

 stances of their education and history, 

 and the two cases were found to 

 agree only in one circumstance : 

 would it follow that this one circum- 

 stance was the cause of the quality 

 which characterised both those indi- 

 viduals ? Not at all ; for the causes 

 which may produce any type of char- 

 acter are very numerous ; and the 

 two persons might equally have 

 agreed in their character, though 

 there had been no manner of resem- 

 blance 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 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 con- 

 clusions which that yields, when the 

 number of instancCvH compared is 

 small, are of no real value, except as, 

 in the character of suggestions, they 

 may lead either to experiments bring- 

 ing them to the test of the Method 

 of Difference, or to reasonings which 

 may explain and verify them deduc- 

 tively. 



It is only when the instances, being 

 indefinitely multiplied and varied, 

 continue to suggest the same result, 

 that this result acquires any high 

 degree of independent value. If there 

 are but two instances, ABC and 

 A D E, though these instances have 

 no antecedent in common except A, 



