PLUEALITY OF CAUSES. 313 



lect, if an opportunity present itself, to exclude A from some one of these 

 combinations, from A H K for instance, and by trying H K separately, ap- 

 peal to the Method of Diffei'ence in aid of the Method of Agreement. By 

 the Method of Difference alone can it be ascertained that A is the cause 

 of a; but that it is either the cause, or another effect of the same cause, 

 may be placed beyond any reasonable doubt by the Method of Agreement, 

 provided the instances are very numerous as well as sufficiently various. 



After how great a multiplication, then, of varied instances, all agreeing 

 in no other antecedent except A, is the supposition of a plurality of causes 

 sufficiently I'ebutted, and the conclusion that a is connected with A divest- 

 ed of the characteristic imperfection, and reduced to a virtual certainty? 

 This is a question which we can not be exempted from answering: but 

 the consideration of it belongs to what is called the Theory of Probability, 

 which will form the subject of a chajjter hereafter. It is seen, however, at 

 once, that the conclusion does amount to a practical certainty after a suffi- 

 cient number of instances, and that the method, therefore, is not radically 

 vitiated by the characteristic imperfection. The result of these considera- 

 tions is only, in the first place, to point out a new source of inferiority in 

 the Method of Agreement as compared with other modes of investigation, 

 and new reasons for never resting contented with the results obtained by 

 it, without attempting to confirm them either by the Method of Difference, 

 or by connecting them deductively with some law or laws already ascer- 

 tained by that superior method. And, in the second place, we learn from 

 this the true theory of the value of mere mimher of instances in inductive 

 inquiry. The Plurality of Causes is the only reason why mere number is 

 of any importance. The tendency of unscientific inquirers is to rely too 

 much on number, without analyzing the instances ; Avithout looking closely 

 enough into their nature to ascertain what circumstances are or are not 

 eliminated by means of them. Most people hold their conclusions with a 

 degree of assurance proportioned to the mere mass of the experience on 

 which they appear to rest; not considering that by the addition of in- 

 stances to instances, all of the same kind, that is, differing from one another 

 only in points already recognized as immaterial, nothing wiiatever is add- 

 ed to the evidence of the conclusion. A single instance eliminating some 

 antecedent which existed in all the other cases, is of more value than the 

 greatest multitude of instances which are reckoned by their number alone. 

 It is necessary, no doubt, to assure ourselves, by repetition of the observa- 

 tion or experiment, that no error has been committed concerning the indi- 

 vidual facts observed ; and until we have assured ourselves of this, instead 

 of varying the circumstances, we can not too scrupulously repeat the same 

 experiment or observation without any change. But when once this as- 

 surance has been obtained, the multiplication of instances which do not ex- 

 clude any more circumstances is entirely useless, provided there have been 

 already enough to exclude the supposition of Plurality of Causes. 



It is of importance to remark, that the peculiar modification of the 

 Method of Agreement, which, as partaking in some degree of the nature 

 of the Method of Difference, I have called the Joint Method of Agreement 

 and Difference, is not affected by the characteristic imperfection now 

 pointed out. For, in the joint method, it is supposed not only that the in- 

 stances in which a is, agree only in containing A, but also that the in- 

 stances in Avhich a is not, agree only in not containing A. Now, if this be 

 so, A must be not only the cause of a, but the only possible cause : for if 

 there were another, as for example B, then in the instances in which a is 



