PLURALITY OF CAUSES. 485 



oftener we repeat the observation, varying the circumstances, 

 the more we advance towards a solution of this doubt. For 

 if we try A F G, A H K, &c., all unlike one another except 

 in containing the circumstance A, and if we find the effect a 

 entering into the result 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 addi 

 tion, therefore, to the number of instances, the presumption is 

 strengthened in favour of A. The inquirer, of course, will not 

 neglect, 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, appeal to the Method of Difference 

 in aid of the Method of Agreement. By the Method of Dif 

 ference 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 nume 

 rous, 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 rebutted, and the conclu 

 sion that a is connected with A divested of the characteristic 

 imperfection, and reduced to a virtual certainty ? This is a 

 question which we cannot 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 chapter hereafter. 

 It is seen, however, at once, that the conclusion does amount 

 to a practical certainty after a sufficient number of instances, 

 and that the method, therefore, is not radically vitiated by the 

 characteristic imperfection. The result of these considerations 

 is only,- in the first place, to point out a new source of infe 

 riority in the Method of Agreement as compared with other 

 modes of investigation, and new reasons for never resting con 

 tented with the results obtained by it, without attempting to 

 confirm them either by the Method of Difference, or by con 

 necting 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 number 



