PLURALITY OF CAUSES. 



287 



yet, as the effect may possibly have 

 been produced in the two cases by 

 different causes, the result is at most 

 only a slight probability in favour 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 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 addition, there- 

 fore, 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 sepa- 

 rately, appeal to the Method of Dif- 

 ference 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 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 rebutted, and the conclu- 

 sion that a is connected with A 

 divested of the characteristic imper- 

 fection, and reduced to a virtual cer- 

 tainty ? This is a question which we 

 cannot be exempted from answering : 

 but the consideration of it belongs to 

 what is called the Theory of Proba- 

 bility, which will form the subject of 

 a chapter hereafter. It is seen, how- 

 ever, at once, that the conclusion 

 does amount to a practical certainty 

 »fter » tufficient 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 

 inferiority in the Method of Agree- 

 ment as compared with other modes 

 of investigation, "and new reasons for 

 never resting contented with the 

 results obtained by it, without at- 

 tempting to confirm them either by 

 the Method of Difference, or by con- 

 necting them deductively with some 

 law or laws already ascertained by 

 that superior method. And, in the 

 second place, we learn from this the 

 true theory of the value of a mere 

 mumher of instances in inductive in- 

 quiry. 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 analysing 

 the instances ; without 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 con- 

 sidering that by the addition of in- 

 stances to instances, all of the same 

 kind, that is, differing from one 

 another only in points already recog- 

 nised as immaterial, nothing whatever 

 is added to the evidence of the con- 

 clusion. 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 

 observation or experiment, that no 

 error has been committed concerning 

 the individual facts observed ; and 

 until we have assured ourselves of 

 this, instead of varying the circum- 

 stances, we cannot too scrupulously 

 repeat the same experiment or obser- 

 vation without any change. But 

 when once this assurance has been 

 obtained, the multiplication of in- 



