252 INDUCTION. 



probable that there was only, as the expression is, 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 

 AFG, AHK, &c., all entirely 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 in- 

 stances. With each addition, therefore, to the number of instances, 

 the presumption is strengthened in favor oi A. The inquirer, of course, 

 will not neglect, if an opportunity present itself, to exclude A from 

 some one of these combinations, from AHK for instance, and by trying 

 H K separately, appeal to the Method of Difference in aid of the Method 

 of Agreement. By the former method 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 gi-eat a multiplication, then, of varied instances, all agree- 

 ing in no other antecedent ex<;ept A, is the supposition of a plurality 

 of causes sufficiently rebutted, and the conclusion that a is the effect of 

 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 here- 

 after. 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 imperfec- 

 tion. The result of these considerations 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 

 resthig contented with the results obtained by it, without attempting to 

 confirm tliem either by the Method of Difference, or by coimecting 

 them deductively with some law or laws already ascertained by that 

 superior method. And, in the second place, we leaj-n from this, the 

 true theory of the value of mere number of instances in inductive 

 inquiry. The tendency of unscientific inquiries is to rely too much 

 upon number, without analyzing the instances ; without looking closely 

 enough into their nature, to ascertain what circumstar^ces 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 expe- 

 rience on which they appear to rest : not considering that by the addi- 

 tion of instances to instances, all of the same kind, that is, differing from, 

 one another only in points already recognized as immaterial, nothing 

 whatever is added 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 a 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 circumstances, we 

 cannot too scrupulously repeat the same experiment or observation 

 without any change. But when once this assurance has been obtained, 

 the multiplication of instances which do not exclude any more cir- 



