486 INDUCTION. 



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 analysing the instances ; without looking 

 closely enough into their nature, to ascertain what circum 

 stances are or are not eliminated by means of them. Most 

 people hold their conclusions with a degree of assurance pro 

 portioned 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 recognised as imma 

 terial, 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 

 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 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 modifica 

 tion 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 instances in 

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

 instances in which 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 not, B must have been 

 absent as well as A, and it would not be true that these 

 instances agree only in not containing A. This, therefore, 



