PLURALITY OF CAUSES. 511 



degree of assurance proportioned to the mere mass of 

 the experience on which they appear to rest : not 

 considering that by the addition of instances to 

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

 one another only in points already recognised 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 repeti- 

 tion 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 obser- 

 vation without any change. But when once this 

 assurance has been obtained, the multiplication of 

 instances which do not exclude any more circum- 

 stances would be entirely useless, were it not for the 

 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 Agree- 

 ment and Difference, is not affected by the charac- 

 teristic 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 



