454 INDUCTION. 



tain among them, the cause : and so, mutatis mutandis, 

 of the effect. As this method proceeds by comparing 

 different instances to ascertain in what they agree, I 

 have termed it the Method of Agreement : and we 

 may adopt as its regulating principle the following 

 canon : 



FIRST CANON. 



If two or more instances of the phenomenon under 

 investigation have only one circumstance in common, the 

 circumstance in which alone all the instances agree, is 

 the cause Cor effect) of the given phenomenon. 



Quitting for the present the Method of Agree- 

 ment, to which we shall almost immediately return, 

 we proceed to a still more potent instrument of the 

 investigation of nature, the Method of Difference. 



2. In the Method of Agreement, we endeavoured 

 to obtain instances which agreed in the given circum- 

 stance but differed in every other : in the present 

 method we require, on the contrary, two instances 

 resembling one another in every other respect, but 

 differing in the presence or absence of the phenomenon 

 we wish to study. If our object be to discover the 

 effects of an agent A, we must procure A in some set 

 of ascertained circumstances, as ABC, and having 

 noted the effects produced, compare them with the 

 effect of the remaining circumstances B C, when A is 

 absent. If the effect of A B C is a b c, and the effect 

 of B C, b c, it is evident that the effect of A is a. So 

 again, if we begin at the other end, and desire to 

 investigate the cause of an effect a, we must select an 

 instance, as a b c, in which the effect occurs, and in 

 which the antecedents were ABC, and we must look 

 put for another instance in which the remaining 



