428 INDUCTION. 



nomenon. We may overlook some of the material circum- 

 stances in an experiment with an electrical machine ; but we 

 shall, at the worst, he hetter acquainted with them than with 

 those of a thunder-storm. 



The mode of discovering and proving laws of nature, which 

 we have now examined, proceeds on the following axiom : 

 Whatever circumstances can he excluded, without prejudice to 

 the phenomenon, or can be absent notwithstanding its 

 presence, is not connected with it in the way of causation. 

 The casual circumstances being thus eliminated, if only one 

 remains, that one is the cause which we are in search of: if 

 more than one, they either are, or contain 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 investiga- 

 tion have only one circumstance in common, the circumstance in 

 which alone all the instances agree, is the cause (or effect) of the 

 given phenomenon. 



Quitting for the present the Method of Agreement, 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 circumstance 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 A B C, 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 



