280 INDUCTION. 



tion. 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 ei- 

 ther 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 principal 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 (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 endeavored 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 circum- 

 stances, 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 5 c, and the effect of B C ^ c, it is evident that the 

 effect of A is a. So again, if we begin at the other end, and desire to in- 

 vestigate the cause of an effect «, we must select an instance, as ahc, in 

 which the effect occurs, and in which the antecedents were ABC, and we 

 must look out for another instance in which the remaining circumstances, 

 6 c, occur without a. If the antecedents, in that instance, are B C, we 

 know that the cause of a must be A: either A alone, or A in conjunction 

 with some of the other circumstances present. 



It is scarcely necessary to give examples of a logical process to which 

 we owe almost all the inductive conclusions we draw in daily life. When 

 a man is shot through the heart, it is by this method we know that it was 

 the gunshot which killed him: for he was in the fullness of life immedi- 

 ately before, all circumstances being the same, except the wound. 



The axioms implied in this method ai*e evidently the following. What- 

 ever antecedent can not be excluded without preventing the phenome- 

 non, is the cause, or a condition, of that phenomenon : whatever consequent 

 can be excluded, with no other difference in the antecedents than the ab- 

 sence of a particular one, is the effect of that one. Instead of comparing 

 different instances of a phenomenon, to discover in what they agree, this 

 method compares an instance of its occurrence with an instance of its non- 

 occurrence, to discover in what they differ. The canon which is tlie regu- 

 lating principle of the Method of Difference may be expressed as follows : 



Second Canon. 



If an instance in which the phenomenon under investigation occurs, 

 and an instance in which it does not occur, have every circumstance in 

 common save one, that one occurring only in the former; the circum- 

 stance in which alone the two i^istances differ, is the effect, or the cause, or 

 an indisjyensable pa7't of the cause, of the phenomenon. 



