THE FOUR EXPERIMENTAL METHODS. 429 



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 A B C., and we 

 must look out for another instance in which the remaining 

 circumstances, b 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 gun-shot 

 which killed him : for he was in the fulness of life imme- 

 diately before, all circumstances being the same, except the 

 wound. 



The axioms implied in this method are evidently the 

 following. Whatever antecedent cannot be excluded without 

 preventing the phenomenon, is the cause, or a condition, of 

 that phenomenon : Whatever consequent can be excluded, 

 with no other difference in the antecedents than the absence 

 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 the regulating 

 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 circumstance in which alone the two instances 

 differ, is the effect, or the cause, or an indispensable part of 

 the cause, of the phenomenon. 



3. The two methods which we have now stated have 

 many features of resemblance, but there are also many dis- 



