THE FOUR EXPERIMENTAL METHODS. 455 



circumstances/ b c, occur without a. If the antece- 

 dents, in that instance, are B C, we know that the 

 cause of a must be A : either A alone, or A in conjunc- 

 tion 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 induc- 

 tive 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 immediately before, all 

 circumstances being the same, except the wound. 



The axioms which are taken for granted in this 

 method are evidently the following : Whatever ante- 

 cedent 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 inves- 

 tigation occurs, and an instance in which it does not 

 occur, have every circumstance save one in common, 

 that one occurring only in the former ; the circumstance 

 in which alone the two instances differ, is the effect, or 

 cause, or a necessary part of the cause, of the pheno- 

 menon. 



