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INDUCTION. 



elusions we draw in early 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 fulness of life immediately 

 before, all circumstances being the 

 same, except the wound. 



The axioms implied in this method 

 are evidently the following. What- 

 ever 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 dif- 

 ference in the antecedents than the 

 absence of a particular one, is the 

 effect of that one. Instead of com- 

 paring different instances of a phe- 

 nomenon, to discover in what they 

 agree, this method compares an in- 

 stance of its occurrence with an 

 instance of its non-occurrence, to dis- 

 cover in what they differ. The canon 

 which is the regulating principle of 

 the Method of Difference may be ex- 

 pressed as follows : — 



Second Canon. 

 If an instance in which the phe- 

 nomenon under investigation occurs, 

 and an instance in which it does not 

 occur, have every circumstance in com- 

 mon 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 indis- 

 pensable part of the cause, of the phe- 

 nomenon. 



§ 3. The two methods which we 

 have now stated have many features 

 of resemblance, but there are also 

 many distinctions between them. 

 Both are methods of elimination. Thi^ 

 term (employed in the theory of equa- 

 tions to denote the process by which 

 one after another of the elements 

 of a question is excluded, and the 

 solution made to depend on the re- 

 lation between the remaining ele- 

 ments only) is well suited to express 

 the operation, analogous to this, 

 which has been understood since the 

 time of Bacon to be the foundation of 



experimental inquiry, namely, the 

 successive exclusion of the various 

 circumstances which are found to ac- 

 company a phenomenon in a given 

 instance, in order to ascertain what 

 are those among them which can be 

 absent consistently with the existence 

 of the phenomenon. The Method of 

 Agreement stands on the ground that 

 whatever can be eliminated is not 

 connected with the phenomenon by 

 any law. The Method of Difference 

 has for its foundation, that whatever 

 cannot be eliminated is connected 

 with the phenomenon by a law. 



Of these methods, that of Differ- 

 ence is more particularly a method of 

 artificial experiment ; while that of 

 Agreement is more especially the re- 

 source employed where experimenta- 

 tion is impossible. A few reflections 

 will prove the fact, and point out the 

 reason of it. 



It is inherent in the peculiar char- 

 acter of the Method of Difference 

 that the nature of the combinations 

 which it requires is much more strictly 

 defined than in the Method of Agree- 

 ment. The two instances which are 

 to be compared with one another 

 must be exactly similar in all circum- 

 stances except the one which we are 

 attempting to investigate : they must 

 be in the relation of A B C and B C,or 

 oi a b c and b c. It is true that this 

 similarity of circumstances needs not 

 extend to such as are already known 

 to be immaterial to the result. And 

 in the case of most phenomena we 

 learn at once, from the commonest 

 experience, that most of the co-existent 

 phenomena of the universe may be 

 either present or absent without affect- 

 ing the given phenomenon ; or, if pre- 

 sent, are present indifferently when 

 the phenomenon does not happen and 

 when it does. Still, even limiting the 

 identity which is required between 

 the two instances, ABC and B C, to 

 such circumstances as are not already 

 known to be indifferent ; it is very 

 seldom that nature affords two in- 

 stances, of which we can be assured 

 that they stand in this precise rela- 



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