430 INDUCTION. 



tinctions between them. Both are methods of elimination. 

 This term (employed in the theory of equations 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 relation 

 between the remaining elements 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 cir- 

 cumstances which are found to accompany 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 con- 

 nected with the phenomenon by a law. 



Of these methods, that of Difference is more particularly 

 a method of artificial experiment; while that of Agreement is 

 more especially the resource employed where experimentation 

 is impossible. A few reflections will prove the fact, and point 

 out the reason of it. 



It is inherent in the peculiar character of the Method of 

 Difference, that the nature of the combinations which it 

 requires is much more strictly defined than in the Method of 

 Agreement. The two instances which are to be compared 

 with one another must be exactly similar, in all circumstances 

 except the one which we are attempting to investigate : they 

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

 phenomena of the universe may be either present or absent 

 without affecting the given phenomenon; or, if present, 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 



