456 INDUCTION. 



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. This term (which is 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 upon 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 circumstances 

 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 exist- 

 ence 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 what- 

 ever can not be eliminated, is connected with the phe- 

 nomenon 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 we 

 employ 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 combi- 

 nations 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. 



