THE FOUR EXPERIMENTAL METHODS. 435 



have by the one proposition A B C, a b c, by the other B C, 

 b c, the positive and negative instances which the Method of 

 Difference requires. 



This method may be called the Indirect Method of Dif- 

 ference, or the Joint Method of Agreement and Difference; 

 and consists in a double employment of the Method of Agree- 

 ment, each proof being independent of the other, and corro- 

 borating it. But it is not equivalent to a proof by the direct 

 Method of Difference. For the requisitions of the Method of 

 Difference are not satisfied, unless we can be quite sure either 

 that the instances affirmative of a agree in no antecedent 

 whatever but A, or that the instances negative of a agree in 

 nothing but the negation of A. Now if it were possible, 

 which it never is, to have this assurance, we should not need 

 the joint method ; for either of the two sets of instances 

 separately would then be sufficient to prove causation. This 

 indirect method, therefore, can only be regarded as a great 

 extension and improvement of the Method of Agreement, but 

 not as participating in the more cogent nature of the Method 

 of Difference. The following may be stated as its canon : 



THIRD CANON. 



If two or more instances in which the phenomenon occurs 

 have only one circumstance in common, while two or more in- 

 stances in which it does not occur have nothing in common 

 save the absence of that circumstance; the circumstance in 

 which alone the two sets of instances differ, is the effect, or the 

 cause, or an indispensable part of the cause, of the phenomenon. 



We shall presently see that the Joint Method of Agree- 

 ment and Difference constitutes, in another respect not yet 

 adverted to, an improvement upon the common Method of 

 Agreement, namely, in being unaffected by a characteristic 

 imperfection of that method, the nature of which still remains 

 to be pointed out. But as we cannot enter into this exposi- 

 tion without introducing a new element of complexity into 

 this long and intricate discussion, I shall postpone it to a sub- 

 sequent chapter, and shall at once proceed to a statement of 

 two other methods, which will complete the enumeration of 



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