512 INDUCTION. 



instances agree only in not containing A. This, there- 

 fore, constitutes an immense advantage of the joint 

 method over the simple Method of Agreement. It 

 may seem, indeed, that the advantage does not belong 

 so much to the joint method, as to one of its two pre- 

 misses (if they may be so called), the negative premiss. 

 The Method of Agreement, when applied to negative 

 instances, or those in which a phenomenon does not 

 take place, is certainly free from the characteristic 

 imperfection which affects it in the affirmative case. 

 The negative premiss, it might therefore be supposed, 

 could be worked as a simple case of the Method of 

 Agreement, without requiring an affirmative premiss 

 to be joined with it. But although this is true in 

 principle, it is generally altogether impossible to work 

 the Method of Agreement by negative instances with- 

 out positive ones : it is so much more difficult to 

 exhaust the field of negation than that of affirmation. 

 For instance, let the question be, what is the cause 

 of the transparency of bodies ; with what prospect of 

 success could we set ourselves to inquire directly in 

 what the multifarious substances which are not trans- 

 parent, agree ? But we might hope much sooner to 

 seize some point of resemblance among the compara- 

 tively few and definite species of objects which are 

 transparent ; and this being attained, we should quite 

 naturally be put upon examining whether the absence 

 of this one circumstance be not precisely the point in 

 which all opaque substances will be found to re- 

 semble. 



The Joint Method of Agreement and Difference, 

 therefore, or, as I have otherwise called it, the Indirect 

 Method of Difference (because, like the Method of 

 Difference properly so called, it proceeds by ascertain- 

 ing how and in what the cases where the phenomenon 



