PLURALITY OF CAUSES. 



487 



constitutes an immense advantage of the joint method over 

 the simple Method of Agreement. It may seem, indeed, that 

 the advantage does not helong so much to the joint method, 

 as to one of its two premises, (if they may he so called,) the 

 negative premise. 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 imper 

 fection which affects it in the affirmative case. The negative 

 premise, it might therefore be supposed, could he worked as 

 a simple case of the Method of Agreement, without requiring 

 an affirmative premise to he joined with it. But though this 

 is true in principle, it is generally altogether impossible to 

 work the Method of Agreement by negative instances without 

 positive ones : it is so much more difficult to exhaust the field 

 of negation than that of affirmation. For instance, let the 

 question he, 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 

 transparent, agree? But we might hope much sooner to 

 seize some point of resemblance among the comparatively few 

 and definite species of objects which are transparent; and this 

 being attained, we should quite naturally be put upon examin 

 ing whether the absence of this one circumstance be not pre 

 cisely the point in which all opaque substances will be found 



to resemble. 



The Joint Method of Agreement and Difference, therefore, 

 or, as I have otherwise called it, the Indirect Method of Diffe 

 rence (because, like the Method of Difference properly so called, 

 it proceeds by ascertaining how and in what the cases where 

 the phenomenon is present, differ from those in which it is 

 absent) is, after the Direct Method of Difference, the most 

 powerful of the remaining instruments of inductive investiga 

 tion ; and in the sciences which depend on pure observation, 

 with little or no aid from experiment, this method, so well ex 

 emplified in the speculation on the cause of dew, is the primary 

 resource, so far as direct appeals to experience are concerned. 



3. We have thus far treated Plurality of Causes only as 



