PLURALITY OF CAUSES. 253 



cumstanccs would be entirely useless, were it not for the Plurality 

 of Causes. 



It is of importance to remark, that the peculiar modification of the 

 Method of AE^recmcnt which, as partaking in some degree of the na- 

 ture of the Method of Diff'ercnce, 1 have called the Joint Method of 

 Agi'eement and Dift'ei-ence, is not afi'ected by the characteristic imper- 

 fection now pointed out. For, in the joint method, it is supposed not 

 only that the instances in which a is, agree only in containing A, but 

 also that the instaTices in which a is not, agi-ee oidy in not contain- 

 ing A. Now, if this be so, A must be not only the cause of a, but the 

 only possible cause : for if there were another, as for example B, 

 then in the instances in which a is not, B must have been absent as 

 well as A, and it would not be true that these instances agree only 

 in not containing A. This, therefore, constitutes an immense advan- 

 tage 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 premisses (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 fi-ee 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 witli it. But although this is true in principle, it is gen- 

 erally altogether impossible to work the Method of Agreement by 

 negative instances without positive ones : it is so much more diffi- 

 cult 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 

 transparent, agree 1 But we might hope much sooner to seize some 

 point of resemblance among the comparatively few and definite 

 species of objects which arc transparent ; and this being attained, 

 we should quite naturally be put upon examining whetlKU- the ab- 

 sence of this one circumstance be not precisely the point in which 

 all opaque substances will be found to resemble. 



The Joint Method of Agreement and Diffijrence, therefore, or, as 

 I have otherwise called it, the Indirect Method of Difference (be- 

 cause, like the Method of Difference properly so called, it proceeds by 

 a.scertaining how and in what the cases where the phenomenon is 

 present, difter from those in which it is absent) is, after the direct 

 Method of Difference, the most powerful of the remaining instru- 

 ments of inductive investigation • and in the sciences which depend 

 on pure obscr\-ation, with little or no aid from experiment, this 

 method, so well exemplified in the beautiful speculation on tin; cause 

 of dew, is the primary resource, so far as direct appeals to experi- 

 ence are concerned. 



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

 supposition, which, until nnnoved, renders our inductiolis uncertain, 

 and have only considered by what means, where the plurality does not 

 really exist, we may be enabled to disprove it. But we must also con- 

 sider it as a case actually occurring in nature, and which, as often as 



