288 



INDUCTION. 



Btances which do not exclude any 

 more circumstances is entirely useless, 

 provided there have been already 

 enough to exclude the supposition of 

 Plurality of Causes. 



It is of importance to remark, 

 that the peculiar modification of the 

 Method of Agreement, which, as par- 

 taking in some degree of the nature 

 of the Method of Difference, I have 

 called the Joint Method of Agree- 

 ment and Difference, is not affected 

 by the characteristic imperfection now 

 pointed out. For, in the joint method, 

 it is supposed not only that the in- 

 stances in which a is, agree only in 

 containing A, but also that the in- 

 stances in which a is not, agree only 

 in not containing 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 con- 

 taining A. This, therefore, consti- 

 tutes 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 premises, (if they may 

 be 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 char- 

 acteristic imperfection which affects 

 it in the affirmative case. The nega- 

 tive premise, it might therefore be 

 supposed, could be worked as a simple 

 case of the Method of Agreement, 

 without requiring an affirmative pre- 

 mise to be joined with it. But though 

 this is true in principle, it is gene- 

 rally 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 affirma- 

 tion. For instance, let the question 

 be, what is the cause of the trans- 

 parency of bodies ; with what prospect 



of success could we set ourselves to 

 inquire directly in what the multi- 

 farious substances which are not trans- 

 parent 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 examining whether the 

 absence of this one circumstance be 

 not precisely the point in which all 

 opaque circumstances will be found 

 to resemble. 



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 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 instru- 

 ments of inductive investigation ; 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 a possible 

 supposition, which, until removed, 

 renders our inductions 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 consider it as a 

 case actually occurring in nature, and 

 which, as often as it does occur, our 

 methods of induction ought to be 

 capable of ascertaining and establish- 

 ing. For this, however, there is re- 

 quired no peculiar method. When an 

 effect is really producible by two or 

 more causes, the process for detecting 

 them is in no way different from that 

 by which we discover single causes. 

 They may (first) be discovered as 

 separate sequences by separate seta 



