PLURALITY OF CAUSES. 513 



is present, differ from those in which it is absent) is, 

 after the direct Method of Difference, the most power- 

 ful of the remaining instruments of inductive investi- 

 gation ; and in the sciences which depend on pure 

 observation, with little or no aid from experiment, 

 this method, so well exemplified in the beautiful 

 speculation on the cause of dew, is the primary re- 

 source, 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 con- 

 sidered 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 ascer- 

 taining and establishing. For this, however, there is 

 required 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 sets of 

 instances. One set of observations or experiments 

 shows that the sun is a cause of heat, another that 

 friction is a source of it, another that percussion, ano- 

 ther that electricity, another that chemical action is such 

 a source. Or (secondly) the plurality may come to light 

 in the course of collating a number of instances, when 

 we attempt to find some circumstance in which they 

 all agree, and fail in doing so. We find it impossible 

 to trace, in all the cases in which the effect is met 

 with, any common circumstance. We find that we 

 can eliminate all the antecedents ; that no one of them 

 VOL. i. 2 L 



