488 INDUCTION. 



a possible supposition, which, until removed, renders our induc- 

 tions uncertain ; and have only considered by what means, where 

 the plurality does not really exist, we may be enabled to dis- 

 prove 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 

 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, another 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 circum- 

 stance 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 is present 

 in all the instances, no one of them indispensable to the effect. 

 On closer scrutiny, however, it appears that though no one is 

 always present, one or other of several always is. If, on fur- 

 ther analysis, we can detect in these any common element, we 

 may be able to ascend from them to some one cause which is 

 the really operative circumstance in them all. Thus it is now 

 thought that in the production of heat by friction, percussion, 

 chemical action, &c., the ultimate source is one and the same. 

 But if (as continually happens) we cannot take this ulterior 

 step, the different antecedents must be set down provisionally 

 as distinct causes, each sufficient of itself to produce the 

 effect. 



We here close our remarks on the Plurality of Causes, and 

 proceed to the still more peculiar and more complex case of 

 the Intermixture of Effects, and the interference of causes 

 with one another : a case constituting the principal part of 

 the complication and difficulty of the study of nature; and 



