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CHAPTER VI. 

 OF THE COMPOSITION OF CAUSES. 



1. To complete the general notion of causation 

 on which the rules of experimental inquiry into the 

 laws of nature must be founded, one distinction still 

 remains to be pointed out : a distinction so funda- 

 mental, and of so much importance, as to require a 

 chapter to itself. 



The preceding discussions have rendered us fami- 

 liar with the case in which several agents, or causes, 

 concur as conditions to the production of an effect ; a 

 case, in truth, almost universal, there being very few 

 effects to the production of which no more than one 

 agent contributes. Suppose, then, that two different 

 agents, operating jointly, are followed, under a certain 

 set of collateral conditions, by a given effect. If either 

 of these agents, instead of being joined with the 

 other, had operated alone, under the same set of condi- 

 tions in all other respects, some effect would probably 

 have followed ; which would have been different from 

 the joint effect of the two, and more or less dissimilar 

 to it. Now, if we happen to know what would be the 

 effects of each cause when acting separately from the 

 other, we are often able to arrive deductively, or a 

 priori, at a correct prediction of what will arise from 

 their conjunct agency. To enable us to do this, it is 

 only necessary that the same law which expresses the 

 effect of each cause acting by itself, shall also cor- 

 rectly express the part due to that cause, of the effect 

 which follows from the two together. This con- 

 dition is realised in the extensive and important 



