CHAPTER VI. 



ON 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 radical, and of so much impor 

 tance, as to require a chapter to itself. 



The preceding discussions have rendered us familiar with 

 the case in which several agents, or causes, concur as condi 

 tions 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 conditions 

 in all other respects, some effect would probably have fol 

 lowed ; which would have been different from the joint effect 

 of the two, and more or less dissimilar to it. Xow, if we 

 happen to know what would be the effect of each cause 

 when acting separately from the other, we are often able to 

 arrive deductively, or d 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 correctly 

 express the part due to that cause, of the effect which follows 

 from the two together. This condition is realized in the 

 extensive and important class of phenomena commonly 

 called mechanical, namely the phenomena of the communi 

 cation of motion ^or of pressure, which is tendency to motion) 

 from one body to another. In this important class of cases 

 of causation, one cause never, properly speaking, defeats or 



