266 INDUCTION. 



from one another, invokes not only as its evidence, but as its sole evidence, the absolute in- 

 conceivability of any theory but itself, we are enabled to measure the worth of this kind of 

 evidence : and when we find the Volitional theory entirely built upon the assertion that by 

 our mental constitution we are compelled to recognize our volitions as efficient causes, and 

 then find other thinkers maintaining that we know that they are not and can not be such 

 causes, and can not conceive them to be so, I think we have a right to say that this supposed 

 law of our mental constitution does not exist. 



Dr. Tulloch (pp. 45-47) thinks it a sufficient answer to this, that Leibnitz and the Cartesians 

 were Theists, and believed the will of God to be an efficient cause. Doubtless they did, and 

 the Cartesians even believed (though Leibnitz did not) that it is the only such cause. Dr. 

 Tulloch mistakes the nature of the question. I was not writing on Theism, as Dr. Tulloch 

 is, but against a particular theory of causation, which, if it be unfounded, can give no effect- 

 ive support to Theism or to any thing else. I found it asserted that volition is the only ef- 

 ficient cause, on the ground that no other efficient cause is conceivable. To this assertion I 

 oppose the instances of Leibnitz and of the Cartesians, who affirmed with equal positiveness 

 that volition as an efficient cause is itself not conceivable, and that omnipotence, which ren- 

 ders all things conceivable, can alone take away the impossibility. This I thought, and think, 

 a conclusive answer to the argument on which this theory of causation avowedly depends. 

 But I certainly did not imagine that Theism was bound up with that theory ; nor expected 

 to be charged with denying Leibnitz and the Cartesians to be Theists because I denied that 

 they held the theory. 



CHAPTER VI. 



ON THE COMPOSITIOX OP CAUSES. 



§ 1. To complete the general notion of causation on vvhicli the rules of 

 experimental inquiry into the laws of nature must be founded, one dis- 

 tinction still remains to be pointed out: a distinction so radical, and of so 

 much importance, 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 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 conditions 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 effect of each cause when acting separately from the 

 other, we are often able to arrive deductively, or a2)riori, at a correct pre- 

 diction of what will arise from their conjunct agency. To "render this pos- 

 sible, 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 call- 

 ed mechanical, namely the phenomena of the communication 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 speak- 

 ing, defeats or frustrates another ; both have their full effect. If a body is 

 propelled in two directions by two forces, one tending to drive it to the 

 north and the other to the east, it is caused to move in a given time exact- 

 ly as far in both directions as the two forces would separately have carried 

 it ; and is left precisely where it would have arrived if it had been acted 



