430 Molecular Motion. [October, 



In physics we have been accustomed to attribute every 

 thing to force ; force, at least, has always been regarded as 

 the all-important element. This, however, is a mistake ; 

 for, as we shall see, far more depends upon the determination 

 of force than upon its existence, and therefore, unless force be 

 determined by force, the most important element in physical 

 causation is a something different from force. 



Motion is not only produced, but it is produced in a par- 

 ticular manner and under particular conditions or determi- 

 nations in regard to time and space and other circumstances. 

 In other words, not only must something produce the 

 motion, but something must determine it also. The causing 

 of, or giving mere existence to the motion, Mr. Croll has 

 called the production of the motion. The causing of it to 

 happen in the particular manner in which it does, rather 

 than in some other manner, he calls the determination of the 

 motion. It must be evident to every one who will consider 

 the matter that these two things are radically distinct. 

 And they are not only radically distinct, but must be sepa- 

 rately accounted for. To account for the mere existence of 

 motion does not account for its happening in one way rather 

 than in some other. It is quite true that the one cannot be 

 produced without the other ; we cannot determine motion 

 unless there is motion to be determined ; we cannot deter- 

 mine that which has got no existence ; neither, on the other 

 hand, can we produce motion without at the same time 

 giving it some particular determination in regard to time, 

 place, or other circumstance. But, although the one cannot 

 be produced without the other, yet they are the result of 

 different agencies ; and to assign a sufficient cause for the 

 one does not in the least degree satisfy the mind as to the 

 presence of the other. To account for the motion of a ball 

 does not account for why it moves, say, east rather than 

 west, or in any other possible direction. A force, it is true, 

 cannot act without at the same time acting in some parti- 

 cular way, nor move a body without moving it in some par- 

 ticular direction ; but to account for the one does not satisfy 

 the mind in regard to the other. The explosion of the 

 powder within a gun is a sufficient cause for the motion of 

 the ball, but the explosion of the powder is not to the mind 

 a sufficient cause why the ball moves east rather than west, 

 or in any other direction. 



The grand and fundamental question then is, What is it 

 that determines or directs the action of the forces concerned 

 in the production of molecular change ? The question 

 therefore regards not Law but Cause, unless we use the 



