x Difficulty seen by Descartes. 173 



I do not propose here to go back on the meta- 

 physical aspect of the controversy, but to treat it 

 only in its relations to physical science. 



The physical or mechanical, as distinguished from 

 the metaphysical, difficulty in recognising Will as an 

 agency capable of acting on matter was, I believe, 

 first seen by Descartes. He taught that matter and 

 spirit, though in union, are absolutely distinct ; that 

 matter acts, and is acted on, according to rigidly 

 mechanical laws ; and that the total quantity of 

 motion in the universe is invariable. From these 

 premises it is an obvious consequence that Will can- 

 not be a source of motive power in the universe of 

 matter; but Descartes solved the difficulty by adding 

 that Will, though unable to produce motion, is able 

 to direct it. I believe this to be in substance the 

 true solution ; and it is substantially that of at least 

 two of the three writers of whose views I have under- 

 taken to give an account ; but it needs to be translated 

 into not only the language, but the ideas of modern 

 science. " Quantity of motion," in this context, is not 

 an accurate expression ; but the truth after which 

 Descartes was groping is what is now known as the 

 doctrine of the Conservation of Energy; that the 

 energy of the universe, though perpetually undergoing 

 transformation, is a constant quantity ; that a given 

 quantity of energy, when it undergoes transformation, 

 does an exactly equivalent quantity of work, which 

 work reappears in some other form of energy. 

 Muscular action presents no exception to this law 



