n THE PROGRESS OF SCIENCE 03 



To all appearance, the phenomena exhibited by 

 the pendulum are not to be accounted for by 

 impact : in fact, it is usually assumed that corre- 

 sponding phenomena would take place if the earth 

 and the pendulum were situated in an absolute 

 vacuum, and at any conceivable distance from 

 one another. If this be so, it follows that there 

 must be two totally different kinds of causes of 

 motion : the one impact a vera causa, of which, 

 to all appearance, we have constant experience ; 

 the other, attractive or repulsive "force" a 

 metaphysical entity which is physically incon- 

 ceivable. Newton expressly repudiated the notion 

 of the existence of attractive forces, in the sense 

 in which that term is ordinarily understood ; and 

 he refused to put forward any hypothesis as to 

 the physical cause of the so-called "attraction 

 of gravitation." As a general rule, his successors 

 have been content to accept the doctrine of 

 attractive and repulsive forces, without troubling 

 themselves about the philosophical difficulties 

 which it involves. But this has not always been 

 the case ; and the attempt of Le Sage, in the last 

 century, to show that the phenomena of attrac- 

 tion and repulsion are susceptible of explanation 

 by his hypothesis of bombardment by ultra- 

 mundane particles, whether tenable or not, has 

 the great merit of being an attempt to get rid of 

 the dual conception of the causes of motion which 

 has hitherto prevailed. On this hypothesis, the 



