IN THE LAST HALF-CEXTUKY. 83 



impact — a vera causa, of which, to all ap- 

 pearance, we have constant experience ; 

 the other, attractive or repulsive 'force' 

 —a metaphysical entity which is physi- 

 cally inconceivable. 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 re- 

 pulsive forces, without troubling them- 

 selves about the philosophical difficulties 

 which it involves. But this has not al- 

 ways been the case ; and the attempt of 

 Le Sage, in the last century, to show that 

 the phenomena of attraction and repul- 

 sion are susceptible of explanation by his 

 hypothesis of bombardment by ultra-mun- 

 dane particles, whether tenable or not, 

 has the great merit of being an attempt 



