76 THE ADVANCE OF SCIENCE 



not move. It is then in a condition total- 

 ly different from that in which it was at 

 first. A cause competent to produce mo- 

 tion is operating upon it, but, for some 

 reason or other, is unable to give rise to 

 motion. If the obstacle is removed, the 

 energy which was there, but could not 

 manifest itself, at once gives rise to mo- 

 tion. While the restraint lasts, the en- 

 ergy of the particle is merely potential ; 

 and the case supposed illustrates what is 

 meant by potential energy. In this con- 

 trast of the potential with the actual, 

 modern physics is turning to account the 

 most familiar of Aristotelian distinctions 

 — that between S1W/U9 and ivepjeta. 



That kinetic energy appears to be im- 

 parted by impact is a fact of daily and 

 hourly experience : we see bodies set in 

 motion by bodies, already in motion, 

 which seem to come in contact with them. 

 It is a truth which could have been learned 

 by nothing but experience, and which 



