II THE PROGRESS OF SCIENCE 89 



once gives rise to motion. While the restraint 

 lasts, the energy of the particle is merely poten- 

 tial ; and the case supposed illustrates what is 

 meant by potential energy. In this contrast of the 

 potential with the actual, modern physics is turn- 

 ing to account the most familiar of Aristotelian 



o 



distinctions that between 8vva/j,i<; and eve'pyeta. 



That kinetic energy appears to be imparted 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 cannot be ex- 

 plained, but must be taken as an ultimate fact 

 about which, explicable or inexplicable, there can 

 be no doubt. Strictly speaking, we have no direct 

 apprehension of any other cause of motion. But 

 experience furnishes innumerable examples of the 

 production of kinetic energy in a body previously 

 at rest, when no impact is discernible as the cause 

 of that energy. In all such cases, the presence of 

 a second body is a necessary condition ; and the 

 amount of kinetic energy, which its presence 

 enables the first to gain, is strictly dependent on 

 the relative positions of the two. Hence the 

 phrase energy of position, which is frequently used 

 as equivalent to potential energy. If a stone is 

 picked up and held, say, six feet above the ground, 

 it has potential energy, because, if let go, it will 

 immediately begin to move towards the earth ; 



