MECHANICS AND PHYSICS 185 



bodies which are subject to the phenomena of birth 

 and death. 



This being so, the local movement can assume two 

 forms, the one natural, the other violent. 



The natural movement arises from the fact that 

 for each body there is a place in which it exists in 

 perfect equilibrium and towards which it naturally 

 tends. This natural movement is necessarily simple 

 like each of the simple substances affected by it. 



Only two kinds of simple movements exist, the move- 

 ment of rotation, which Aristotle calls the circular, 

 and the movement of translation, which he called the 

 rectilinear 1 (Phys. 261 b). The circular movement is 

 that which belongs by its nature to celestial bodies, 

 for it is, like them, perfect. The rectilinear movement, 

 on the contrary, is the movement of bodies situated 

 in the sublunar regions, which are subject to generation 

 and corruption. 



The simple movements of translation are of two 

 kinds, some are directed towards the centre of the 

 universe, others follow directions issuing from this 

 point ; the rectilinear centripetal movement (down- 

 ward movement) naturally affects the heavy or weighty 

 bodies whose position of equilibrium is the centre of 

 the universe ; the rectilinear centrifugal movement 

 (upward movement) belongs to the light bodies which 

 are situated in the concavity of the lunar orbit. Of 

 the four elements which exist in the sublunar region, 

 two are heavy, namely earth and water, and two 

 are light, air and fire. 



Thus heaviness and lightness impart rectilinear 

 movements to the bodies possessing these qualities ; 

 but these movements cease as soon as the bodies have 

 reached their position of equilibrium, that is to say 

 the region of space in which they are naturally in 

 equilibrium. So this position is not only a reality 

 1 13 Duhem, Systeme, I, p. 205. 



