184 SCIENCE IN GRECO-ROMAN ANTIQUITY 



the theory of motion ; he does not assign to the former 

 its own principles quite independent of the latter ; 

 he deals generally with the movements which can take 

 place in a mechanism ; when no movement takes place 

 the mechanism remains in equihbrium. 1 It must not 

 be forgotten, moreover, that, for Aristotle, mechanics 

 as a whole rested mainly on philosophical doctrines 

 regarding the nature of movement and of natural 

 position, the distinction between celestial and sublunar 

 bodies, the opposition of natural and " violent " move- 

 ments, etc. 



The idea of motion had primarily a much wider 

 meaning than that which we give it. 2 As a matter 

 of fact, by motion Aristotle understood : 



1. A substantial change, which, for a given body, 

 can take place in two opposite senses : the passage 

 from form to formlessness which causes corruption, 

 or, inversely, a passage from formlessness to form 

 which gives rise to a birth. 



2. A quantitative change, owing to which a body 

 is diminished or increased in volume. 



3. A qualitative change which causes in a body a 

 transformation of its properties. 



4. A local movement which brings about the dis- 

 placement of a body from one position to another. 



Of these four species of motion, the qualitative 

 change presents a special character because it cannot 

 be reduced to a mechanism or to a simple study of 

 spatial ratios. A substance which changes in quality 

 does so, not by a displacement of its molecules, but 

 by an internal variation of its nature. The changes 

 in quantity and substance, on the contrary, imply a 

 local movement. This latter is therefore the most 

 important in mechanics. Besides, it concerns the in- 

 corruptible celestial bodies as well as the terrestrial 



1 n Duhem, Origines, I, p. 5. 

 2 13 Duhem, Sys*eme, I, p. 161. 



