54 THE GREEK SCHOOL PHILOSOPHY. 



knowledge, was that in which he asserts 15 "That 

 body is heavier than another which in an equal bulk 

 moves downward quicker." The opinion maintained 

 by the Aristotelians at the time of Galileo was, that 

 bodies fall quicker exactly in proportion to their 

 weight. The master himself asserts this in express 

 terms, and reasons upon it 16 . Yet in another passage 

 he appears to distinguish between weight and actual 

 motion downwards 17 . "In physics, we call bodies 

 heavy and light from their power of motion ; but 

 these names are not applied to their actual opera- 

 tions (evepyeiais) except any one thinks momentum 

 (poTrrj) to be a word of both applications. But 

 heavy and light are, as it were, the embers or sparks 

 of motion, and therefore proper to be treated of 

 here." 



The distinction just alluded to between Power or 

 Faculty of Action, and actual Operation or Energy, 

 is one very frequently referred to by Aristotle ; and 

 though not by any means useless, may easily be so 

 used as to lead to mere verbal refinements instead 

 of substantial knowledge. 



The Aristotelian distinction of Causes has not 

 any very immediate bearing upon the parts of phy- 

 sics of which we have here mainly spoken ; but it 

 was so extensively accepted, and so long retained, 

 that it may be proper to notice it 18 . " One kind of 



15 De Coelo, iv. 1, p. 308. 16 De Ccelo, iii. 2. 



17 De Coelo, iv. 1, p. 307- 18 Phys. ii. 3. 



