<2 THE GREEK SCHOOL PHILOSOPHY. 



momentum (f>o<7r^) 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 physics 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. 19 " One kind of Cause is the matter 

 of which any thing is made, as bronze of a statue, and silver of a 

 vial ; another is the form and pattern, as the Cause of an octave is 

 the ratio of two to one ; again, there is the Cause which is the origin 

 of the production, as the father of the child ; and again, there is the 

 End, or that for the sake of which any thing is done, as health is the 

 cause of walking." These four kinds of Cause, the material, informal, 

 the efficient, and the final, were long leading points in all speculative 

 inquiries ; and our familiar forms of speech still retain traces of the 

 influence of this division. 



It is my object here to present to the reader in an intelligible shape, 

 the principles and mode of reasoning of the Aristotelian philosophy, 

 not its results. If this were not the case, it would be easy to excite a 

 smile by insulating some of the passages which are most remote from 

 modern notions. I will only mention, as specimens, two such passages, 

 both very remarkable. 



In the beginning of the book " On the Heavens," he proves 20 the 

 world to be perfect, by reasoning of the following kind : " The bodies 

 of which the world is composed are solids, and therefore have three 

 dimensions : now three is the most perfect number ; it is the first of 

 numbers, for of one we do not speak as a number 5 of two we say 

 both ; but three is the first number of which we say all ; moreover, it 

 has a beginning, a middle, and an end." 



The reader will still perceive the verbal foundations of opinions thus 

 supported. * 



*' The simple elements must have simple motions, and thus fire and 

 air have their natural motions upwards, and water and earth have 



l Phys. ii. 3. 20 De Coelo, i. 1. 



