THE GREEK ATTITUDE 



tively and to explain phenomena by causes postulated 

 as necessary and from a priori grounds. 



Before we can understand his ideas on evolution 

 we must look first at his concepts of physics, Aristotle 

 accepts the four elements — earth, water, air, and fire 

 — first proposed by Empedocles and adds to them a 

 fifth essence, the aether. He reviews the atomic 

 theor)^ of Democritus and finally discards it as in- 

 volving the principle of chance rather than the teleo- 

 logical basis of purpose to which he was unwavering- 

 ly committed. These five elements are, as he expresses 

 it, the substratum of reality, the stuff out of which the 

 universe was fashioned. The centre of the earth is the 

 centre of the universe and the first four elements have 

 their natural places in the order mentioned from this 

 centre. If by external causes the elements become 

 mixed, as for example, earth and water, they en- 

 deavour to return to their natural levels. The proof 

 of this law is that water exists on the surface of the 

 earth; while above it is the shell of air; and furthest 

 from the solid globe is the region of celestial fire. The 

 fifth element is the essence of the celestial bodies and 

 is perfect and unchangeable in contradistinction to 

 the mutable and corruptible four terrestrial essences. 



In order that there may be individual things or 

 bodies, he postulates form as the cause by which the 

 elements change from potentiality to actuality. Thus, 

 the proportionate combination of the elements and 

 form determine the individual object. 



C 53 3 



