166 SCIENCE IN GRECO-ROMAN ANTIQUITY 



of the universe was revived by Aristotle and affirmed 

 until the Renaissance. 



The bodies which exist above the moon are composed 

 of pure fire or pure elements, which cannot be impaired 

 or changed ; they are therefore eternal, and, being un- 

 created, are imperishable. 



The sub-lunar bodies, on the contrary, are all com- 

 plex ; they are subject to generation and destruction, 

 since the mixtures of which they are formed are 

 subject to all sorts of changes. 



The earth is in the Uranus as well as its opposite 

 the counter-earth (Antichthon) , which was postulated 

 to satisfy the law of perfection which required that the 

 number of the heavenly bodies in circular motion 

 should reach the perfect figure ten. The existence of 

 the counter-earth was also necessary to explain the 

 greater frequency of eclipses of the moon than of the 

 sun. 



The earth and the counter-earth turn around the 

 central fire as if they were rigidly fixed to the extremi- 

 ties of one diameter. This is why we cannot see either 

 the central fire or the counter-earth from the side on 

 which we live. The ten celestial bodies (sphere of the 

 stars, five planets, sun, moon, earth and counter-earth) 

 move around the central fire, the hearth of the universe, 

 after the manner of a chorus on the stage ; moving at 

 different speeds, they produce by their revolution a 

 perfect musical harmony. The earth is not the only 

 heavenly body inhabited. The moon is also inhabited, 

 but the lunar beings are more beautiful and fifteen 

 times as big as the terrestrial beings. (Aetius, Diels, 

 Vor., 237, 43.) 



Hicetas and Ecphantus, two disciples of Pythagoras 

 later than Philolaus, abandoned the hypothesis of the 

 counter-earth; they placed the central fire in the 

 interior of the earth and the earth itself at the centre 

 of the universe. Furthermore, to explain the move- 



