ASTRONOMY 169 



the mathematical hypotheses which attempt to account 

 for their movements. The physical hypotheses, al- 

 though in some degree supplemented by Aristotle, 

 remained in antiquity and the Middle Ages practically 

 the same as in the time of Plato and his immediate 

 predecessors. At this epoch, as we have seen, the air 

 and humidity were no longer confused ; darkness was 

 considered as a shadow and no more as a material fact. 

 It was also admitted that although the sun, planets 

 and stars shine by themselves, the moon has a borrowed 

 light. 



This being so, the physical hypotheses may be 

 reduced to four : 



1. The universe forms a finite and finished whole. 

 To suppose it illimitable, is to contradict both reason 

 and fact. Our reason cannot in fact conceive of some- 

 thing which exists in reality and does not occupy a 

 definite place. On the other hand, if the universe 

 were infinite, its extremities would have to traverse 

 infinite spaces in a finite period of twenty-four hours, 

 which is actually impossible. 



2. Since the universe is finite, it has a spherical form 

 and a centre, and it is the earth which must occupy 

 this centre. If we consider the earth alone, we see 

 that it is motionless. Besides, of all the elements 

 known to us, it is the terrestrial element which is 

 heaviest and consequently must occupy the centre of 

 the universe. 



3. The universe as a whole is composed of two 

 regions : one celestial, the other sublunar. The sub- 

 lunar region comprises the bodies formed by the 

 mixture of the four elements, water, air, earth and fire, 

 and which are therefore subject to birth and death. 

 The celestial region is occupied by the heavenly bodies, 

 which, being formed of a fifth and unique element 

 (quintessence), are, like this element, incorruptible. 



4. Physically there is but one possible movement, 



