28 HISTORY OF SCIENCE. 



Atlas. Empedocles also was wrong in supposing that the heavens 

 are sustained by the extreme velocity of their rotation in the same way 

 as water sustains itself in a vessel rapidly whirled round. 



Aristotle's argument to prove that the heavens are incorruptible is 

 conducted after a similar fashion, somewhat as follows. It was one 

 of his doctrines that the natural motions of contraries are contrary. 

 Thus, of the pair of opposites earth and fire, the natural motion of 

 the former is downward towards the centre, that of the latter upwards 

 from the centre. He urges that to motion round a centre there can 

 be no opposite motion, inasmuch as there being but three simple 

 motions, viz., towards a centre, from a centre, and round a centre, and 

 the first and second modes of motion forming a pair of contraries, 

 there remains no kind of motion to which the circular motion can be 

 opposed, for each thing can have but one opposite. He meets by 

 some curious reasoning the anticipated objection that circular mo- 

 tion in one direction may have its opposite in circular motion in the 

 other direction. For example, he says, the purpose of circular motion 

 is to convey a thing from one part of a circle to another part, and 

 whatever thus moves necessarily comes uniformly round to each part 

 in succession ; the circular motions by which a body may be carried 

 from the point A to the point B are infinite in number, since an infinite 

 number of circles may be drawn so as to pass through the fixed points 

 A and B ; and in a circle there are no contraries of place, as above 

 or below, before or behind, right or left, on which opposition of 

 direction can be founded. Circular motion being, therefore, without 

 a contrary, so the heavenly body to whose nature this kind of motion 

 is conformable can have no contrary; and, having no contrary, the 

 celestial body must be indestructible, since destruction occurs by a 

 body changing into its opposite. And for the same reason it is not 

 possible for the celestial body to have been produced or generated, 

 since in production a body arises from its opposite. 



Aristotle advances similar arguments to prove that there can be 

 only one sphere of the heavens, and that no body beyond can exist 

 outside of that sphere. He has also long metaphysical discussions 

 about the Finite and the Infinite, etc. Sometimes, but rarely, he cites 

 in support of his theories a fact which he considers analogous. Thus, 

 to illustrate the assumption that light and heat arise from the stars, by 

 reason of the friction against the air which their rapid motion pro- 

 duces, he says that it is the nature of motion to set on fire pieces of 

 wood, stone, and iron, and that arrows become heated by their rapid 

 motion through the air. 



That the earth can itself revolve, either in the centre of the world 

 or about the centre, appears to Aristotle impossible, because such a 

 motion would necessarily be violent that is, against the nature of the 

 earth. " We know," he says, " that it is the nature of portions of the 

 earth to move in straight lines" (he refers here to falling bodies), "and 



