BOOK III. 



THE GEEEK ASTRONOMY. 



INTRODUCTION. 



THE mathematical opinions of Plato respecting the philosophy of 

 nature, and especially respecting what we commonly call "the 

 heavenly bodies," the Sun, Moon, and Planets, were founded upon the 

 view which I have already described : namely, that it is the business of 

 philosophy to aim at a truth higher than observation can teach ; and 

 to solve problems which the phenomena of the universe only suggest. 

 And though the students of nature in more recent times have learnt 

 that this is too presumptuous a notion of human knowledge, yet the 

 very boldness and hopefulness which it involved impelled men in the 

 pursuit of truth, with more vigor than a more timorous temper could 

 have done ; and the belief that there must be, in nature, mathematical 

 laws more exact than experience could discover, stimulated men often 

 to discover true laws, though often also to invent false laws. Plato's 

 writings, supplying examples of both these processes, belong to the 

 Prelude of true Astronomy, as well as to the errors of false philosophy. 

 We may find specimens of both kinds in those parts of his Dialogues 

 to which we have referred in the preceding Book of our History. 



To Plato's merits in preparing the way for the Theory of Epicycles, 

 I have already referred in Chapter ii. of this Book. I conceive that 

 he had a great share in that which is an important step in every dis- 

 covery, the proposing distinctly the problem to be solved ; which was, 

 in this case, as he states it, To account for the apparent movements of 

 the planets by a combination of two circular motions for each : — the 

 motion of identity, and the motion of difference. (Tim. 39, a.) In 

 the tenth Book of the Republic, quoted in our text, the spindle which 

 Destiny or Necessity holds between her knees, and on which are rings, 

 by means of which the planets revolve round it as an axis, is a step 

 towards the conception of the problem, as the construction of a machine. 



It will not be thought surprising that Plato expected that Astron- 



