THE GOLDEN AGE OF GREECE 83 



below the moon's sphere is subject to continual change, and motions 

 within it are in general rectilinear a theory destined long to 

 block progress in mechanics. Of the four elements, earth is near- 

 est the centre, water comes next, fire and air form the atmosphere, 

 fire predominating in the upper part, air in the lower. In this 

 region of fire are generated shooting stars, auroras, and comets, 

 the latter consisting of ignited vapors, such as constitute the 

 Milky Way. 



Against any orbital motion of the earth Aristotle urges the ab- 

 sence of any apparent displacement of the stars. Reviewing his 

 astronomical theories, Dreyer says : 



His careful and critical examination of the opinions of previous phi- 

 losophers makes us regret all the more that his search for the causes 

 of. phenomena was often a mere search among words, a series of vague 

 and loose attempts to find what was ' according to nature ' and what 

 was not; and even though he professed to found his speculations 

 on facts, he failed to free his discussion of these from purely metaphys- 

 ical and preconceived notions. It is, however, easy to understand 

 the great veneration in which his voluminous writings on natural 

 science were held for so many centuries, for they were the first, and 

 for many centuries the only, attempt to systematize the whole 

 amount of knowledge of nature accessible to mankind; while the 

 tendency to seek for the principles of natural philosophy by con- 

 sidering the meaning of the words ordinarily used to describe the 

 phenomena of nature, which to us is his great defect, appealed strongly 

 to the mediaeval mind, and, unfortunately, finally helped to retard 

 the development of science in the days of Copernicus and Galileo. 



At times Aristotle shows consciousness that his theories are 

 based on inadequate knowledge of facts. 



' The phenomena are not yet sufficiently investigated. When they 

 once shall be, then one must trust more to observation than to spec- 

 ulation, and to the latter no farther than it agrees with the phe- 

 nomena.' 



' An astronomer ' he says ' must be the wisest of men ; his mind 

 must be duly disciplined in youth ; especially is mathematical study 

 necessary; both an acquaintance with the doctrine of number, 



