THE theories established in one generation become the starting- 

 points of successors. Newton, with all his genius, would not have 

 detected the law of gravitation had not Kepler and Galileo preceded 

 him ; nor could they have made their discoveries had not Greek 

 mathematicians supplied the means. It was by the bold and happy 

 identification of celestial with terrestrial physics that the great 

 thinkers of the seventeenth century made physical Astronomy an 

 exact science, making it a part of Mechanics, explaining its pheno- 

 mena by those very Laws of Motion which were proved to regulate 

 the phenomena of terrestrial bodies. 



LEWES, Aristotle. 



