MOTOR FORCE OF THE WORLD MACHINE 259 



It is quite probable that Newton did not realise at their 

 just value the contributions of others. His own ideas, as we 

 know, were icily clear ; he thought things out until every trace 

 of turbidity was lost. Probably he had a very human im- 

 patience with that confusion of thought which all of us find 

 so troubling in the mental processes of others. Hooke's ideas 

 never reached complete clarity. 



That a letter of Hooke's was the especial incitement which 

 led Newton to take up his old calculations and conjectures and 

 give to the law complete and irrefragable proof that will last 

 with time, seems clear from Newton's own letters. That Hooke 

 was deserving of much credit is certain. That the law would 

 have been discovered within a very few years without a Newton 

 is patent beyond question. But Hooke did not write the 

 Principia Newton did. In the Principia the law of gravita- 

 tion is not put forth as a special fact it is but a part of a vast 

 scheme. What Descartes bravely attempted Newton achieved. 

 With the aid of a single principle he disclosed our planetary or 

 solar system as a vast yet simple mechanism, all of whose motions 

 were not merely fixed but calculable. This principle he applied 

 everywhere, alike to the flight of the planets, the revolutions 

 of their satellites, the flux and reflux of the waters of the earth. 

 In the Principia, the human intellect reached for the first time 

 a clear mental presentation of the workings of the world amid 

 which its dream-like existence is passed. So far as the mechanics 

 of the solar system is concerned nothing of consequence has been 

 added since, and nothing has been taken away. It is this, and 

 not simply the discovery or the proof of the law of mass-action, 

 which is the true glory of Newton. 



Yet the discovery alone was a great deed. Its precise nature 

 is often strangely befuddled. One may read in many a book 

 that Newton did not discover gravity but simply the law of 

 its action that the force of gravity had been known for thou- 

 sands of years. This is trifling. What Newton discovered, and 

 demonstrated, was Attraction, not merely by the law but the 

 fact, not merely the attractive force of the planets but of all 

 matter whatsoever, from a molecule to a moon, from a grain 

 of sand to the colossal mass of the sun. This discovery was 

 wholly independent of gravity or any other consideration. It 

 was, and might so have remained, complete in itself. So even 

 does it appear to have been for a time in Newton's own mind. 



