THE EXPLANATION OF LAWS 



although I can hardly hope that I shall succeed where 

 so many writers have failed, and although the attempt 

 transgresses the strict limit of an introduction, I should 

 like to try to tell again the familiar story of one of the 

 most wonderful romances of science the story of 

 Newton and the apple ! l 



The early chapters of the story must be greatly abbrevi- 

 ated. Copernicus and Kepler, a century before Newton, 

 had shown clearly what were the paths in which the 

 planets move about the sun and the satellites, such as 

 our moon, about the planets. It does not seem clear 

 whether anyone before Newton had thought of inquiring 

 why they should move in such paths, or had ever con- 

 templated the possibility of explaining the laws which 

 Kepler had laid down. In science, as in many other 

 tilings, i; \\ much harder to ask questions than to 



pie might have said, and many 

 probably did say : The planets have to move somehow ; 

 the paths Kepler describes are quite simple ; why should 

 i he planets move in them ? It is as ridiculous to ask 

 v so move as to ask why a man's hair is yellow 

 i or red, and not blue or green. The mere con- 

 cept i lining the paths of the planets was itself 



achievement. 



c can see now what suggested it to Newton. 

 \ty years before Galileo had, for the HIM time, 

 discovered some t\vs which govern the m<>ti; 



bodi- ; forces. 1I- bad -h<>\vn that, in ->ine simple 



re such tilings as " lav 



dynamics." a occun > \\ton, May not the 



..uujts aiul their satellites be Sill 

 dynamics as Galileo had discovered 

 he ordinary bodi U we see and handle. If so, 



be mythic.il like all .ncal 



to be Ctr 

 n really thought. But his thought might have followed the 



