THE EXPLANATION OF LAWS 99 



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 I 1 



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 

 things, it is often much harder to ask questions than to 

 answer them. People might have said, and many 

 probably did say : The planets have to move somehow ; 

 the paths Kepler describes are quite simple ; why should 

 not the planets move in them ? It is as ridiculous to ask 

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

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

 ception of explaining the paths of the planets was itself 

 an immense achievement. 



And we can see now what suggested it to Newton. 

 Some sixty years before Galileo had, for the first time, 

 discovered some of the laws which govern the motions of 

 bodies under forces. He had shown that, in some simple 

 instances at least, there were such things as " laws of 

 dynamics." The idea occurred to Newton, May not the 

 movements of the planets and their satellites be subject 

 to just such laws of dynamics as Galileo had discovered 

 for the ordinary bodies which we see and handle. If so, 



1 Of course the apple may be mythical like all the historical 

 objects of our childhood. And it is impossible to be certain what 

 Newton really thought. But his thought might have followed the 

 line suggested here. 



