100 WHAT IS SCIENCE? 



we ought to be able to find a set of forces such that if they 

 act on the planets according to Galileo's laws, the planets 

 will move as Kepler has shown that they do move. That 

 seems to us very obvious now ; but it was not obvious 

 then. Galileo, as far as we know, never thought of it ; 

 nor did anyone in the two generations between him 'and 

 Newton. And perhaps one reason why nobody thought 

 of it was that they realized instinctively that if they had 

 thought of it they could have got no further. To-day any 

 clever schoolboy could solve the next problem which 

 presents itself, namely that of finding what forces, acting 

 according to Galileo's laws, would make the planets move 

 as they do ; but that is only because Newton has shown 

 him the way. In order to solve this problem which seems 

 to us now so easy, Newton had to invent modern mathe- 

 matics ; he had to make a greater advance in mathe- 

 matics than had been made in all the time since the high- 

 water of Egyptian civilization. This achievement of 

 his was quite as wonderful as any other ; but as it was 

 not characteristically scientific (in the modern sense) it 

 may be left on one side here. 



So he solved his problem. He showed that the planets 

 can be regarded as subject to Galileo's laws, and that the 

 force on a planet must be directed towards the sun, and 

 that on the moon towards the earth ; and that these forces 

 must vary in a certain simple way with the distance 

 between planet and sun or moon and earth. The moon 

 follows the course she does because there is a pull between 

 it and the earth just as, when a stone is whirled round at 

 the end of a string, there is a pull between the stone and 

 the hand. 



And now as I like to think he had ended his labour. 

 He realized that he had made a stupendous discovery, 

 which must revolutionize, as it actually has done, the 

 whole science of astronomy. He had shown that the 

 laws of dynamics apply to planets as well as to ordinary 



