102 WHAT IS SCIENCE? 



with the distance and with the size of the moon. If 

 the moon were brought to the surface of the earth and 

 reduced to the size of the apple would the force on it be 

 such that it would fall with the speed of the apple ? The 

 answer is, Yes I 1 The motion of the planets is therefore 

 explained both by generalization and by familiarity. 

 That motion is merely one particular instance of a general 

 principle of which the very familiar fall of heavy objects 

 is another. 



What I want to impress on the reader is how purely 

 personal was Newton's idea. His theory of universal 

 gravitation, suggested to him by the trivial fall of an 

 apple, was a product of his individual mind, just as much 

 as the Fifth Symphony (said to have been suggested by 

 another trivial incident, the knocking at a door) was a 

 product of Beethoven's. The analogy seems to me exact. 

 Beethoven's music did not exist before Beethoven wrote 

 it, and Newton's theory did not exist before he thought 

 of it. Neither resulted from a mere discovery of some- 

 thing that was already there ; both were brought into 

 being by the imaginative creation of a great artist. How- 

 ever there is one apparent difference ; Beethoven having 



1 The reader who knows the story and who does not ? will see that 

 here I deviate widely from history. Newton did not know how far 

 the moon was from the earth ; current estimates were wrong ; and at 

 first he was therefore doubtful of his theory.' But when the distance 

 was measured more accurately, he found it agreed perfectly with his 

 theory. I hesitate to suggest that it was Newton's theory that had 

 changed the distance ! 



I feel, too, that some people will think that I must be very antiquated 

 in my knowledge if I glorify Newton so greatly when the daily Press 

 has been assuring us lately that his ideas have been completely over- 

 thrown by Einstein. This is not the place to discuss what Einstein 

 has proved. I admire his work as much as anyone, but he has not 

 invalidated in the smallest degree the great discovery of Newton 

 which is discussed in the text. It is still as certain as ever it was that 

 the fall of the apple and the " fall " of the moon are merely two 

 examples of the very same fundamental principle ; and it is as certain 

 as ever that the motions of the planets are subject to the same laws 

 as those of terrestrial bodies. What is now not quite certain is whether 

 Galileo's laws are strictly applicable to circumstances very different 

 to those of the experiments by which he proved them. 



