MATTER IN MOTION 109 



the way do^^^l, it is possible he will find a path 

 shorter than any other, and this is the path that 

 he will take." He imagines now an observer 

 watcliing the man's progress from an airplane. 

 To him the crater looks flat and he wonders why 

 the man goes so far out of his course, and finally 

 forms the hypothesis that someone in the house 

 is directing a hose at the intruder, forcing him 

 to make a detour. More detailed observation 

 leads to more complicated assumptions, and 

 finally it is seen that all the difficulties are re- 

 moved by assuming a crater. 



Of course it will be noted that in this illus- 

 tration it is assumed that there is a crater and 

 that there is not a hose, whereas in the prob- 

 lem of gravitation it seems at present optional 

 whether we shall choose to use a simple geome- 

 try and introduce the alien concept of a force, 

 which moreover becomes more complex as our 

 observations become more accurate, or whether 

 we shall eliminate this alien concept and employ 

 a complex geometry, which perhaps also will 

 become still more complex with an increase in 

 our experimental and observational knowledge. 

 In other words, it is far too early to decide, even 

 tentatively, that the one is illusion, the other 

 reality. At present the advantage seems to be 



