152 GRAVITATION— THE EXPLANATION 



I shall have to emphasise elsewhere that the whole of 

 our physical knowledge is based on measures and that 

 the physical world consists, so to speak, of measure- 

 groups resting on a shadowy background that lies 

 outside the scope of physics. Therefore in conceiving 

 a world which had existence apart from the measure- 

 ments that we make of it, I was trespassing outside the 

 limits of what we call physical reality. I would not 

 dissent from the view that a vagary which by its very 

 nature could not be measurable has no claim to a physical 

 existence. No one knows what is meant by such a 

 vagary. I said that the earth might go anywhere it 

 chose, but did not provide a "where" for it to choose; 

 since our conception of "where" is based on space 

 measurements which were at that stage excluded. But 

 I do not think I have been illogical. I am urging that, 

 do what it will, the earth cannot get out of the track 

 laid down for it by the law of gravitation. In order to 

 show this I must suppose that the earth has made the 

 attempt and stolen nearer to the sun; then I show that 

 our measures conspire quietly to locate it back in its 

 proper orbit. I have to admit in the end that the earth 

 never was out of its proper orbit;* I do not mind that, 

 because meanwhile I have proved my point. The fact 

 that a predictable path through space and time is laid 

 down for the earth is not a genuine restriction on its 

 conduct, but is imposed by the formal scheme in which 

 we draw up our account of its conduct. 



* Because I can attach no meaning to an orbit other than an orbit in 

 space and time, i.e. as located by measures. But I could not assume that 

 the alternative orbit would be meaningless (inconsistent with possible 

 measures) until I tried it. 



