256 POINTER READINGS 



It is obviously one of the conditions of the problem 

 that the same elephant should be concerned in the 

 weighing experiment and in the tobogganing experi- 

 ment. How can this identity be expressed in a descrip- 

 tion of the world by pointer readings only? Two 

 readings may be equal, but it is meaningless to inquire 

 if they are identical; if then the elephant is a bundle of 

 pointer readings, how can we ask whether it is continu- 

 ally the identical bundle ? The examiner does not confide 

 to us how the identity of the elephant was ensured; we 

 have only his personal guarantee that there was no 

 substitution. Perhaps the creature answered to its name 

 on both occasions; if so the test of identity is clearly 

 outside the present domain of physics. The only test 

 lying purely in the domain of physics is that of con- 

 tinuity; the elephant must be watched all the way from 

 the scales to the hillside. The elephant, we must remem- 

 ber, is a tube in the four-dimensional world demarcated 

 from the rest of space-time by a more or less abrupt 

 boundary. Using the retina of his eye as an indicator 

 and making frequent readings of the oudine of the 

 image, the observer satisfied himself that he was fol- 

 lowing one continuous and isolated world-tube from 

 beginning to end. If his vigilance was intermittent he 

 took a risk of substitution, and consequently a risk of 

 the observed time of descent failing to agree with the 

 time calculated.* Note that we do not infer that there 

 is any identity of the contents of the isolated world-tube 

 throughout its length; such identity would be meaning- 



* A good illustration of such substitution is afforded by astronomical 

 observations of a certain double star with two components of equal 

 brightness. After an intermission of observation the two components 

 were inadvertently interchanged, and the substitution was not detected 

 until the increasing discrepancy between the actual and predicted orbits 

 was inquired into. 



