514 Mr. Norman Campbell on the 



oldest and least satisfactory metaphysical doctrines, are so 

 enamoured of the conception of " reality '■' that they refuse to 

 give it up. Finding that the observations of different 

 observers do not agree, tliey define a new function of those 

 observations, such that it is the same for all observers, and 

 proceed to call this the " real rate." This function, according 

 to the Principle of Relativity, is /3n\ where n' is the rate of 

 the clock as seen by an observer relative to whom it is 

 travelling with the velocity v : according to that Principle, if 

 we substitute in that function the appropriate values for 

 any one observer, the resulting number will always be the 

 same. 



So far no overwhelming objection can be raised. The 

 function is important in the theory, and, if care is taken to 

 note the precise meaning now attributed to the vvord u real/ 5 

 there is no harm in calling it by that name. But now certain 

 writers commit an extraordinary series of blunders. They 

 not only inquire whether the real rate changes with the 

 velocity, a question which, as the real rate is defined as that 

 function which does not change with the velocity, is utterly 

 trivial, but they actually give a negative answer. They see 

 that the expression for the real rate contains v explicitly and 

 rush to the absurd conclusion that the real rate changes with 

 the velocity. No wonder that they soon involve themselves 

 in a hopeless maze of paradox. 



As a matter of fact the " crude argument' 5 given above 

 shows that the second definition of "real 5 ' had been intro- 

 duced before the Principle of Relativity. It had been recog- 

 nised already that observers would not agree as to the rate of 

 a clock : the conception of the clock " as it really is, 55 intro- 

 duced in that argument, means (if it means anything) that 

 function of the observed rate of the clock and its velocity 

 relative to the observer which is the same for all observers. 

 But the logical order of the argument is reversed. Instead 

 of proving from the " real rate" of the clock, which we do 

 not know, the observed rate, which we do know, we should 

 say that the observed rate of the clock is n, and that our 



theory of light leads to the conclusion that n/ ( 1 ) will be 



the same for all observers. Whether that conclusion or the 

 conclusion reached by the Principle of Relativity is correct 

 can only be determined by experiment, and the experiment 

 has not yet been tried. 



It is the great merit of the Principle of Relativity that it 

 forces on our attention the true nature of the concepts of 



