Common Sense of Relativity. 515 



" real time " and " real space " which have caused such end- 

 less confusion. If we mean by them quantities which are 

 directly observed to be the same by all observers, there simply 

 is no real space and. real time. If we mean by them, as 

 apparently we do mean nowadays, functions of the directly 

 observed quantities which are the same for all observers, then 

 they are derivative conceptions which depend for their 

 meaning on the acceptance of some theory as to how the 

 directly observed quantities will vary with the motion, position, 

 etc. of the observers. " Real " quantities can never be the 

 starting point of a scientific argument ; by their very nature 

 they are not quantities which can be determined by a single 

 observation : the term " real " has always kept its original 

 meaning of some property of a body which is not observed 

 simply. 



All the difficulties and apparent paradoxes of the Principle 

 of Relativity will vanish it the attention is kept rigidly fixed 

 upon the quantities which are actually observed. If anyone 

 thinks he discovers that that Principle predicts some experi- 

 mental result which is incomprehensible, let him dismiss 

 utterly from his mind the conception of reality. Let him 

 imagine himself in the laboratory actually performing the 

 experiment : let him consider the numbers which he will 

 record in his note-book and the subsequent calculation which 

 he will make. He may then find that the result is somewhat 

 unexpected — to meet with unexpected results is the usual end 

 of performing experiments, — but he will not find any contra- 

 diction or any conclusion which is not quite as simple as that 

 which he expected. 



8. There is one further point sometimes raised in con- 

 nexion with the Principle on which a few words may be said. 



It is sometimes objected that the Principle " has no physical 

 meaning," that it destroys utterly the old theory of light 

 ba^ed on an elastic sother and puts nothing in its place, that, 

 in fact, it sacrifices the needs of the physical to the needs of 

 the mathematical instinct. That the statement is true there 

 can be no doubt, but the absence of any substitute for the 

 elastic sether theory of light may simply be due to the fact 

 that the Principle has been developed so far chiefly by people 

 who are primarily mathematicians. It is well to ask, can any 

 physical theory of light be produced which is consistent with 

 the Principle ? 



The answer depends on what is meant by a " physical 

 theory/'' Hitherto the term has always meant a " mechanical 

 theory," a theory of which the fundamental propositions are 



