176 Mr. N. Campbell on the 



guesses, to devise assumptions of which none are actually 

 ever proved to be inconsistent with each other. Even the 

 greatest genius fails sometimes at the first guess, as in the 

 case of the Foucault pendulum, but then, as in that case, the 

 second guess is successful. 



Much of this is trite and has been said in other words 

 often before. But the conclusion is important that the pro- 

 position of theoretical dynamics (even if relation A between 

 the Distances and Times on the one hand and the distances 

 and times on the other is maintained strictly) cannot be 

 either proved or disproved by experiment. In order to apply 

 those propositions to experiment it is necessary to introduce 

 a certain number of assumptions, which are wholly inde- 

 pendent of those of theoretical dynamics : the connexion 

 between theoretical and practical dynamics will be broken 

 only when it is impossible to find assumptions, suitable for 

 the purpose, which are not inconsistent with those adopted on 

 other grounds. 



§ 11. Let us now consider the vexed questions of dynamics 

 — those concerning " absolute and relative motion." 



It is sometimes asked, "Is motion absolute or relative "? 

 From what has been said it is clear that the question does 

 not admit of a definite answer without further explanation. 

 The word " motion " might mean any one of three things : 

 (1) a concept defined by the Absolute Coordinates and the 

 Time ; (2) a concept defined by the Relative Distances and 

 the Time ; (3) a concept defined by the relative distances 

 and the time. There is no doubt that the word is otten 

 used in the senses (1) and (2), which we may term Absolute 

 Motion and Relative Motion respectively; but I do not think 

 that I have ever seen it used in the sense (3). The "motion" 

 round which almost all discussion takes place is defined in 

 terms of mathematical quantities, to which the operations of 

 mathematics can be applied significantly. But " motion " in 

 the sense (3) is not a mathematical quantity : it is denned in 

 terms of conceptions which are wholly foreign to that stud}'. 

 It is no more significant to talk of adding (in the mathema- 

 tical sense) two distances than of adding two colours. All 

 that we can do in this direction is to define Distances bearing 

 the relation A to the two distances, add these Distances, 

 and define a new distance to which the sum bears the 

 relation A. 



Now Absolute Motion and Relative Motion are both valid 

 concepts: they are related, but the relation is neither identity 

 nor contradiction ; they happen to be commonly called by 

 the same name because they have developed from the same 



