THE CONTINUITY OF MOTION. 191 



latent activity in the medium occupying space, and that 

 when the body falls, this is re-transformed into its equiva- 

 lent of perceptible activity. If we conceive the process at 

 all, we must conceive it thus: otherwise, we have to con- 

 ceive that sl power is changed into a space-relation, and this 

 is inconceivable. 



Here, then, is the solution of the difficulty. The space- 

 element of Motion is not in itself a thing. Change of posi- 

 tion is not an existence, but the manifestation of an exist- 

 ence. This existence may cease to display itself as transla- 

 tion; but it can do so only by displaying itself as strain. 

 And this principle of activity, now shown by translation, 

 now by strain, and often by the two together, is alone that 

 which in Motion we can call continuous. 



§ 58. What is this principle of activity? Vision gives 

 us no idea of it. If by a mirror we cast the image of an 

 illuminated object on to a dark wall, and then suddenly 

 changing the attitude of the mirror, make the reflected 

 image pass from side to side, the image, if recognized as 

 such, does not raise the thought that there is present in it 

 a principle of activity. Before we can conceive the presence 

 of this, we must regard the impression yielded through 

 our eyes as symbolizing something tangible — something 

 which offers resistance. Hence the principle of activity 

 as known by sight, is inferential: visible translation sug- 

 gests by association the presence of a principle of activity 

 which would be appreciable by our skin and muscles did 

 we lay hold of the body. Evidently, then, this principle 

 of activity which Motion shows us, is the objective corre- 

 late of our subjective sense of effort. By pushing and pull- 

 ing, we get feelings which, generalized and abstracted, yield 

 our ideas of resistance and tension. Now displayed by 

 changing position and now by unchanging strain, this prin- 

 ciple of activity is ultimately conceived by us under the 

 single form of its equivalent muscular effort. So that the 



