126 THE WOKLD-ENEKGY 



no distinguishing points whatever in space, considered 

 merely as space, it is wholly "unscientific to distinguish 

 between rest and motion as between two different 

 states of a body in itself." And still further; we know 

 absolutely that if the actual distance between two bodies 

 increases or diminishes, one or other, or both the bodies, 

 must move. By the law of consistency, thought must 

 accept this as true and must utterly repudiate any 

 asserting by which it is contradicted. 



Motion, therefore, is primarily a change in the space- 

 relations of two or more bodies. And this, too, we may 

 fairly claim to know absolutely. 



But now another phase of the subject presents itself. 

 We have just seen that all our knowledge of motion is 

 a knowledge of change in space-relations between 

 actual bodies. But change of any kind can only take 

 place in time. Whence it appears that our knowledge 

 of motion is a complex knowledge, involving the rela- 

 tions both of time and of space. At the same time, 

 however, it is to be noted that though our knowledge 

 of motion is, in its nature, a knowledge of relations, it 

 by no means necessarily follows from this that all we 

 can know of motion is to be counted as merely relative 

 knowledge. 



It seems well worth while to notice, by the way, too, 

 that the ambiguity just noticed is precisely that which 

 underlies the whole theory of the relativity of knowl- 

 edge the advocates of which seem to find not the 

 slightest difficulty in knowing with absolute certainty 

 that absolutely nothing can ever be certainly known. 

 Nor are they likely to become aware of such difficulty 



