20 ORIGIN OF PHYSICAL CONCEPTS 



Now we might perhaps begin by taking the idea 

 of Time as a concept constantly employed in 

 Discourse, but of which it would be absurd to 

 suggest that it is supplied to us by Sensation. 

 It might, however, be urged in reply that the 

 idea of Time is not derived from the external 

 world at all, but is furnished to us directly by 

 the operations of the Mind, and that therefore 

 its intellectual origin need not involve any excep- 

 tion to the general rule that the materials of our 

 Knowledge of the world are furnished by Sensation 

 alone. Without, therefore, entering upon any dis- 

 cussion of the interesting question as to what is 

 the real nature of Time, we shall pass to the idea 

 of Space. 



Mach, the writer whom we have already quoted, in 

 his essay on Space and Geometry speaks constantly 

 and freely of sensations of Space, and as there 

 can be no denial of the fact that Space is a con- 

 stitutent of the external world, it would seem 

 to follow that those who hold Sensation to be the 

 only source of our Knowledge must be obliged 

 to affirm the possibility of sensations of Space. 

 Mach indeed claims to distinguish physiological 

 Space, geometrical Space, visual Space, tactual 

 Space as all different and yet apparently harmoni- 

 ously blended in our Experience. He is, however, 



