ORIGIN OF PHYSICAL CONCEPTS 21 



sadly wanting in clearness of statement. He 

 never tells us when and where exactly we do 

 have a sensation of Space. In truth he never 

 gets behind the postulate of an all-enveloping 

 tridimensional world ; so that he throughout 



^ assumes Space as a datum, and his inquiry is an 

 v effort to rediscover Space where he has already 

 placed it. 



Let us, however, consider for a moment what 



, can be meant by a sensation of Space. Does it 

 not look very like a contradiction in terms ? Pure 

 Space, if it means anything, means absolute material 



^ emptiness and vacuity. How, then, by any possi- 

 bility can it give rise to a sensation ? What 

 sensory organ can it be conceived as affecting ? 

 How and in what way can it be felt ? 



The truth is the idea of Space is essentially 

 negative. It represents absence of physical ob- 

 struction of every kind. No doubt, we may describe 



, it positively as a possibility of free movement, 

 and such a description is at once true and im- 

 portant. Yet even it involves a negative. The 



term " free " is in reality, though not in form, 

 f 



a negative term and means " unconstrained." 

 And the reason why such a term is necessarily 

 negative is to be found in the fact that a state 

 of dynamic constraint is the essential condition 



