THE RELATIVITY OF KNOWLEDGE 



to conceive one particle acting upon another 

 through a space that is utterly empty ; and we 

 can in no wise conceive any such action. How 

 shall we escape this difficulty ? Shall we assume 

 that the intervals between the particles are filled 

 by a fluid of excessive tenuity, like the so-called 

 imponderable ether to which physicists are in 

 the habit of appealing ? We shall soon find that 

 the problem is only shifted. As soon as we in- 

 quire into the constitution of this hypothetical 

 intermolecular fluid, we are no better ofi^ than 

 before. For we have no alternative but to re- 

 gard this fluid as itself an extremely rarefied 

 form of matter : since it does not perceptibly 

 affect the weights of bodies, we must regard it 

 as possessed of a density that is almost infini- 

 tesimal, — th^t is, its constituent particles must 

 be separated from each other by regions of 

 empty space that are even greater in proportion 

 to the size of the particles than are the spaces 

 that intervene between the molecules of that 

 relatively dense form of matter which we call 

 ponderable. With regard to the ether, as before 

 with regard to the matter, we have to ask. How 

 can its particles act upon each other through 

 space that is utterly empty ? How can a thing 

 act where it is not? How can motion be trans- 

 mitted, in the absence of any medium of trans- 

 mission ? and to this question no answer ever 

 has been, or ever can be devised. 



7 



