242 THE GRAMMAR OF SCIENCE 



than motion. It is in the field of conception solely that 

 we can properly talk of the motion of bodies ; it is there, 

 and there only, that geometrical forms change their 

 position in absolute time — that is, move. In the field of 

 perception motion is but a popular expression to describe 

 the mixed mode in which we discriminate and distinguish 

 groups of sense-impressions. 



8 2. — The Three Problems 



That we speak of the motion of bodies as a fact of 

 perceptual experience is largely due to the constructive 

 elements associated with immediate sense -impression ^ 

 (p. 41). These constructive elements are drawn from 

 our conceptual notions of change, which again flow very 

 naturally from a limited perception ; a deeper perceptual 

 experience is required to demonstrate their purely ideal 

 character (p. 170). But the reader will, perhaps, hardly 

 be prepared to accept the conclusion that change is per- 

 ceptual, motion conceptual, without closer analysis. This 

 analysis may be summed up in the three questions : What 

 is it that moves ? Why does it move ? How does it 

 move ? 



In the first place we must settle whether we are asking 

 these questions of the conceptual or of the perceptual 

 sphere. If it be of the former, the world of symbolic 

 motions by aid of which science describes the sequences 

 of our sense-impressions, then these questions are easy to 

 answer. The things which move are points, rigid bodies 

 and strainable media, geometrical concepts one and all. 

 To ask why they move is to ask why we form concep- 

 tions at all, and ultimately to question why science exists. 

 Finally, the manner in which they move is that which 

 enables us most effectually to describe the results of our 

 perceptual experience. 



1 The writer is not objecting to the current use of such expressions as 

 "the sun moves" or "the train moves." Both do move — in conception; 

 in perception there is a change of sense-impressions. So soon as space is 

 recognised as a mode of perception, and not itself a phenomenon, this con- 

 clusion cannot be avoided. 



