252 THE GRAMMAR OF SCIENCE 



of sense-impression ; it is as idle as any other thing-in- 

 itself, as any other projection into the superscnsuous, be 

 it the force of the materialists or the infinite mind of the 

 philosophers. The classification of certain groups of 

 sense-impressions as material groups is, on the other hand, 

 scientifically of value ; it throws no light, however, on 

 matter as that which perceptually moves. 



Conceptually all motion is the motion of geometrical 

 ideals, which are so chosen as best to describe those 

 changes of sense-impression which in ordinary language 

 we term perceptual motion. 



§ 5- — ^/^^ " Coinvion-sense " View of Matter as 

 Impetietrable and Hard 



Now the reader may feel inclined, on the basis of his 

 daily experience, to assert that both the physicists above 

 referred to and the author are really quibbling about 

 words, and that we can sufficiently describe matter by 

 saying that it is impenetrable and hard. Now these terms 

 describe important classes of sense-impressions, and the 

 sense-impressions of impenetrability and hardness are very 

 frequently factors of what we have called material groups 

 of sense-impressions. But it is very doubtful whether we 

 can consider them as invariably associated with these 

 material groups. At any rate if we do we shall find our- 

 selves again involved in the antinomies which result when 

 we pass incautiously to and fro from the field of percep- 

 tion to that of conception. When we say a thing is im- 

 penetrable, we can only mean that something else will 

 not pass through it, or that there are two groups of sense- 

 impressions which, in our perceptual experience, we have 

 always been able to distinguish under the mode space. 

 Impenetrability, therefore, can only be a relative term ; 

 one thing is impenetrable for a second. When we say 

 that matter is impenetrable we cannot mean that nothing 

 whatever can pass through it. A bird cannot fly through 

 a sheet of plate glass, but a ray of light does penetrate it 

 perfectly easily. A ray of light cannot pass through a 



