172 THE GRAMMAR OF SCIENCE 



of our sense-impressions. They are the scientific short- 

 hand by which we describe, classify, and formulate the 

 characteristics of that mode of perception which we term 

 perceptual space. Their validity, like that of all other 

 conceptions, lies in the power they give us of codifying 

 past and predicting future experience. 



We speak of a spherical or cubical body, and say that 

 it is of such and such a capacity. But no perceptual 

 body is ever truly spherical or cubical, and the size we 

 attribute to it is at best an approximate one. Further 

 analysis of our sense-impressions leads us in each case 

 to find variations from the geometrical definition and 

 measurement. Yet the conceptions of sphere and cube 

 are frequently sufficient to enable us to classify and 

 identify various bodies and predict the different types of 

 sense - impression to which these bodies correspond.'^ 

 Perhaps no better instance than geometry can be taken to 

 show how science describes the world of phenomena by aid 

 of conceptions corresponding to no reality in phenomena 

 themselves. That our geometrical conceptions enable us 

 on the whole to so effectually describe perceptual space is 

 only a striking instance of the practically equal develop- 

 ment of our perceptive and reasoning faculties (p. 103). 



^ 8. — Surfaces as Boundaries 



Although perceptual boundaries do not, on ultimate 

 analysis, in any way correspond to any special geo- 

 metrical definition such as that of plane or sphere, we 

 have still to inquire whether they answer to our concep- 

 tion of surface at all. By surface in this sense we are to 

 consider, not something of which it would be possible to 

 analyse the properties by any of the known processes of 

 geometry, but any contijiuous boundary between two 

 groups of sense-impressions or bodies." Is there a con- 



1 Our whole system of measuring size will be found to be based on 

 geometrical conceptions having no actuality in perception. 



2 " That which has position, length and breadth but fwt thickness, is called 

 stirface. 



" The word surface in ordinary language conveys the idea of extension in 

 two directions ; for instance, we speak of the surface of the earth, the surface 



