154 THE GRAMMAR OF SCIENCE 



mental expression for the fact that the perceptive faculty 

 has separated coexisting sense-impressions into groups of 

 associated impressions. This separation of immediate 

 sense-impressions into groups, this discriminating power 

 of the perceptive faculty, is, at any rate in the early stages 

 of man's development, most clearly recognised and closely 

 associated with the senses of sight and touch. Hence it 

 comes about that the invisible and intangible "inside 

 skin " is at first not considered as in space. Later, for 

 example, as we localise pain, or associate it with other 

 sense-impressions classified as visible and tangible, we 

 treat " inside skin " as belonging to space. Yet we still 

 frequently consider the presence of visible and tangible 

 members a condition for a spacial group of sense-impres- 

 sions. Space, says Thomas Reid, is known directly by 

 the senses of sight and touch. But probably a like, if 

 less powerful, means of discriminating groups of sense- 

 impressions lies in the senses of sound and smell.^ We 

 localise sounds and smells without necessarily associating 

 them with visible and tangible resounding and smelling 

 bodies. It will, I think, be admitted on reflection that 

 whenever we concentrate our attention on a limited group 

 of associated sense-impressions, then we consider them as 

 spacial, or " existing in space." We join together, owing 

 to past experience, certain sense-impressions as a per- 

 manetit group, and we then mentally separate this group 

 from other groups. The actual boundary of the group, 

 however, when we attempt to define it, is found in reality 

 to be vague (p. 72). The group, although in the main a 

 permanent association, has a continual flow in and out of 

 junior partners ; while some of the partners belong, on 

 closer examination, as much to one association as another. 

 The separation is thus rather practical than real ; it 

 arises, in the first place, from the fact that in our per- 

 ception certain sense - impressions are more or less 



1 My baby when three days old was able to distinguish between the 

 snapping of the fingers of the right and left hands, and to follow with the 

 ear the direction of the sound. She would turn to a voice long before she 

 paid any attention to bodies moving quite close to her eyes. Difference of 

 position was thus associated with sound. 



