THE PERCEPTION OF SPACE. 161 



fciou ou the otlier, were able to yield ; so here we shall 

 emerge from our more complicated quest with the convic- 

 tion that all the facts can be accounted for on the supposi- 

 tion that no other mental forces have been at work save 

 those we find everywhere else in psychology : sensibility, 

 namely, for the data ; and discrimination, association, 

 memory, and choice for the rearrangements and combina- 

 tions which they undergo. 



1. The Suhdivision of the Original Sense-spojces. 



How are spatial subdivisions brought to consciousness ? 

 in other words, How does spatial discrimination occur? 

 The general subject of discrimination has been treated in 

 a previous chapter. Here we need only inquire what are 

 the conditions that make spatial discrimination so much 

 finer in sight than in touch, and in touch than in hearing, 

 smell, or taste. 



The first great condition is, that different points of the 

 surface shall differ in tJie qiiality of their immanent sensibility, 

 that is, that each shall carry its special local-sign. If the 

 skin felt everywhere exactly alike, a foot-bath could be dis- 

 tinguished from a total immersion, as being smaller, but 

 never distinguished from a wet face. The local-signs are 

 indispensable ; two points which have the same local-sign 

 will always be felt as the same point. We do not judge 

 them two unless we have discerned their sensations to be 

 difi"erent.* Granted none but homogeneous irritants, that 

 organ would then distinguish the greatest multiplicity of 

 irritants — would count most stars or compass-points, or 

 best compare the size of two wet surfaces — whose local 

 sensibility was the least even. A skin whose sensibility 

 shaded rapidly off from a focus, like the apex of a, boil, 

 would be better than a homogeneous integument for spatial 

 perception. The retina, with its exquisitely sensitive fovea, 

 has this peculiarity, and undoubtedly owes to it a great part 



*M. Binet (Revue Philosophique, Sept. 1880, page 291) says we judge 

 them locally different as soon as their sensations differ enough for us to 

 distinguish them as qualitatively different when successively excited. This 

 Is not strictly true. Skin-sensations, different enough to be discriminated 

 »¥hen successive, may still fuse locally if excited both at once. 



