184 w. JAMES :

(a) Their Subdivision. Let us take subdivision first. How are spatial subdivisions brought to consciousness ? in other words, How does spatial discrimination occur ? I must reserve a general treatment of the subject of discrimination for another place. 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 the quality of their immanent sensi- bility, that is, that each shall carry its special local-sign. If the skin felt everywhere exactly alike, a foot-bath could be distinguished 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. 1 We do not judge them two unless we have discerned their sensations to be different. 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 of the minuteness with which we are able to subdivide the total bigness of the sensation it yields. On its periphery the local differences do not shade off very rapidly, and we can count their fewer subdivisions. But these local differences of feeling, so long as the surface is unexcited from without, are almost null. I cannot feel them by a pure mental act of attention unless they belong to quite distinct parts of the body, as the nose and the lip, the finger- tip and the ear ; their contrast needs the reinforcement of outward excitement to be felt. In the spatial muchness of a colic or, to call it by the more spacious-sounding verna- cular, of a ' bellyache ' I can with difficulty distinguish the north-east from the south-west corner, but can do so much more easily if, by pressing my finger against the 1 A. Binet (Revue Philosophise, 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 discrimi- nated when successive, may still fuse locally if excited both at once.