§ 3. Extensity. — So far we have only attempted to show that extensity is a constituent of the fully developed percept of extension, and can be distinguished in it by analysis. The next question is whether extensity is not merely distinguishable, but actually separable from extension. Is there such a thing as an extensive quantum in which local sign differences are not distinguished by positions separated and connected by distances? Can the quantitative aspect of space exist without a spatial order? The answer is that though it is perhaps not possible to give examples of absolutely pure extensity, we can nevertheless exhibit many approximations to it. We can point to sensible experiences which have extensive diffusion with a relatively vague spatial arrangement.

The first set of examples is supplied by experiments on tactile sensibility. If the skin receives two punctiform impressions, such as those produced by the points of two needles, or of the legs of a pair of compasses, it is found, as Weber first showed, that the points must be at a certain distance apart, if the two impressions are to be distinguished.* At less than this distance only one everyday observation to have high tactual sensibility. It is much finer in the mobile parts, hands, feet, and lips, than in the comparatively fixed parts (the trunk). It is about twice as fine on the anterior as on the posterior surface of the fingers." (Sully, The Human Mind, vol. i., p. 106.)

* The distance varies with the part of the skin affected. "In general, it is finest in those regions, as the fingers and lips, which are known by continuous tactile impression is felt. It might be supposed that this is due to an absence of any local sign differences in the sensation : but the facts prove that this is not the case. Even when the two points are not discriminated, the sensation is often recognised as having a certain indefinite diffusion; and this may even happen when only one point is used. It is also found that the power of discrimination varies greatly according as the impressions are applied successively or simultaneously. When one needle point is removed before another is applied, the two contacts can be distinguished at a very much smaller distance than when they occur simultaneously. This shows that they may be simultaneously applied without being distinguished, although the sensation they produce contains a complexity of local sign differences. The local sign differences unite in one continuous extensive quantum, without internal distinction of position, direction, and distance. When the needle points are successively applied, they may be distinguished without their relative position being apprehended. For apprehension of their relative position or direction they must be a certain distance apart. The experiment may be further varied by applying one needle and afterwards applying the other without removing the first. In this case, they must be somewhat further apart to be distinguished. But what most interests us is that the application of the second point is sometimes only recognised as producing a blunter or more diffuse contact. Finally, the skin may be stimulated by continuous lines such as the edge of a strip of cardboard, instead of by separate points. It is found that this linear impression can be distinguished from a punctiform impression when its endpoints are separated by a much smaller distance than what is required for discriminating two separate points simultaneously applied. The linear impression does not give rise to the impression of a line unless its endpoints are a certain distance apart; at shorter distances the experience is merely one of indefinite diffusion. Longitudinal direction gradually comes to be perceived as the linear impression is lengthened: but in this process the line is apprehended as such before its direction is apprehended.*

It thus appears that in these experiments on tactile sensibility we have all kinds of gradations between pure extensity and fully definite extension. If we turn from touch to sight, there is more difficulty in exhibiting intermediate stages of this kind, because the optical perception of space is far more completely developed in the adult consciousness than the tactile. But there is a marked distinction in this respect between the central parts of the field of vision and its margin. There is an outer zone of dim imagery which has extensive diffusion, but a comparatively vague spatial order. This is best seen in the gray field which is presented when the eyes are closed. The edges of this field have no definite outline. It is impossible to assign its shape. But perhaps the best example is supplied when one eye is kept open in a fairly strong light, and the other closed.

* "Ueber die Wahrnehmung zweier Punkte mittelst des Tastsinnes, etc." Von G. A. Tawney. Philosophische Studien, Band xiii., 2. Heft.

A diffused blackness is presented to the closed eye, but its partial determinations are of the vaguest kind.

Typical cases of extensive diffuseness or massiveness are afforded by organic sensations. Professor James speaks of the "vast discomfort of a colic or a lumbago."* Let us consider such a sensation as that of hunger or of stomachache. This is spatial in so far as it is localised in a certain portion of the body, but it is almost entirely without internal spatial arrangement. We do not distinguish in it points of hunger having definite relations of position to each other, — points of hunger separated and connected by hungerdistances. At any rate, if we do so at all it is in the very vaguest way. But there is no doubt that the hunger has extensive quantity. It is, as James says, "voluminous or massive."

It is also sometimes urged that the other special senses, such as sound or smell, have an extensive character. "The reverberations of a thunderstorm," says James, "are more voluminous than the squeaking of a slatepencil."+ But this voluminousness is not a purely auditory experience. It is urged with some show of reason that the extensity belongs not to the sound as such, but to accompanying tactile sensation due to vi brations of the tympanum, and of the external ear, and to similar conditions. However this may be, the fact remains that the voluminousness is present, and that it has no distinctively spatial character, no internal order of positions and distances.

* Principles of Psychology, vol. ii., p. 134.              + Ibid.