this point of view we may say that more partial presentations are concerned in the sensation corresponding to two stamps than in that corresponding to one. The fact that these partial presentations, though identical in quality and intensity, on the one hand are not wholly identical, and on the other are presented only as a quantity and not as a plurality, is explained by the distinctness along with the continuity of their local signs. Assuming that to every distinguishable part of the body there corresponds a local sign, we may allow that at any moment only a certain portion of this continuum is definitely within the field of consciousness; but no one will maintain that a part of one hand is ever felt as continuous with part of the other or with part of the face. Local signs have thus an invariable relation to each other: two continuous signs are not one day coincident and the next widely separate.[1] This last fact is only implied in the mere massiveness of a sensation in so far as this admits of differentiation into local signs. We have, then, when the differentiation is accomplished, a plurality of presentations constituting an extensive continuum, presented simultaneously, and having certain fixed and invariable relations to each other. Of such experience the typical case is that of passive touch, though the other senses exemplify it. It must be allowed that our concept of space in like manner involves a fixed continuity of positions; but then it involves, further, the possibility of movement. Now in the continuum of local signs there is nothing whatever of this; we might call this continuum an implicit plenum. It only becomes the presentation of occupied space after its several local signs are complicated in an orderly way with active touches, when in fact we have experienced the contrast of movements with contact and movements without, i.e. in vacuo. It is quite true that we cannot now think of this plenum except as a space, because we cannot divest ourselves of these motor experiences by which we have explored it. We can, however, form some idea of the difference between the perception of space and this one element in the perception by contrasting massive internal sensations with massive superficial ones, or the general sensation of the body as “an animated organism” with our perception of it as extended. Or we may express the difference by remarking that extension implies the distinction of here and there, while extensity rather suggests ubiquity.
It must seem strange, if this conception of extensity is essential to a psychological theory of space, that it has escaped notice so long. The reason may be that in investigations into the origin of our knowledge of space it was always the concept of space and not our concrete space percepts that came up for examination. Now in space as we conceive it one position is distinguishable from another solely by its co-ordinates, i.e. by the magnitude and signs of certain lines and angles, as referred to a certain datum, position or origin; and these elements our motor experiences seem fully to explain. But on reflection we ought, surely, to be puzzled by the question, how these coexistent positions could be known before those movements were made which constitute them different positions. The link we thus suspect to be missing is supplied by the more concrete experiences we obtain from our own body, in which two positions have a qualitative difference or “local colour” independently of movement. True, such positions would not be known as spatial without movement; but neither would the movement be known as spatial had those positions no other difference than such as arises from movement. In a balloon drifting steadily in a fog we should have no more experience of change of position than if it hung becalmed and still.
We may now consider the part which movement plays in elaborating the presentations of this dimensionless continuum Positional signs. into percepts of space. In so doing we must bear in mind that while this continuum implies the incopresentability of two impressions having the same local sign, it allows not only of the presentation of sensations of varying massiveness, but also of a sensation involving the whole continuum simultaneously, as in Bain's classic example of the warm bath. As regards the motor element itself, the first point of importance is the incopresentability and invariability of a successive series of auxiliomotor or kinaesthetic presentations, P₁, P₂, P₃, P₄. P₁ cannot be presented along with P₂, and from P₄ it is impossible to reach P₁ again save through P₃ and P₂. Such a series, taken alone, could afford us, it is evident, nothing but the knowledge of an invariable sequence of impressions which it was in our own power to produce. Calling the series of P's “positional signs,” the contrast between them and local signs is obvious. Both are invariable, but succession characterizes the one, simultaneity the other; the one yields potential position without place, the other potential place (τόπος) without position; hence we call them both merely signs.[2] But in the course of the movements necessary to the exploration of the body—probably our earliest lesson in spatial perception—these positional signs receive a new significance from the active and passive touches that accompany them, just as they impart to these last a significance they could never have alone.
It is only in the resulting complex that we have the presentations of actual position and of spatial magnitude. For space, though conceived as a coexistent continuum, excludes the notion of omnipresence or ubiquity; two positions ld and lg must coexist, but they are not strictly distinct positions so long as we conceive ourselves present in the same sense in both. But, if Fd and Fg are, e.g. two impressions produced by compass points touching two different spots as ld and lg on the hand or arm, and we place a finger upon ld and move it to lg, experiencing thereby the series P₁, P₂, P₃, P₄, this series constitutes ld and lg into positions and also invests Fd and Fg with a relation not of mere distinctness as τόποι but of definite distance. The resulting complex perhaps admits of symbolization as follows:—
. . . . . FaFbFc Fd FeFf Fg FhFk . . . . .
T t t t
P₁p₂p₃p₄
Here the first line represents a portion of the tactual continuum, Fd and Fg being distinct “feels,” if we may so say, or passive touches presented along with the fainter sensations of the continuum as a whole, which the general “body-sense” involves; T stands for the active touch of the exploring finger and P₁ for the corresponding kinaesthetic sensation regarded as “positional sign”; the rest of the succession, as not actually present at this stage but capable of revival from past explorations, is symbolized by the t t t and p₂p₃p₄.
When the series of movements is accompanied by active touches without passive there arises the distinction between one's own body and foreign bodies; when the initial movement of a series is accompanied by both active and passive touches, the final movement by active touches only, and the intermediate movements are unaccompanied by either, we get the further presentation of empty space lying between us and them—but only when by frequent experience of contacts along with those intermediate movements we have come to know all movement as not only succession but change of position. Thus active touches come at length to be projected, passive touches alone being localized in the stricter sense. But in actual fact, of course, the localization of one impression is not perfected before that of another is begun, and we must take care lest our necessarily meagre exposition give rise to the mistaken notion
- ↑ The improvements in the sensibility of our “spatial sense” consequent on practice, its variations under the action of drugs, &c., are obviously no real contradiction to this; on the contrary, such facts are all in favour of making extensity a distinct factor in our space experience and one more fundamental than that of movement.
- ↑ Thus a place may be known topographically without its position being known geographically, and vice versa.
the nerve-endings but in the variety of the underlying parts—in one place bone, in another fatty tissue, in others tendons or muscles variously arranged—we find ample ground for diversity in “the local colouring” of sensations. And comparative zoology helps us to see how such diversity has been developed as external impressions and the answering movements have gradually differentiated an organism originally almost homogeneous and symmetrical. Between one point and another on the surface of a sphere there is no ground of difference; but this is no longer true if the sphere revolves round a fixed axis, still less if it also runs in one direction along its axis.
