(E) Perception (cont.)
Attributing this property of extensity to the presentation-continuum as a whole, we may call the relation of any particular sensation to this larger whole its local sign, and can see that, so long as the extensity of a presentation admits of diminution without the presentation becoming nil, such presentation has two or more local signs,its parts, taken separately, though identical in quality and intensity, having a different relation to the whole. Such difference of relation must be regarded fundamentally as a ground or possibility of distinctness of signwhether as being the ground or possibility of different complexes or otherwiserather than as being form the beginning such an overt difference as the term "local sigh," when used by Lotze, is meant to imply. [Footnote 54-1] From this point of view we may say that more partial presentations are concerned in the sensations caused by two stamps than in that caused by one. The fact 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 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. This we can only represent by saying that the local signs have an invariable relation to each other : two continuous signs are not one day coincident and the next widely separate. [Footnote 54-2] This last fact is hardly perhaps implied in the mere massiveness of a sensation, but it will be convenient to include it when speaking of the continuum of local signs as extensive. We have, then, a plurality of presentations constituting a continuum, presented simultaneously as impressions 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 e allowed that our conception 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 or "associated" 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 ideas 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.
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 conception of space and not our concrete space perceptions that came up for examination. Now is 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 reflexion we ougth, 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 than such as arises from movement.
54-1 To illustrate what is meant by different complexes it will be enough to refer to the psychological implications of the fact that scarcely two portions of the sensitive surface of the human body are anatomically alike. Not only in the distribution and character of the nerve-endings but in the variety of the underlying partsin one place bone, in another fatty tissue, in others tendons or muscles variously arrangedwe 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.
54-2 The improvements in the sensibility of our "spatial sense" consequent on its variations under practice, 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.
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