(E) Perception (cont.)
Extensity of Sensation
The first condition of spatial experience seem to lie in what has been noted above (p. 46) as the extensity of sensations varying only in intensity and quality, not even if motor presentations are added, will account for the space-elements in our perceptions. A series of touches a, b, c, d may be combined with a series of movements m1, m2, m3, m4 ; both series may be reversed ; and finally the touches may be presented simultaneously. IN this way we can attain the knowledge of the coexistence of object that have a certain quasi-distance between them, and such experience is an important element in our perception of space ; but it is not the whole of it. For, as has been already remarked by critics of the associationist psychology, we have an experience very similar to this in singing and hearing musical notes or the chromatic scale. The most elaborate attempt to get extensity out of succession and coexistence is that of Mr Herbert Spencer. He has done, perhaps, all that can be done, only to make it the more plain that the entire procedure is a husteron proteron [Gk.]. We do not first experience a succession of touches or of retinal excitations by of movements, and then, when these impressions are simultaneously presented, regard them as extensive, because they are associated with or symbolize the original of movements ; but, before and apart from movement altogether, we experience that massiveness or extensity of impressions in which movements enable us to find positions, and also to measure. [Footnote 53-1] But it will be objected, perhaps not without impatience, that this amounts to the monstrous absurdity of making the contents of consciousness extended. The edge of this objection will be best turned by rendering the conception of extensity more precise. Thus, suppose a postage stamp pasted on the back of the hand; we have in consequence a certain sensation. If another be added beside it, the new experience would not be adequately described by saying we have a greater quantity of sensation, for intensity involves quantity, and increased intensity is not what is meant. For a sensation of a certain intensity, say a sensation of red, cannot be changed into one having two qualities, red and blue, leaving the intensity unchanged ; but with extensity this change is possible. For one of the postage stamps a piece of wet cloth of the same size might be substituted and the massiveness of the compound sensation remain very much the same. Intensity belongs to what be called graded quantity : it admits of increment of decrement, but is not a sum of parts. Extensity, on the other hand, does imply plurality : we might call it latent or merged plurality or a ground " of plurality, inasmuch as to say that a single presentation has massiveness is to say that a portion of the presentation continuum at the moment undifferentiated is capable of differentiation.
53-1 We are ever in danger of exaggerating the competence of a new discovery ; and the associationists seem to have fallen into this mistake, not only in the use they have made of the conception of association in psychology in general, but in the stress they have laid upon the fact of movement when explaining our space-perceptions in particular. Indeed, bot ideas have here conspired, against themassociation in keeping up the notion that we have only to deal with a plurality of discrete impressions, and movement in keeping to the p. 266 sq.) surely ought to convince us that, unless we are prepared to say, as Mill seems to do, "the idea of space is at bottom one of time" (p. 276), we must admit the inadequacy of our experience of movement to explain the origin of it.
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