THE PEECEPTION OF SPACE. (l.) 17

Let us take the problem of Locality first, and begin with the simple case of a sensitive surface, only two points of which happen to be recipients of stimulation from without. How, first, are these two points felt as alongside of each other with an interval of space between them ? We must be conscious of two things for this : of the duality of the excited points, and of the extensiveness of the unexcited interval. The duality alone, although a necessary, is not a sufficient condition of the spatial separation. We may, for instance, discern two sounds in the same place, sweet and sour in the same lemonade, warm and cold, round and pointed contact in the same place on the skin, &C. 1 In all discrimination the recognition of the duality of two feelings by the mind is the easier the more strongly the feelings are contrasted in quality. If our two excited points awaken identical qualities of sensation, they must, perforce, appear to the mind as one; and, not distinguished at all, they are, a fortiori, not localised apart. Spots four centimetres distant on the back have no qualitative contrast at all, and fuse into a single sensation. Points less than three-thousandths of a millimetre apart awaken on the retina sensations so con- trasted that we apprehend them immediately as two. Now these unlikenesses which arise so slowly when we pass from one point to another in the back, so much faster on the tongue and finger-tips, but with such inconceivable rapidity on the retina, what are they? Can we discover anything about their intrinsic nature ? The most natural and immediate answer to make is that they are unlikeness of place pure and simple. In the words of a German physiologist, 2 to whom psychophysics owes much : " The sensations are from the outset (vonvornhereiri) localised. . . . Every sensation as such is from the very beginning affected with the spatial quality, so that this quality is nothing like an external attribute coming to the sensation from a higher faculty, but must be regarded as something immanently residing in the sensation itself." And yet the moment we reflect on this answer an insuper- able logical difficulty seems to present itself. No single quale of sensation can, by itself, amount to a consciousness of position. Suppose no feeling but that of a single one of the points ever to be awakened. Could that possibly be the 1 This often happens when the warm and cold points, or the round and pointed ones, are applied to the skin within the limits of a single " Emp- findungskreis ". 2 Vierordt, Grundriss der Physioloyie, 5te Auflage, 1877, pp. 326, 436. 2