So many of the critical applications of aesthetic, as well as of the historical problems of art-evolution are connected with this fact or rather the continued misunderstanding of it, that it is well to remind the Reader of what general Psychology can teach us of the perception of the Third Dimension. A very slight knowledge of cubic existence, in the sense of _relief,_ is undoubtedly furnished as the stereoscope furnishes it, by the inevitable slight divergence between the two eyes; an even more infinitesimal dose of such knowledge is claimed for the surfaces of each eye separately. But whatever notions of three-dimensional space might have been developed from such rudiments, the perception of cubic existence which we actually possess and employ, is undeniably based upon the incomparably more important data afforded by locomotion, under which term I include even the tiny pressure of a finger against a surface, and the exploration of a hollow tooth by the tip of the tongue. The muscular adjustments made in such locomotion become associated by repetition with the two-dimensional arrangements of colour and light revealed by the eye, the two-dimensional being thus turned into the three-dimensional in our everyday experience. But the mistakes we occasionally make, for instance taking a road seen from above for a church-tower projecting out of the plain, or the perspective of a mountain range for its cubic shape, occasionally reveal that we do not really _see_ three-dimensional objects, but merely _infer_ them by connecting visual data with the result of locomotor experience. The truth of this commonplace of psychology can be tested by the experiment of making now one, now the other, colour of a floor pattern seem convex or concave according as we think of it as a light flower on a dark ground, or as a white cavity banked in by a dark ridge. And when the philistine (who may be you or me!) exclaims against the "out of drawing" and false perspective of unfamiliar styles of painting, he is, nine times out of ten, merely expressing his inability to identify two-dimensional shapes as "representing" three-dimensional things; so far proving that we do not decipher the cubic relations of a picture until we have guessed what that picture is supposed to stand for. And this is my reason for saying that visible shapes, though they may be aspects of cubic objects, have no body; and that the thought of their volume, their weight and their texture, is due to an interruption of our contemplation of shape by an excursion among the recollections of qualities which shapes, _as_ shapes, cannot possess.