In terms of mental science, colour and sound, like temperature, texture, taste and smell, are _sensations_; while _shape_ is, in the most complete sense, a _perception._ This distinction between _sensation_ and _perception_ is a technicality of psychology; but upon it rests the whole question why shapes can be contemplated and afford the satisfaction connected with the word _beautiful,_ while colours and sounds, except as grouped or groupable into shapes, cannot. Moreover this distinction will prepare us for understanding the main fact of all psychological aesthetics: namely that the satisfaction or the dissatisfaction which we get from shapes is satisfaction or dissatisfaction in what are, directly or indirectly, activities of our own. Etymologically and literally, _perception_ means the act of _grasping_ or _taking_ in, and also the result of that action. But when we thus _perceive_ a shape, what is it precisely that we grasp or take in? At first it might seem to be the _sensations_ in which that form is embodied. But a moment's reflection will show that this cannot be the case, since the sensations are furnished us simply without our performing any act of perception, thrust on us from outside, and, unless our sensory apparatus and its correlated brain centre were out of order, received by us passively, nilly willy, the Man on the Hill being invaded by the sense of that blue, that lilac and that russet exactly as he might have been invaded by the smell of the hay in the fields below. No: what we grasp or take in thus actively are not the sensations themselves, but the _relations_ between these sensations, and it is of these relations, more truly than of the sensations themselves, that a shape is, in the most literal sense, _made up._ And it is this _making up of shapes,_ this grasping or taking in of their constituent relations, which is an active process on our part, and one which we can either perform or not perform. When, instead of merely _seeing_ a colour, we _look at_ a shape, our eye ceases to be merely passive to the action of the various light-waves, and becomes active, and active in a more or less complicated way; turning its differently sensitive portions to meet or avoid the stimulus, adjusting its focus like that of an opera glass, and like an opera glass, turning it to the right or left, higher or lower.