and know no other sensation than that which is specifically peculiar to them, and which serves the purpose of mere apprehension. Thus the retina, as also the optic nerve, is insensible to every injury; and this is also the case with the nerve of hearing. In both organs pain is only felt in their other parts, the surroundings of the nerve of sense which is peculiar to them, never in this nerve itself. In the case of the eye such pain is felt principally in the conjunctiva; in the case of the ear, in the meatus auditorius. Even with the brain this is the case, for if it is cut into directly, thus from above, it has no feeling. Thus only on account of this indifference with regard to the will which is peculiar to them are the sensations of the eye capable of supplying the understanding with such multifarious and finely distinguished data, out of which it constructs in our head the marvellous objective world, by the application of the law of causality upon the foundation of the pure perceptions of space and time. Just that freedom from affecting the will which is characteristic of sensations of colour enables them, when their energy is heightened by transparency, as in the glow of an evening sky, in painted glass, and the like, to raise us very easily into the state of pure objective will-less perception, which, as I have shown in my third book, is one of the chief constituent elements of the æsthetic impression. Just this indifference with regard to the will fits sounds to supply the material for denoting the infinite multiplicity of the conceptions of the reason.
Outer sense, that is, receptivity for external impressions as pure data for the understanding, is divided into five senses, and these accommodate themselves to the four elements, i.e., the four states of aggregation, together with that of imponderability. Thus the sense for what is firm (earth) is touch; for what is fluid (water), taste; for what is in the form of vapour, i.e., volatile (vapour, exhalation), smell; for what is permanently elastic (air), hearing; for what is imponderable (fire, light), sight. The second im-
