The senses are merely the channels through which the brain receives from without (in the form of sensations) the materials which it works up into ideas of perception. Those sensations which principally serve for the objective comprehension of the external world must in themselves be neither agreeable nor disagreeable. This really means that they must leave the will entirely unaffected. Otherwise the sensation _itself_ would attract our attention, and we would remain at the _effect_ instead of passing to the _cause_, which is what is aimed at here. For it would bring with it that marked superiority, as regards our consideration, which the will always has over the mere idea, to which we only turn when the will is silent. Therefore colours and sounds are in themselves, and so long as their impression does not pass the normal degree, neither painful nor pleasurable sensations, but appear with the indifference that fits them to be the material of pure objective perception. This is as far the case as was possible in a body which is in itself through and through will; and just in this respect it is worthy of admiration. Physiologically it rests upon the fact that in the organs of the nobler senses, thus in sight and hearing, the nerves which have to receive the specific outward impression are quite insusceptible to any sensation of pain, 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.