The purpose of my writings is not to repeat what others have said: so I will give only a few of my own observations concerning the senses.

The senses are merely the outlets through which the brain receives material from the outside (in the form of sensation), and it works this matter up into intuitive representation. Those sensations that are supposed to serve primarily for the objective apprehension of the external world cannot themselves be either pleasant or unpleasant: this really means that they must have absolutely no effect on the will. Otherwise, the sensation itself would capture our attention and we would stay with the effect instead of proceeding at once to the cause, as is the purpose here: this is due to the decisive preference that our attention always gives to the will over mere representation: we turn to representation only when the will is silent. Accordingly, colours and sounds are intrinsically neither painful nor pleasant sensations, as long as their impression is within normal bounds, and they arise with an indifference appropriate for the material of purely objective intuitions. This is as much the case as could possibly be expected in a body that is through and through will, and is remarkable in this respect. Physiologically it is due to the fact that in the organs of the nobler senses, which is to say seeing and hearing, the nerves that receive the specific external impression are not even capable of feeling pain, they are only familiar with the sensations specifically characteristic of them, the ones that serve pure perception. Thus the retina as well as the optical nerve is insensible to injury, as is the auditory nerve: in both organs pain is felt only elsewhere, in the areas surrounding the distinctive sense nerves and never in the sense nerves themselves: in the eye primarily in the conjunctiva; in the ear in the meatus auditorius. The same thing is true even with the brain, since if it is cut directly, which is to say from above, it will not have any sensation of this.