5. That external sensation is according to the nature of its communication with the internal sensory. It is not the organ of external sensation that sensates, but only the soul, since the soul understands the nature of the sensation. Consequently, the organ of the external sense is nothing more than an instrument receiving the first impulses and contacts, that is, the forces that come to it. Therefore, when the eye is closed and the ear at rest, as during sleep, we still seem to see and hear; and when, in the brain, the faculty of perception is lost, the external organs are straightway deprived of their sensation-though not the reverse. From this cause it is that our sensations become dull or acute, or obscure or distinct. That the sense itself varies according to the changed state of the brain, is apparent from diseases of the head. For the fiber is either relaxed, as in sleep, or is tensed and elevated and rendered distinct for the reception of the sensation, as in wakefulness; or it is inflamed and heated, or affected in other ways; and according to the state thus induced on the fibers, or into which the fibers are reduced, so the sense itself is at once varied.
