In considering the third kind of sense, hearing, we must speak of the causes
in which it originates. We may in general assume sound to be a blow which
passes through the ears, and is transmitted by means of the air, the brain,
and the blood, to the soul, and that hearing is the vibration of this blow,
which begins in the head and ends in the region of the liver. The sound which
moves swiftly is acute, and the sound which moves slowly is grave, and that
which is regular is equable and smooth, and the reverse is harsh. A great body
of sound is loud, and a small body of sound the reverse. Respecting the
harmonies of sound I must hereafter speak.
There is a fourth class of sensible things, having many intricate varieties,
which must now be distinguished. They are called by the general name of
colours, and are a flame which emanates from every sort of body, and has
particles corresponding to the sense of sight. I have spoken already, in what
has preceded, of the causes which generate sight, and in this place it will be
natural and suitable to give a rational theory of colours.