timaeus   

carried upwards to the sensations of the head, and cut all that
comes in their way, by reason of these qualities in them, are all
termed pungent. But when these same particles, refined by
putrefaction, enter into the narrow veins, and are duly proportioned
to the particles of earth and air which are there, they set them
whirling about one another, and while they are in a whirl cause them
to dash against and enter into one another, and so form hollows
surrounding the particles that enter-which watery vessels of air
(for a film of moisture, sometimes earthy, sometimes pure, is spread
around the air) are hollow spheres of water; and those of them which
are pure, are transparent, and are called bubbles, while those
composed of the earthy liquid, which is in a state of general
agitation and effervescence, are said to boil or ferment-of all
these affections the cause is termed acid. And there is the opposite
affection arising from an opposite cause, when the mass of entering
particles, immersed in the moisture of the mouth, is congenial to
the tongue, and smooths and oils over the roughness, and relaxes the
parts which are unnaturally contracted, and contracts the parts
which are relaxed, and disposes them all according to their
nature-that sort of remedy of violent affections is pleasant and
agreeable to every man, and has the name sweet. But enough of this.
The faculty of smell does not admit of differences of kind; for
all smells are of a half formed nature, and no element is so
proportioned as to have any smell. The veins about the nose are too
narrow to admit earth and water, and too wide to detain fire and
air; and for this reason no one ever perceives the smell of any of
them; but smells always proceed from bodies that are damp, or
putrefying, or liquefying, or evaporating, and are perceptible only in
the intermediate state, when water is changing into air and air into
water; and all of them are either vapor or mist. That which is passing
out of air into water is mist, and that which is passing from water
into air is vapour; and hence all smells are thinner than water and
thicker than air. The proof of this is, that when there is any
obstruction to the respiration, and a man draws in his breath by
force, then no smell filters through, but the air without the smell
alone penetrates. Wherefore the varieties of smell have no name, and
they have not many, or definite and simple kinds; but they are
distinguished only painful and pleasant, the one sort irritating and
disturbing the whole cavity which is situated between the head and the
navel, the other having a soothing influence, and restoring this
same region to an agreeable and natural condition.
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.
Of the particles coming from other bodies which fall upon the sight,
some are smaller and some are larger, and some are equal to the
parts of the sight itself. Those which are equal are imperceptible,
and we call them transparent. The larger produce contraction, the
smaller dilation, in the sight, exercising a power akin to that of hot
and cold bodies on the flesh, or of astringent bodies on the tongue,