timaeus   

causes of every affection, whether of sense or not, to be of the
following nature, remembering that we have already distinguished
between the nature which is easy and which is hard to move; for this
is the direction in which we must hunt the prey which we mean to take.
A body which is of a nature to be easily moved, on receiving an
impression however slight, spreads abroad the motion in a circle,
the parts communicating with each other, until at last, reaching the
principle of mind, they announce the quality of the agent. But a
body of the opposite kind, being immobile, and not extending to the
surrounding region, merely receives the impression, and does not
stir any of the neighbouring parts; and since the parts do not
distribute the original impression to other parts, it has no effect of
motion on the whole animal, and therefore produces no effect on the
patient. This is true of the bones and hair and other more earthy
parts of the human body; whereas what was said above relates mainly to
sight and hearing, because they have in them the greatest amount of
fire and air. Now we must conceive of pleasure and pain in this way.
An impression produced in us contrary to nature and violent, if
sudden, is painful; and, again, the sudden return to nature is
pleasant; but a gentle and gradual return is imperceptible and vice
versa. On the other hand the impression of sense which is most
easily produced is most readily felt, but is not accompanied by
Pleasure or pain; such, for example, are the affections of the
sight, which, as we said above, is a body naturally uniting with our
body in the day-time; for cuttings and burnings and other
affections which happen to the sight do not give pain, nor is there
pleasure when the sight returns to its natural state; but the
sensations are dearest and strongest according to the manner in
which the eye is affected by the object, and itself strikes and
touches it; there is no violence either in the contraction or dilation
of the eye. But bodies formed of larger particles yield to the agent
only with a struggle; and then they impart their motions to the
whole and cause pleasure and pain-pain when alienated from their
natural conditions, and pleasure when restored to them. Things which
experience gradual withdrawings and emptyings of their nature, and
great and sudden replenishments, fail to perceive the emptying, but
are sensible of the replenishment; and so they occasion no pain, but
the greatest pleasure, to the mortal part of the soul, as is
manifest in the case of perfumes. But things which are changed all of
a sudden, and only gradually and with difficulty return to their own
nature, have effects in every way opposite to the former, as is
evident in the case of burnings and cuttings of the body.
Thus have we discussed the general affections of the whole body, and
the names of the agents which produce them. And now I will endeavour
to speak of the affections of particular parts, and the causes and
agents of them, as far as I am able. In the first place let us set
forth what was omitted when we were speaking of juices, concerning the
affections peculiar to the tongue. These too, like most of the other
affections, appear to be caused by certain contractions and dilations,
but they have besides more of roughness and smoothness than is found
in other affections; for whenever earthy particles enter into the
small veins which are the testing of the tongue, reaching to the
heart, and fall upon the moist, delicate portions of flesh-when, as
they are dissolved, they contract and dry up the little veins, they
are astringent if they are rougher, but if not so rough, then only
harsh. Those of them which are of an abstergent nature, and purge
the whole surface of the tongue, if they do it in excess, and so
encroach as to consume some part of the flesh itself, like potash
and soda, are all termed bitter. But the particles which are deficient
in the alkaline quality, and which cleanse only moderately, are called
salt, and having no bitterness or roughness, are regarded as rather
agreeable than otherwise. Bodies which share in and are made smooth by
the heat of the mouth, and which are inflamed, and again in turn
inflame that which heats them, and which are so light that they are