Now the relations of these must necessarily
vary, because the principal masses of the different elements hold
opposite positions; for that which is light, heavy, below or
above in one place will be found to be and become contrary and
transverse and every way diverse in relation to that which is
light, heavy, below or above in an opposite place. And about all
of them this has to be considered:-that the tendency of each
towards its kindred element makes the body which is moved heavy,
and the place towards which the motion tends below, but things
which have an opposite tendency we call by an opposite name. Such
are the causes which we assign to these phenomena. As to the
smooth and the rough, any one who sees them can explain the
reason of them to another. For roughness is hardness mingled with
irregularity, and smoothness is produced by the joint effect of
uniformity and density.
The most important of the affections which concern the whole
body remains to be considered-that is, the cause of pleasure and
pain in the perceptions of which I have been speaking, and in all
other things which are perceived by sense through the parts of
the body, and have both pains and pleasures attendant on them.
Let us imagine the 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.
