r 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 clearest 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