In Plato’s explanation of sensation we are struck by the fact
that he has not the same distinct conception of organs of sense
which is familiar to ourselves. The senses are not instruments,
but rather passages, through which external objects strike upon
the mind. The eye is the aperture through which the stream of
vision passes, the ear is the aperture through which the
vibrations of sound pass. But that the complex structure of the
eye or the ear is in any sense the cause of sight and hearing he
seems hardly to be aware.
The process of sight is the most complicated (Rep.), and consists
of three elements—the light which is supposed to reside within
the eye, the light of the sun, and the light emitted from
external objects. When the light of the eye meets the light of
the sun, and both together meet the light issuing from an
external object, this is the simple act of sight. When the
particles of light which proceed from the object are exactly
equal to the particles of the visual ray which meet them from
within, then the body is transparent. If they are larger and
contract the visual ray, a black colour is produced; if they are
smaller and dilate it, a white. Other phenomena are produced by
the variety and motion of light. A sudden flash of fire at once
elicits light and moisture from the eye, and causes a bright
colour. A more subdued light, on mingling with the moisture of
the eye, produces a red colour. Out of these elements all other
colours are derived. All of them are combinations of bright and
red with white and black. Plato himself tells us that he does not
know in what proportions they combine, and he is of opinion that
such knowledge is granted to the gods only. To have seen the
affinity of them to each other and their connection with light,
is not a bad basis for a theory of colours. We must remember that
they were not distinctly defined to his, as they are to our eyes;
he saw them, not as they are divided in the prism, or
artificially manufactured for the painter’s use, but as they
exist in nature, blended and confused with one another.
We can hardly agree with him when he tells us that smells do not
admit of kinds. He seems to think that no definite qualities can
attach to bodies which are in a state of transition or
evaporation; he also makes the subtle observation that smells
must be denser than air, though thinner than water, because when
there is an obstruction to the breathing, air can penetrate, but
not smell.
The affections peculiar to the tongue are of various kinds, and,
like many other affections, are caused by contraction and
dilation. Some of them are produced by rough, others by
abstergent, others by inflammatory substances,—these act upon the
testing instruments of the tongue, and produce a more or less
disagreeable sensation, while other particles congenial to the
tongue soften and harmonize them. The instruments of taste reach
from the tongue to the heart. Plato has a lively sense of the
manner in which sensation and motion are communicated from one
part of the body to the other, though he confuses the affections
with the organs. Hearing is a blow which passes through the ear
and ends in the region of the liver, being transmitted by means
of the air, the brain, and the blood to the soul. The swifter
sound is acute, the sound which moves slowly is grave. A great
body of sound is loud, the opposite is low. Discord is produced
by the swifter and slower motions of two sounds, and is converted
into harmony when the swifter motions begin to pause and are
overtaken by the slower.
The general phenomena of sensation are partly internal, but the
more violent are caused by conflict with external objects.
Proceeding by a method of superficial observation, Plato remarks
that the more sensitive parts of the human frame are those which
are least covered by flesh, as is the case with the head and the
elbows. Man, if his head had been covered with a thicker pulp of
flesh, might have been a longer-lived animal than he is, but
could not have had as quick perceptions. On the other hand, the
tongue is one of the most sensitive of organs; but then this is
made, not to be a covering to the bones which contain the marrow
or source of life, but with an express purpose, and in a separate
mass.