On Sense And The Sensible
water, and this in undeveloped embryos is remarkably cold and
glistening. In sanguineous animals the white of the eye is fat and
oily, in order that the moisture of the eye may be proof against
freezing. Wherefore the eye is of all parts of the body the least
sensitive to cold: no one ever feels cold in the part sheltered by the
eyelids. The eyes of bloodless animals are covered with a hard scale
which gives them similar protection.
It is, to state the matter generally, an irrational notion that
the eye should see in virtue of something issuing from it; that the
visual ray should extend itself all the way to the stars, or else go
out merely to a certain point, and there coalesce, as some say, with
rays which proceed from the object. It would be better to suppose this
coalescence to take place in the fundament of the eye itself. But even
this would be mere trifling. For what is meant by the 'coalescence' of
light with light? Or how is it possible? Coalescence does not occur
between any two things taken at random. And how could the light within
the eye coalesce with that outside it? For the environing membrane
comes between them.
That without light vision is impossible has been stated elsewhere;
but, whether the medium between the eye and its objects is air or
light, vision is caused by a process through this medium.
Accordingly, that the inner part of the eye consists of water is
easily intelligible, water being translucent.
Now, as vision outwardly is impossible without [extra-organic]
light, so also it is impossible inwardly [without light within the
organ]. There must, therefore, be some translucent medium within the
eye, and, as this is not air, it must be water. The soul or its
perceptive part is not situated at the external surface of the eye,
but obviously somewhere within: whence the necessity of the interior
of the eye being translucent, i.e. capable of admitting light. And
that it is so is plain from actual occurrences. It is matter of
experience that soldiers wounded in battle by a sword slash on the