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