engendered, in all the parts of the nether body. Then one and the same
spirit is named by divers names. For by working in the liver it is
called the natural spirit, in the heart the vital spirit, and in the
head, the animal spirit. We may not believe that this spirit is man's
reasonable soul, but more soothly, as saith Austin, the car therof and
proper instrument. For by means of such a spirit the soul is joined to
the body: and without the service of such a spirit, no act the soul
may perfectly exercise in the body. And therefore if these spirits be
impaired, or let of their working in any work, the accord of the body
and soul is resolved, the reasonable spirit is let of all its works in
the body. As it is seen in them that be amazed, and mad men and
frantic, and in others that oft lose use of reason.

The sight is most simple, for it is fiery, and knoweth suddenly things
that be full far. The sight is shapen in this manner. In the middle of
the eye, that is, the black thereof, is a certain humour most pure and
clear. The philosophers call it crystalloid, for it taketh suddenly
divers forms and shapes of colours as crystal doth. The sight is a wit
of perceiving and knowing of colours, figures, and shapes, and outer
properties. Then to make the sight perfect, these things are needful,
that is to wit, the cause efficient, the limb of the eye convenient to
the thing that shall be seen, the air that bringeth the likeness to
the eye, and taking heed, and easy moving. The cause efficient is that
virtue that is called animal. The instrument and limb is the humour
like crystal in either eye clear and round. It is clear that by the
clearness thereof the eye may beshine the spirit, and air; it is round
that it be stronger to withstand griefs. The outer thing helping to
work, is the air, without which being a means, the sight may not be
perfect. It needeth to take heed, for if the soul be occupied about
other things than longeth to the sight, the sight is the less perfect.