4. What is the mutual relation between the light that emanates from
the eye, and the light which is exterior to the eye, and which extends
between the eye and the object?[170] Light has no need of air as a
medium, unless indeed somebody should undertake to say that there
is no light without air, in which case air would be a medium only
accidentally. Light itself, however, is an unaffected medium, for
there is no necessity here for an affection, but only for a medium;
consequently, if light be not a body, there is no need of a body (to
act as medium). It might be objected that sight has no need either of a
foreign light nor of a medium to see near by, but has need of them for
vision at a distance. Later[171] we shall consider whether or not light
without air be possible. Now let us consider the first point.