"There is, then, we assume, something transparent; and by this I mean that
which, though visible, is not, properly speaking, visible in itself, but by
reason of the colour of something else.  Air, water and many solid bodies
answer to this description.  For they are not transparent <i>qua</i> air or
<i>qua</i> water, but because there is a certain nature present in both of them
which is present also in the eternal body on high.  Light is the activity of
this, the transparent <i>qua</i> transparent.  But where the transparent is
only potentially present, there darkness is actually."<p>
<p>
	(<i>De Anima</i>, II, 7, 418b 4 ff)<p>