"At present this much is clear, that the object seen in light is colour, and
this is why it is not seen without light.  For what it is to be colour is, as
we say, just this, that it is capable of exciting change in the operantly
(actual) transparent medium:  and the actuality of the transparent is light.
There is clear evidence of this.  If you lay the coloured object upon your eye,
you will not see it.  On the contrary, what the colour sets in motion is the
transparent medium, say, the air, and by this, which is continuous, the
sense-organ is stimulated."<p>
<p>
	(From <i>De Anima</i>, II, 7, 418b 26 ff)<p>