Whatever is visible is colour and
colour is what lies upon what is in its own nature visible; 'in its
own nature' here means not that visibility is involved in the
definition of what thus underlies colour, but that that substratum
contains in itself the cause of visibility. Every colour has in it the
power to set in movement what is actually transparent; that power
constitutes its very nature. That is why it is not visible except with
the help of light; it is only in light that the colour of a thing is
seen. Hence our first task is to explain what light is.
Now there clearly is something which is transparent, and by
'transparent' I mean what is visible, and yet not visible in itself,
but rather owing its visibility to the colour of something else; of
this character are air, water, and many solid bodies. Neither air
nor water is transparent because it is air or water; they are
transparent because each of them has contained in it a certain
substance which is the same in both and is also found in the eternal
body which constitutes the uppermost shell of the physical Cosmos.
Of this substance light is the activity-the activity of what is
transparent so far forth as it has in it the determinate power of
becoming transparent; where this power is present, there is also the
potentiality of the contrary, viz. darkness. Light is as it were the
proper colour of what is transparent, and exists whenever the
potentially transparent is excited to actuality by the influence of
fire or something resembling 'the uppermost body'; for fire too
contains something which is one and the same with the substance in
question.
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