Tahafut_al-Tahafut-transl-Engl-van-den-Bergh

I say:
and a distinctive mark, but indeed there is no proof of it.
There remains therefore your question whether this specific
difference is a condition of the causeless character of this
causeless existent, and this is nonsense. For we have
shown that there is no cause for its being without a cause,
so as to make it possible to ask for its condition. It would be
like asking whether blackness is a condition for the colour’s
becoming a colour, and if it is a condition, why redness is
then a colour. And the answer is: as to the essential nature
of colour, i. e. in so far as the essence of colouredness is
asserted in the intellect, neither of them is a condition, < and
as to its existence, each of them is a condition for its
existence, but not individually, since a genus cannot exist in
reality without a specific differences And likewise the man
who accepts two causes as starting-points of the series must
say that they are differentiated through a specific difference,
and both differences are a condition for their existence, no
doubt, though not through their individuality.
The summary of what he says here of the proof of the philosophers is
that they say that the specific difference through which the duality in the
necessary existent occurs is either a condition or not a condition for
necessary existence. If the specific difference through which they are
distinguished is a condition for both the necessary existents, they will no
longer be separated in their necessary existence and the necessary
existent will be of necessity one and the same, just as, if black were to be
a condition for the necessity of colour and white a condition for
colouredness, they could not differ in colouredness. If, on the other hand,
the specific difference does not enter into the essence of necessary
existence, then both these necessary existents will have necessary
existence only by accident, and their duality will not be based on their both
being necessary existents. This, however, is not true, for the species are a
condition for the existence of the genus, and both colours are a condition
for the existence of the genus, though not individually (for in this case they
could not exist together in the existence of the colour).
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