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

black must be a colour either by black itself or through an entity additional
to black. And this is what Avicenna meant by his assertion that the
necessary existent must be a necessary existent, either through its own
special character or through an addition which is not peculiar to it; if
through the former, there cannot be two existents which are both
necessary existents; if through the latter, both existents must be
composed of a universal and of a peculiar entity, and the compound is not
a necessary existent through itself. And if this is true, the words of Ghazali
: ‘What prevents us from representing two existents which should both be
of a necessary existence?’ are absurd.
And if it is objected, ‘You have said that this statement comes near
being a proof, but it seems to be a proper proof’, we answer: We said this
only because this proof seems to imply that the difference between those
two assumed necessary existents must lie either in their particularity, and
then they participate in their specific quality, or in their species, and then
they participate in their generic quality, and both these differences are
found only in compounds, and the insufficiency of this proof lies in this,
that it has been demonstrated that there are existents which are
differentiated, although they are simple and differ neither in species nor
individually, namely, the separate intellects. ‘ However, it appears from
their nature that there must be in their existence a priority and posteriority
of rank, for no other differentiation can be imagined in them. Avicenna’s
proof about the necessary existent must be therefore completed in this
way: If there were two necessary existents, the difference between them
must consist either in a numerical difference, or in a specific difference, or
in rank. In the first case they would agree in species; in the second case in
genus, and in both cases the necessary existent would have to be
composite. In the third case, however, the necessary existent will have to
be one, and will be the cause of all the separate existents. And this is the
truth, and the necessary existent is therefore one. For there is only this
tripartite disjunction, two members of which are false, and therefore the
third case, which necessitates the absolute uniqueness of the necessary
existent, is the true one. ‘
Ghazali says:
The second proof of the philosophers is that they say: If
we assumed two necessary existents, they would have to be
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