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

I say:
colour because of a cause which has made it a colour, then
black can be thought of as being without a colour, i. e. as not
having been made a colour by a cause, for a determination
added to an essence through a cause can be represented in
the imagination as absent, even if it exists in reality. “ ‘But’, it
will be objected, ‘this disjunction is false in itself, for one
cannot say of black that it is a colour because of its essence,
meaning by this that it cannot be through anything but its
essence, and in the same way one cannot say that this
existent is necessary because of its essence, i. e. that it has
no cause because of its own essence, meaning by this that it
cannot exist through anything but its essence. ‘
This method of proving the unity of God is peculiar to Avicenna, and is
not found in any of the ancient philosophers; its premisses are commonsense
premisses, and the terms are used in a more or less equivocal way.
For this reason many objections can be urged against it. Still, when those
terms and the aim they intend are properly analysed, this statement
comes near to being a proof.
That this primary disjunction is faulty, as Ghazali asserts, is not true. He
says that the meaning of ‘necessary existent’ is ‘that which has no cause’,
and that the statement ‘that what has no cause, has no cause, either
because of its own essence or through another cause’, and similarly the
statement ‘that the necessary existent is a necessary existent, either
because of its own essence or through another cause’ are meaningless
statements. But this is by no means the case. For the meaning of this
disjunction is only whether the necessary existent is such, because of a
nature which characterizes it, in so far as it is numerically one, ‘ or
because of a nature which it has in common with others-for instance,
when we say that Amr is a man because lie is Amr, or because of a nature
he has in common with Khalid. If he is a man because he is Amr, then
humanity does not exist in anyone else, and if he is a man because of a
general nature, then he is composed of two natures, a general one and a
special one and the compound is an effect; but the necessary existent has
no cause, and therefore the necessary existent is unique. And when
Avicenna’s statement is given in this form it is true.
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