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

should terminate in an absolutely necessary existent in which there is no
potency at all, either in its substance, or locally or in any of the other forms
of movement; and that which is of this description is necessarily simple,
because if it were a compound, it would be possible, not necessary, and it
would require a necessary existent. And this method of proving it is in my
opinion sufficient, and it is true.
However, what Avicenna adds to this proof by saying that the possible
existent must terminate either in an existent necessary through another or
in an existent necessary through itself, and in the former case that the
necessary through another should be a consequence of the existent
necessary through itself, for he affirms that the existent necessary through
another is in itself a possible existent and what is possible needs
something necessary-this addition, is to my mind superfluous and
erroneous, for in the necessary, in whatever way you suppose it, there is
no possibility whatsoever and there exists nothing of a single nature of
which it can be said that it is in one way possible and in another way
necessary in its existence. ‘ For the philosophers have proved that there is
no possible whatsoever in the necessary; for the possible is the opposite
of the necessary, and the only thing that can happen is that a thing should
be in one way necessary, in another way possible, as they believed for
instance to be the case with the heavenly body or what is above the body
of the heavens, namely that it was necessary through its substance and
possible in its movement and in space. What led Avicenna to this division
was that he believed that the body of the heavens was essentially
necessary through another, possible by itself, and we have shown in
another place that this is not true. And the proof which Avicenna uses in
dealing with the necessary existent, when this distinction and this
indication are not made, is of the type of common dialectical notions;
when, however, the distinction is made, it is of i the type of demonstrative
proof.
You must know further that the becoming of which the Holy Law speaks
is of the kind of empirical becoming in this world, and this occurs in the
forms of the existents which the Ash’arites call mental qualitiesand the
philosophers call forms, and this becoming occurs only through another
thing and in time, and the Holy Words: ‘Have not those who have
disbelieved considered that the heavens and the earth were coherent, and
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