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

I say:
The objection is that the possibility of which they speak is
a judgement of the intellect, and anything whose existence
the intellect supposes, provided no obstacle presents itself to
the supposition, we call possible and, if there is such an
obstacle, we call it impossible and, if we suppose that it
cannot be supposed not to be, we call it necessary. These
are rational judgements which need no real existent which
they might qualify. There are three proofs of this. The first is:
If possibility needed an existent to which it could be related,
and of which it could be said that it is its possibility,
impossibility also would need an existent of which it might be
said that it is its impossibility; but impossibility has no real
existence, and there is no matter in which it occurs and to
which it could be related.
That possibility demands an existing matter is clear, for all true
intellectual concepts need a thing outside the soul, for truth, as it has been
defined, is the agreement of what is in the soul with what is outside the
soul.’ And when we say that something is possible, we cannot but
understand that it needs something in which this possibility can be. As
regards his proof that the possible is not dependent on an existent,
because the impossible is not dependent on an existent, this is sophistical.
Indeed the impossible demands a substratum just as much as the
possible does, and this is clear from the fact that the impossible is the
opposite of the possible and opposite contraries undoubtedly require a
substratum. For impossibility is the negation of possibility, and, if
possibility needs a substratum, impossibility which is the negation of this
possibility requires a substratum too, e.g. we say that the existence of
empty space is impossible, because the existence of independent
dimensions outside or inside natural bodies is impossible, or that the
presence of opposites at the same time in the same substratum is
impossible, or that the equivalence of one to two is impossible, i.e. in
reality. All this is self-evident, and it is not necessary to consider the errors
here committed.
Ghazali says:
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