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

‘becoming’ or ‘change’; nor is the thing that has become actual described
in this way, for what becomes loses the quality of becoming, change, and
possibility when it has become actual. Therefore there must necessarily
be something that can be described by ‘becoming’ and ‘change’ and
‘transition from nonexistence to existence’, as happens in the passage of
opposites into opposites; that is to say, there must be a substratum for
them in which they can interchange-with this one difference, however, that
this substratum exists in the interchange of all the accidents in actuality,
whereas in the substance it exists in potency.’
And we cannot think of regarding what is described by ‘possibility’ and
‘change’ as identical with the actual, i.e. which belongs to the becoming in
so far as it is actual, for the former again vanishes and the latter must
necessarily be a part of the product. Therefore there must necessarily be
a substratum which is the recipient for the possibility and which is the
vehicle of the change and the becoming, and it is this of which it is said
that it becomes, and alters, and changes from non-existence into
existence. Nor can we think of making this substratum of the nature of the
actualized, for if this were the case the existent would not become, for
what becomes comes into being from the non-existent not from the
existent.’ Both philosophers and Mu’tazilites agree about the existence of
this entity; only the philosophers are of the opinion that it cannot be
exempt from a form actually existent, i.e. that it cannot be free from
existence, like the transition, for example, from sperma to blood and the
transition from blood to the members of the embryo. The reason is that if it
were exempt from existence it would have an existence of its own, and if it
had an existence of its own, becoming could not come from it. This entity
is called by the philosophers ‘lyle’, and it is the cause of generation and
corruption. And according to the philosophers an existent which is free
from Kyle is neither generable nor corruptible.
Ghazali says:
The third proof is that the souls of men, according to the
philosophers, are substances which subsist by themselves’
without being in a body or in matter or impressed on matters
they had a beginning in time, according to the theory of
Avicenna and the acknowledged philosophers, they had
possibility before their beginning, but they have neither
106