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

meaning is to ask whether a thing possesses a necessary attribute which
determines its existence. ‘ And when by ‘existent’ is meant what is
understood by ‘thing’ and ‘entity’, it follows the rule of the genus which is
predicated analogically, and whatever it is in this sense is attributed in the
same way to that which has a cause and to that which has none, and it
does not signify anything but the concept of the existent, and by this is
meant ‘the true’, and if it means something additional to the essence, it is
only in a subjective sense which does not exist outside the soul except
potentially, as is also the case with the universal. And this is the way in
which the ancient philosophers considered the First Principle, and they
regarded it as a simple existent. As to the later philosophers in Islam, they
stated that, in their speculation about the nature of the existent qua
existent, they were led to accept a simple existent of this description.
The best method to follow, in my opinion, and the nearest to strict proof,
is to say that the actualization of existents which have in their substance a
possible existence necessarily occurs only through an actualizer which is
in act, i. e. acting, and moves them and draws them out of potency into
act. And if this actualizer itself is also of the nature of the possible, i. e.
possible in its substance, there will have to be another actualizer for it,
necessary in its substance and not possible, so that this sublunary world
may be conserved, and the nature of the possible causes may remain
everlastingly, proceeding without end. And if these causes exist without
end, as appears from their nature, and each of them is possible,
necessarily their cause, i. e. that which determines their permanence,
must be something necessary in its substance, and if there were a
moment in which nothing was moved at all, there would be no possibility
of an origination of movements The nexus between temporal existence
and eternal can only take place without a change affecting the First
through that movement which is partly eternal, partly temporal. b And the
thing moved by this movement is what Avicenna calls ‘the existence
necessary through another’, and this ‘necessary through another’ must be
a body everlastingly moved, and in this way it is possible that the
essentially temporal and corruptible should exist in dependence on the
eternal, and this through approach to something and through recession
from it, as you observe it happen to transitory existents in relation to the
heavenly bodies. ? And since this moved body is necessary in its
substance, possible in its local movement, it is necessary that the process
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