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

I say:
the acceptance of one single existent quiddity. The
philosophers, however, say that every existent quiddity is a
plurality, for it contains quiddity and existence, and this is an
extreme confusion; for the meaning of a single existent is
perfectly understandable-nothing exists which has no
essence, and the existence of an essence does not annul its
singleness.
Ghazali does not relate Avicenna’s doctrine literally as he did in his
book The Aims of the Philosophers. ‘ For since Avicenna believed that the
existence of a thing indicated an attribute additional to its essence, he
could no longer admit that its essence was the agent of its existence out of
the possibles, for then the thing would be the cause of its own existence
and it would not have an agent. It follows from this, according to Avicenna,
that everything which has an existence additional to its essence has an
efficient cause, and since according to Avicenna the First has no agent, it
follows necessarily that its existence is identical with its essence. z And
therefore Ghazali’s objection that Avicenna assimilates existence to a
necessary attribute of the essence is not true, because the essence of a
thing is the cause of its necessary attribute and it is not possible that a
thing should be the cause of its own existence, because the existence of a
thing is prior to its quiddity. To identify the quiddity and the existence of a
thing is not to do away with its quiddity, as Ghazali asserts, but is only the
affirmation of the unity of quiddity and existence. If we regard existence as
an accidental attribute of the existent, and it is the agent which gives
possible things their existence, necessarily that which has no agent either
cannot have an existence (and this is absurd), or its existence must be
identical with its essence.
But the whole of this discussion is built on the mistake that the
existence of a thing is one of its attributes. For the existence which in our
knowledge is prior to the quiddity of a thing is that which signifies the true.
Therefore the question whether a thing exists, either (i) refers to that which
has a cause that determines its existence, and in that case its potential
meaning is to ask whether this thing has a cause or not, according to
Aristotle at the beginning of the second chapter of the Posterior
Analytics;s or (2) it refers to that which has no cause, and then its
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