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

I say:
constitution of the essence out of the parts of its quiddity,
and this community is only the sharing of a necessary
common attribute consecutive to the essence, not a
community of genus. Things therefore are only defined by
their constituents, and if they are defined by the necessary
attributes this is only a description’ to differentiate them, not
to define their essential forms; for the triangle is not defined
by the fact that its angles are equal to two right angles,
although this is a necessary and common attribute of all
triangles, but it is defined as a figure bounded by three sides.
And the same applies to its being a substance, and the
meaning of its being a substance is that it is an existent
which does not exist in a substratum. ‘ And the existent is
not a genus, since, as it is related to a negation, namely not
being in a substratum, it cannot become a constituent genus;
indeed, even if it could be brought into a relation to
something positive and it could be said that it existed in a
substratum, it could not become a genus in the accident.
And the reason is that the man who knows substance by its
definition, which is rather its description, namely that it is an
existent which does not exist in a substratum, does not know
whether it exists, and a fortiori does not know whether it
exists in a substratum or not; no, the meaning of the
description of substance is that it is the existent which does
not exist in a substratum, i. e. that it is a certain reality which,
when it does exist, does not exist in a substratum, but we do
not mean that it actually exists at the time of the definition,
and its community is not the community of the genus, for
only the constituents of the quiddity form the community of
the genus which needs also a specific differences But the
First has no other quiddity, except necessary existence, and
necessary existence is its real nature and its own quiddity,
exclusively confined to it, and since necessary existence is
exclusively confined to the First, it cannot be shared by
others, it cannot have a specific difference, and it cannot
have a definition.
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