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

First, to what can undergo division actually or in
imagination, and therefore the single body is not absolutely
one-it is one through the continuity which exists in it, which
can suffer a decrease and can be quantitatively divided in
imagination. This is impossible in the First Principle.
Secondly: a thing may be divided by thought, not
quantitatively, into two different concepts, as for instance the
division of body into matter and form, for although neither
matter nor form can subsist separately, they are two different
things in definition and in reality, and it is by their
composition that a unity results, namely body. This also must
be denied of God, for God cannot be a form or a matter in a
body, or be the compound of both. There are two reasons
why God cannot be their compound, first because this
compound can be divided into quantitative parts, actually or
in imagination, secondly, because this compound can be
divided conceptually into form and matter, and God cannot
be matter, because matter needs a form, and the necessary
existent is self-sufficient in every respect and its existence
cannot be conjoined with the condition of something else
besides it, and God cannot be form, because form needs
matter.
Thirdly: the plurality through attributes implied in
knowledge, power, and will; if these attributes had a
necessary existence, the essence and these attributes would
participate in necessary existence and the necessary
existent must be a plurality, and its uniqueness would be
denied.
Fourthly: the rational plurality which results from the
composition of genus and species. For black is black and
colour, and blackness is not colouredness for the intellect,
but colouredness is a genus, and blackness a specific
difference, and therefore black is composed of genus and
species; and animality is for the mind something different
from humanity, for man is a rational animal, animal is a
genus and rational a specific difference, and man is
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