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

similar in every way or different. If they were similar in every
way, they could not be thought to be a plurality or a duality,
since two blacks can have only a duality, when they are in
two places, or in one place at different times, for black and
movement can only exist in one place and be two at the
same time, because they differ essentially. When the two
essences, like the two blacks, do not differ and at the same
time are simultaneous and in one place, they cannot be
thought to be a plurality; if one could speak of two
simultaneous blacks as being in the same place, any
individual could be said to be two, although not the slightest
difference could be perceived between the two. Since they
cannot be absolutely similar, they must be different, but they
cannot differ in time or in place, and they can therefore only
differ in essence. But two things which differ in something
must either participate in something or not participate in
anything. The latter is impossible, for it would mean that they
would participate neither in existence, ‘ nor in the necessity
of existence, nor in being subsistent in themselves and not
inhering in a substratum. But if they agree in something and
differ in something, that in which they agree must be
different from that through which they differ; there will
therefore be composition in them, and it will be possible to
analyse them in thought. But there is no composition in the
necessary existent, and just as it cannot be divided
quantitatively, so it cannot be analysed by thought either, for
its essence is not composed of elements which intellectual
analysis could enumerate. ‘ The words ‘animal’ and ‘rational’,
for instance, mean that which constitutes the essence of
man, namely, animal and rational, and what is meant by the
word ‘animal’ when one speaks of a man is different from
what is meant by ‘rational’, and therefore man is composed
of parts which are ordered in the definition by words which
indicate these parts, and the term ‘man’ is applied to the
whole of them. ‘ This composition, however, cannot be
imagined in the necessary existent, while duality cannot be
imagined except in this way.
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