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

necessary existent, and this is impossible and absurd. And this is a proof
which comes very near to being an absolute truth, when it is conceded
that the ‘necessary existent’ must indicate an immaterial existent, and in
such existents, which subsist by themselves without being bodies, there
cannot be imagined essential attributes of which their essence is
constituted, not to speak of attributes which are additional to their
essence, that is, the so-called accidents, for when accidents are imagined
to be removed, the essence remains, which is not the case with the
essential attributes. And therefore it is right to attribute essential attributes
to their subject, since they constitute its identity, but it is not right to
attribute non-essential attributes to it, except through derivative words, for
we do not say of a man that he is knowledge, but we only say that he is an
animal and that he is knowing;; however, the existence of such attributes
in what is incorporeal is impossible, since the nature of these attributes is
extraneous to their subject, and for this reason they are called accidents
and are distinct from what is attributed essentially to the subject, be it a
subject in the soul or in the external world. If it is objected that the
philosophers believe that there are such attributes in the soul, since they
believe that the soul can perceive, will, and move, although at the same
time they hold that the soul is incorporeal, we answer that they do not
mean that these attributes are additional to the essence, but that they are
essential attributes, and it is of the nature of essential attributes not to
multiply the substratum which actually supports them; they are a plurality
only in the sense that the thing defined becomes a plurality through the
parts of the definitions, that is, they are only a subjective plurality in the
mind according to the philosophers, not an actual plurality outside the
soul. For instance, the definition of man is ‘rational animal’, but reason and
life are not actually distinguishable from each other outside the soul in the
way colour and shape are. And therefore he who concedes that matter is
not a condition for the existence of the soul must concede that in the
separate existences there is a real oneness existing outside the soul,
although this oneness becomes a plurality through definition . This is the
doctrine of the Christians concerning the three hypostases in the divine
Nature. They do not believe that they are attributes additional to the
essence, but according to them they are only a plurality in the definitionthey
are a potential, not an actual, plurality. Therefore they say that the
three are one, i. e. one in act and three in potency. We shall enumerate
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