We should now see how essences exist in separated substances, that is, in the soul, in the intelligences, and in the first cause. Now, while everyone concedes the simplicity of the first cause, some people have tried to introduce into the intelligences and the soul a composition of form and matter, a position that seems to have begun with Avicebron, the author of the book called Fons Vitae . But this view is repugnant to the common teaching of the philosophers, for they call these things substances separated from matter, and they prove them to be wholly without matter. The most cogent demonstration of this proceeds from the excellence of understanding found in these substances. For we see that forms are not actually intelligible except as they are separated from matter and its conditions, and forms are not made actually intelligible except by virtue of an intelligent substance, which educes the forms and receives them in itself. Hence, in any intelligent substance there is a complete absence of matter in such a way that the substance has neither a material part itself nor even is the substance like a form impressed in matter, as is the case with material forms.
Nor can someone say that only corporeal matter, and not some other kind of matter, impedes intelligibility. For, if it were only corporeal mater that impedes intelligibility, then since matter is called corporeal only insofar as it exists under a corporeal form, matter's impeding intelligibility would come from the corporeal form; and this is impossible, for the corporeal form is actually intelligible just like any other form, insofar as it is abstracted from matter. Hence, in no way is there a composition of matter and form in either the soul or the intelligences, such that an essence is received in these as in corporeal substances. Nevertheless, in separate substances there is a composition of form and existence, and so in the Liber de Causis , prop. 9, com., it is said that the intelligences have form and existence, and in this place form is taken in the sense of a simple quiddity or nature.
It is easy to see how this is the case. Whenever two things are related to each other such that one is the cause of the other, the one that is the cause can have existence without the other, but not conversely. Now, we find that matter and form are related in such a way that form gives existence to matter, and therefore it is impossible that matter exist without a form; but it is not impossible that a form exist without matter, for a form, insofar as it is a form, is not dependent on matter. When we find a form that cannot exist except in matter, this happens because such forms are distant from the first principle, which is primary and pure act. Hence, those forms that are nearest the first principle are subsisting forms essentially without matter, for not the whole genus of forms requires matter, as said above, and the intelligences are forms of this type. Thus, the essences or quiddities of these substances are not other than the forms themselves.
Therefore, the essence of a composite substance and that of a simple substance differ in that the essence of a composite substance is not form alone but embraces both form and matter, while the essence of a simple substance is form alone. And from this two other differences arise. One is that the essence of a composite substance can be signified as a whole or as a part, which happens because of the designation of the matter, as said above. Hence, in one way, the essence of a composite thing is not predicated of the composite thing itself, for we cannot say that a man is his own quiddity. But the essence of a simple thing, which is its form, cannot be signified except as a whole, as in this case there is nothing beyond the form that might receive the quiddity, and so, however we take the essence of a simple thing, the essence is predicated of it. Hence, Avicenna says in Metaphysicae V, cap. 5 that "the quiddity of a simple thing is the simple thing itself," because there is no other thing to receive the form. The second difference is that the essences of composite things, because they are received in designate matter, are multiplied according to the division of matter, and so it happens that some things are the same in species but different in number. But since the essence of a simple thing is not received in matter, there can be no such multiplication in this case, and so among such substances we do not find many individuals of the same species, as Avicenna expressly says in Metaphysicae V, cap.
Although substances of this kind are form alone and are without matter, they are nevertheless not in every way simple, and they are not pure act;rather, they have an admixture of potency, and this can be seen as follows.
