Since in these substances the quiddity is not the same as existence, these substances can be ordered in a predicament, and for this reason we find among these things genera, species, and differences, although their proper differences are hidden from us. In sensible things even the essential differences are unknown to us, and so they are signified through accidental differences that arise from the essential ones, just as a cause is signified through its effect. We take bipedality, for example, as the difference of man. The proper accidents of immaterial substances, however, are unknown to us, and thus we can signify their differences neither per se nor through their accidental differences.
We should note, though, that the genus and difference in immaterial substances are not taken in the same way as in sensible substances, for in sensible substances the genus is taken from that which is material in the thing, while the difference is taken from that which is formal in the thing. Hence, Avicenna says, De Anima I, cap.1, that, in things composed of form and matter, the form "is its simple difference because the thing is constituted from it," not, however, because the form is the difference but rather because it is the principle of the difference, as Avicenna himself says in his Metaphysicae V, cap. 6. Further, this difference is called a simple difference because it is taken from that which is a part of the quiddity of the thing, namely, from the form. But since immaterial substances are simple quiddities, in such substances the difference cannot be taken from that which is a part of the quiddity but only from the whole quiddity, and so in De Anima I, cap. 1, Avicenna says that substances "have no simple difference except for those species of which the essences are composed of matter and form."Similarly, in immaterial things the genus is taken from the whole essence, though not in the same way as the difference is. One separated substance is like another with respect to their immateriality, but they differ one from another with respect to their grade of perfection according to how far each recedes from potentiality and approaches pure act. And so, in such substances, the genus is taken from that which arises in these substances insofar as they are immaterial, as intellectuality and such things; the difference, however, is taken from that which arises in these substances from their grade of perfection, although these differences are unknown to us. Nor are these differences accidental because they arise from greater and lesser perfection, which do not diversify the species. For, while the grade of perfection in receiving the same form does not diversify the species (as whiter and less white in participating in whiteness of the same type), nevertheless, a different grade of perfection in these participated forms or natures does diversify the species, just as nature proceeds by grades from plants to animals through those things that are median between plants and animals, as the Philosopher says in VIII De Historia Animalium cap. 1 (588b4-12). Nor is it necessary that the division of intellectual substances always be made through two true differences, for it is impossible that this happen in all things, as the Philosopher says in I De Partibus Animalium cap. 2 (642b5-7).
In a third way, essence is found in substances composed of matter and form, in which existence is both received and limited because such substances have existence from another, and again because the nature or quiddity of such substances is received in signate matter. And thus such substances are finite in both a superior way and an inferior way, and among such substances, because of the division of signate matter, there can be a multiplication of individuals in one species. The ways in which the essence in such substances is related to the logical intentions we have explained above.
