Chapter 41

Form is the substance which has been, as it were, given form and made specific by the essential differences, and which signifies the most specific species. Thus, for example, the substance which has been given form and made specific by the animate and sentient body constitutes the animal. And again, when this last has taken on the rational and the mortal, it constitutes the species of man. It is precisely this most specific species which is called form, an informed substance, as it were.

And so the holy Fathers apply the terms substance, and nature, and form to the most specific species