1. First Opinion

303. Some say [Alexander of Hales, Thomas Aquinas] that the first reason is unitability with matter and non-unitability with matter.

304. On the contrary: form is the end of matter, from Physics 2.8.199a30-32, and so the distinction of matter is for the distinction of form and not conversely (hence the bodily members of a deer are different from the bodily members of a lion, because soul differs from soul [1 d.2 n.332]); so the first distinction of this thing and of that will not be through matter and non-matter, but will be prior in itself to those acts.

305. There is a confirmation; for because this nature is such and that nature is not, so this nature is not that nature; therefore this idea of perfecting and of not perfecting [matter] will not be the first reason for distinction.