Chapter 77 — That in them there is order and gradation according to perfection of nature

As intellectual substances occupy a higher place in the scale of perfection than other substances, so these same substances must also occupy difference places than each other. For they cannot differ from one another by material differentiation, since they lack matter; hence if any plurality is found among them, it must be caused by formal distinction, which establishes diversity of species. And among beings that differ in species, one must note a certain gradation and order. The reason is that just as addition or subtraction of a unit causes variation of species in numbers, so natural things are found to vary in species by the addition or subtraction of differences. For instance, what is merely alive differs from what is both alive and endowed with sense perception; and what is endowed only with life and sensation differs from what is alive, endowed with sense, and rational. Therefore, the immaterial substances under discussion must be distinct according to various degrees and orders.