Intellectual substances are superior to other substances in the scale of perfection. These same substances must also differ from one another in degree. They cannot differ from one another by material differentiation, since they lack matter; if any plurality is found among them, it must be caused by formal distinction, which establishes diversity of species. In beings that exhibit diversity of species, the degree and order existing in them must be taken into consideration. The reason is that, just as addition or subtraction of a unit causes variation of species in numbers, so natural entities 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 the latter 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.
St. Thomas Aquinas. (1265-1274). Compendium Theologiae: Order and Degree among Intellectual Beings, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947
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