This is from his book "Reality"
"Chapter 23: Angelic Nature And Knowledge
1. Nature Of Angels
St. Thomas [596] teaches clearly that the angels are creatures purely spiritual, subsistent forms without any matter. Scotus says they are composed of form and incorporeal matter, without quantity, because, being creatures, they must have an element of potentiality. The Thomistic reply runs thus: This potential element is first the angelic essence, really distinct, as in all creatures, from existence. Secondly, the real distinction between person and existence, between quod est and existence. Thirdly, real distinction of substance from faculties, and of faculties from acts. All these distinctions are explicitly formulated by St. Thomas himself. [597].
From their pure spirituality St. Thomas concludes that there cannot be two angels of the same species, because the only principle by which a substantial form can be individualized is matter, matter capable of this quantity rather than any other. Thus, to illustrate, two drops of water, perfectly similar, are by their matter and quantity two distinct individuals. But angels have no matter. [598].
Scotus, on the contrary, since he admits a certain kind of matter in the angels, maintains also that there can be many angels of one and the same species. Suarez, in his eclecticism, admits this conclusion of Scotus, although he sides with St. Thomas in maintaining that the angels are purely spiritual and immaterial beings. Thomists reply: if the angels are purely spiritual, you can find in them no principle of individuation, no principle capable of multiplying within one and the same species.
Form unreceived in matter, they say with St. Thomas, is simply unique. Whiteness, for example, if conceived as unreceived in this or that white thing, would be one and unique. If you deny this, then you simultaneously deny the principle which demonstrates the unicity of God, the principle, namely, which St. Thomas thus formulates: [599] Existence unreceived is necessarily subsistent and unique."
Faber's commentary: Now, this isn't really Garrigou's fault...Probably when he wrote this there was still a great deal of dispute as to whether or not the "De rerum principio" was a genuine work of Scotus. In this work, the author says, "I return to the position of Avicebron" and endorses spiritual matter. But in point of fact, this has been proven to be not by Scotus at all, but by Vital du four, most of which is copied out of other others, such as Godfrey of Fontaines. This leads Garrigou astray as to Scouts holding to spiritual matter. He (Scotus) respects the opinion, and as far as I know never attacks it as it is part of the Franciscan tradition, but neither does he explicitly endorse it (possible exception in the QQ de anima, though he seems there to be contrasting the relative merits of spiritual matter against certain Thomist views). However he is dead wrong on this being the reason for multiple angels per species, as this is permitted by Scotus's theory of individuation being by means of a further determination of the species form to a singularity. There are obviously a number of other issues at play as well, relating to the relation of form and matter. I will say, however, that Scotus does not hold to the real distinction between essence and existence, which has caused no end of scandal to Thomists and their Cambridge offspring, Radical Orthodoxy.
