ltiplication
of the angels is not to be taken according to matter, nor according to
bodies, but according to the divine wisdom devising the various orders
of immaterial substances.
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FOURTH ARTICLE [I, Q. 50, Art. 4]

Whether the Angels Differ in Species?

Objection 1: It would seem that the angels do not differ in species.
For since the "difference" is nobler than the 'genus,' all things
which agree in what is noblest in them, agree likewise in their
ultimate constitutive difference; and so they are the same according
to species. But all angels agree in what is noblest in them--that is
to say, in intellectuality. Therefore all the angels are of one
species.

Obj. 2: Further, more and less do not change a species. But the
angels seem to differ only from one another according to more and
less--namely, as one is simpler than another, and of keener
intellect. Therefore the angels do not differ specifically.

Obj. 3: Further, soul and angel are contra-distinguished mutually
from each other. But all souls are of the one species. So therefore
are the angels.

Obj. 4: Further, the more perfect a thing is in nature, the more
ought it to be multiplied. But this would not be so if there were but
one individual under one species. Therefore there are many angels of
one species.

_On the contrary,_ In things of one species there is no such thing as
"first" and "second" [prius et posterius], as the Philosopher says
(Metaph. iii, text 2). But in the angels even of the one order there
are first, middle, and last, as Dionysius says (Hier. Ang. x).
Therefore the angels are not of the same species.

_I answer that,_ Some have said that all spiritual substances, even
souls, are of the one species. Others, again, that all the angels are
of the one species, but not souls; while others allege that all the
angels of one hierarchy, or even of one order, are of the one species.

But this is impossible. For such things as agree in species but differ
in number, agree in form, but are distinguished materially. If,
therefore, the angels be not composed of matter and form, as was said
above (A. 2), it follows that it is impossible for two angels to
be of one species; just as it would be impossible for there to be
several whitenesses apart, or several humanities, since whitenesses
are not several, except in so far as they are in several substances.
And if the angels had matter, not even then could there be several