r it cannot be
said that one angel knows another by the essence of the angel known;
because that whereby the intellect understands is something within
the intellect; whereas the Trinity alone can penetrate the mind.
Again, it cannot be said that one angel knows the other by a species;
because that species would not differ from the angel understood,
since each is immaterial. Therefore in no way does it appear that one
angel can understand another.

Obj. 4: Further, if one angel did understand another, this would be
either by an innate species; and so it would follow that, if God were
now to create another angel, such an angel could not be known by the
existing angels; or else he would have to be known by a species drawn
from things; and so it would follow that the higher angels could not
know the lower, from whom they receive nothing. Therefore in no way
does it seem that one angel knows another.

_On the contrary,_ We read in _De Causis_ that "every intelligence
knows the things which are not corrupted."

_I answer that,_ As Augustine says (Gen. ad lit. lit. ii), such things
as pre-existed from eternity in the Word of God, came forth from Him
in two ways: first, into the angelic mind; and secondly, so as to
subsist in their own natures. They proceeded into the angelic mind in
such a way, that God impressed upon the angelic mind the images of the
things which He produced in their own natural being. Now in the Word
of God from eternity there existed not only the forms of corporeal
things, but likewise the forms of all spiritual creatures. So in every
one of these spiritual creatures, the forms of all things, both
corporeal and spiritual, were impressed by the Word of God; yet so
that in every angel there was impressed the form of his own species
according to both its natural and its intelligible condition, so that
he should subsist in the nature of his species, and understand himself
by it; while the forms of other spiritual and corporeal natures were
impressed in him only according to their intelligible natures, so that
by such impressed species he might know corporeal and spiritual
creatures.

Reply Obj. 1: The spiritual natures of the angels are distinguished
from one another in a certain order, as was already observed (Q. 50,
A. 4, ad 1, 2). So the nature of an angel does not hinder him from
knowing the other angelic natures, since both the higher and lower
bear affinity to his nature, the only difference being a