soul,
since it is not human flesh before it has a rational soul.

Reply Obj. 1: Human flesh depends upon the soul for its being; and
hence, before the coming of the soul, there is no human flesh, but
there may be a disposition towards human flesh. Yet in the conception
of Christ, the Holy Ghost, Who is an agent of infinite might,
disposed the matter and brought it to its perfection at the same time.

Reply Obj. 2: The form actually gives the species; but the matter in
itself is in potentiality to the species. And hence it would be
against the nature of a form to exist before the specific nature. And
therefore the dissimilarity between our origin and Christ's origin,
inasmuch as we are conceived before being animated, and Christ's
flesh is not, is by reason of what precedes the perfection of the
nature, viz. that we are conceived from the seed of man, and Christ
is not. But a difference which would be with reference to the origin
of the soul, would bespeak a diversity of nature.

Reply Obj. 3: The Word of God is understood to be united to the flesh
before the soul by the common mode whereby He is in the rest of
creatures by essence, power, and presence. Yet I say "before," not in
time, but in nature; for the flesh is understood as a being, which it
has from the Word, before it is understood as animated, which it has
from the soul. But by the personal union we understand the flesh as
united to the soul before it is united to the Word, for it is from
its union with the soul that it is capable of being united to the
Word in Person; especially since a person is found only in the
rational nature.
_______________________

FIFTH ARTICLE [III, Q. 6, Art. 5]

Whether the Whole Human Nature Was Assumed Through the Medium of the
Parts?

Objection 1: It would seem that the Son of God assumed the whole
human nature through the medium of its parts. For Augustine says (De
Agone Christ. xviii) that "the invisible and unchangeable Truth
assumed the soul through the medium of the spirit, and the body
through the medium of the soul, and in this way the whole man." But
the spirit, soul, and body are parts of the whole man. Therefore He
assumed all, through the medium of the parts.

Obj. 2: Further, the Son of God assumed flesh through the medium of
the soul because the soul is more like to God than the body. But the
parts of human nature, since they are simpler than the body, would
seem to be more like to God, Who is most simple,