to the nature in Christ
must be two; and that those belonging to the hypostasis in Christ
must be only one. Now being pertains both to the nature and to the
hypostasis; to the hypostasis as to that which has being--and to the
nature as to that whereby it has being. For nature is taken after the
manner of a form, which is said to be a being because something is by
it; as by whiteness a thing is white, and by manhood a thing is man.
Now it must be borne in mind that if there is a form or nature which
does not pertain to the personal being of the subsisting hypostasis,
this being is not said to belong to the person simply, but
relatively; as to be white is the being of Socrates, not as he is
Socrates, but inasmuch as he is white. And there is no reason why
this being should not be multiplied in one hypostasis or person; for
the being whereby Socrates is white is distinct from the being
whereby he is a musician. But the being which belongs to the very
hypostasis or person in itself cannot possibly be multiplied in one
hypostasis or person, since it is impossible that there should not be
one being for one thing.

If, therefore, the human nature accrued to the Son of God, not
hypostatically or personally, but accidentally, as some maintained,
it would be necessary to assert two beings in Christ--one, inasmuch
as He is God--the other, inasmuch as He is Man; even as in Socrates
we place one being inasmuch as he is white, and another inasmuch as
he is a man, since "being white" does not pertain to the personal
being of Socrates. But being possessed of a head, being corporeal,
being animated--all these pertain to the one person of Socrates, and
hence there arises from these only the one being of Socrates. And if
it so happened that after the person of Socrates was constituted
there accrued to him hands or feet or eyes, as happened to him who
was born blind, no new being would be thereby added to Socrates, but
only a relation to these, i.e. inasmuch as he would be said to be,
not only with reference to what he had previously, but also with
reference to what accrued to him afterwards. And thus, since the
human nature is united to the Son of God, hypostatically or
personally as was said above (Q. 2, AA. 5, 6), and not accidentally,
it follows that by the human nature there accrued to Him no new
personal being, but only a new relation of the pre-existing personal
being to the human nature, in such a way that the Person is said to
s