Reply Obj. 2: The "individual substance," which is included in the definition of a person, implies a complete substance subsisting of itself and separate from all else; otherwise, a man's hand might be called a person, since it is an individual substance; nevertheless, because it is an individual substance existing in something else, it cannot be called a person; nor, for the same reason, can the human nature in Christ, although it may be called something individual and singular.
Reply Obj. 3: As a person signifies something complete and self-subsisting in rational nature, so a hypostasis, suppositum, and being of nature in the genus of substance, signify something that subsists of itself. Hence, as human nature is not of itself a person apart from the Person of the Son of God, so likewise it is not of itself a hypostasis or suppositum or a being of nature. Hence in the sense in which we deny that "Christ as Man is a person" we must deny all the other propositions. _______________________
QUESTION 17
OF CHRIST'S UNITY OF BEING
(In Two Articles)
We must now consider what pertains to Christ's unity in common. For, in their proper place, we must consider what pertains to unity and plurality in detail: thus we concluded (Q. 9) that there is not only one knowledge in Christ, and it will be concluded hereafter (Q. 35, A. 2) that there is not only one nativity in Christ.
Hence we must consider Christ's unity (1) of being; (2) of will; (3) of operation.
Under the first head there are two points of inquiry:
(1) Whether Christ is one or two?
(2) Whether there is only one being in Christ? _______________________
FIRST ARTICLE [III, Q. 17, Art. 1]