Objection 1. It would seem that Christ as Man is a hypostasis or person. For what belongs to every man belongs to Christ as Man, since He is like other men according to Philippians 2:7: "Being made in the likeness of men." But every man is a person. Therefore Christ as Man is a person.
Objection 2. Further, Christ as Man is a substance of rational nature. But He is not a universal substance: therefore He is an individual substance. Now a person is nothing else than an individual substance of rational nature; as Boethius says (De Duab. Nat.). Therefore Christ as Man is a person.
Objection 3. Further, Christ as Man is a being of human nature, and a suppositum and a hypostasis of the same nature. But every hypostasis and suppositum and being of human nature is a person. Therefore Christ as Man is a person.
On the contrary, Christ as Man is not an eternal person. Therefore if Christ as Man is a person it would follow that in Christ there are two persons--one temporal and the other eternal, which is erroneous, as was said above (2, 6; 4, 2).
I answer that, As was said (10,11), the term "Man" placed in the reduplication may refer either to the suppositum or to the nature. Hence when it is said: "Christ as Man is a person," if it is taken as referring to the suppositum, it is clear that Christ as Man is a person, since the suppositum of human nature is nothing else than the Person of the Son of God. But if it be taken as referring to the nature, it may be understood in two ways. First, we may so understand it as if it belonged to human nature to be in a person, and in this way it is true, for whatever subsists in human nature is a person. Secondly it may be taken that in Christ a proper personality, caused by the principles of the human nature, is due to the human nature; and in this way Christ as Man is not a person, since the human nature does not exist of itself apart from the Divine Nature, and yet the notion of person requires this.
Reply to Objection 1. It belongs to every man to be a person, inasmuch as everything subsisting in human nature is a person. Now this is proper to the Man Christ that the Person subsisting in His human nature is not caused by the principles of the human nature, but is eternal. Hence in one way He is a person, as Man; and in another way He is not, as stated above.
Reply to Objection 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 to Objection 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.
Source: Summa Theologica (New Advent)