Thomas Aquinas

“ In Christ the soul gives being to the body, inasmuch as it makes it actually animated, which is to give it the complement of its nature and species. But if we consider the body perfected by the soul, without the hypostasis having both---this whole, composed of soul and body, as signified by the word "humanity," does not signify "what is," but "whereby it is." Hence being belongs to the subsisting person, inasmuch as it has a relation to such a nature, and of this relation the soul is the cause, inasmuch as it perfects human nature by informing the body. ”

“ 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. ”

“ 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. ”