How of necessity the Word only can unite in one person with man.
Anselm. Now must we inquire further, in what person God, who exists in three persons, shall take upon himself the nature of man. For a plurality of persons cannot take one and the same man into a unity of person. Wherefore in one person only can this be done. But, as respects this personal unity of God and man, and in which of the Divine persons this ought to be effected, I have expressed myself, as far as I think needful for the present inquiry, in a letter on the Incarnation of the Word, addressed to my lord, the Pope Urban.
Boso. Yet briefly glance at this matter, why the person of the Son should be incarnated rather than that of the Father or the Holy Spirit.
Anselm. If one of the other persons be incarnated, there will be two sons in the Trinity, viz., the Son of God, who is the Son before the incarnation, and he also who, by the incarnation, will be the son of the virgin; and among the persons which ought always to be equal there will be an inequality as respects the dignity of birth. For the one born of God will have a nobler birth than he who is born of the virgin. Likewise, if the Father become incarnate, there will be two grandsons in the Trinity; for the Father, by assuming humanity, will be the grandson of the parents of the virgin, and the Word, though having nothing to do with man, will yet be the grandson of the virgin, since he will be the son of her son. But all these 251things are incongruous and do not pertain to the incarnation of the Word. And there is yet another reason which renders it more fitting for the Son to become incarnate than the other persons. It is, that for the Son to pray to the Father is more proper than for any other person of the Trinity to supplicate his fellow. Moreover, man, for whom he was to pray, and the devil, whom he was to vanquish, have both put on a false likeness to God by their own will. Wherefore they have sinned, as it were, especially against the person of the Son, who is believed to be the very image of God. Wherefore the punishment or pardon of guilt is with peculiar propriety ascribed to him upon whom chiefly the injury was inflicted. Since, therefore, infallible reason has brought us to this necessary conclusion, that the Divine and human natures must unite in one person, and that this is evidently more fitting in respect to the person of the Word than the other persons, we determine that God the Word must unite with man in one person.
Boso. The way by which you lead me is so guarded by reason that I cannot deviate from it to the right or left.
Anselm. It is not I who lead you, but he of whom we are speaking, without whose guidance we have no power to keep the way of truth.
Source: Cur Deus Homo (CCEL)