I answer that, opinions differ on this question. For some, considering only the cause of filiation, which is nativity, put two filiations in Christ, just as there are two nativities. On the contrary, others, considering only the subject of filiation, which is the person or hypostasis, put only one filiation in Christ, just as there is but one hypostasis or person. Because the unity or plurality of a relation is considered in respect, not of its terms, but of its cause or of its subject. For if it were considered in respect of its terms, every man would of necessity have in himself two filiations---one in reference to his father, and another in reference to his mother. But if we consider the question aright, we shall see that every man bears but one relation to both his father and his mother, on account of the unity of the cause thereof. For man is born by one birth of both father and mother: whence he bears but one relation to both. The same is said of one master who teaches many disciples the same doctrine, and of one lord who governs many subjects by the same power. But if there be various causes specifically diverse, it seems that in consequence the relations differ in species: wherefore nothing hinders several such relations being in the same subject. Thus if a man teach grammar to some and logic to others, his teaching is of a different kind in one case and in the other; and therefore one and the same man may have different relations as the master of different disciples, or of the same disciples in regard to diverse doctrines. Sometimes, however, it happens that a man bears a relation to several in respect of various causes, but of the same species: thus a father may have several sons by several acts of generation. Wherefore the
paternity cannot differ specifically, since the acts of generation are specifically the same. And because several forms of the same species cannot at the same time be in the same subject, it is impossible for several paternities to be in a man who is the father of several sons by natural generation. But it would not be so were he the father of one son by natural generation and of another by adoption.
Now, it is manifest that Christ was not born by one and the same nativity, of the Father from eternity, and of His Mother in time: indeed, these two nativities differ specifically. Wherefore, as to this, we must say that there are various filiations, one temporal and the other eternal. Since, however, the subject of filiation is neither the nature nor part of the nature, but the person or hypostasis alone; and since in Christ there is no other hypostasis or person than the eternal, there can be no other filiation in Christ but that which is in the eternal hypostasis. Now, every relation which is predicated of God from time does not put something real in the eternal God, but only something according to our way of thinking, as we have said in the FP, Question [13], Article [7]. Therefore the filiation by which Christ is referred to His Mother cannot be a real relation, but only a relation of reason.
Consequently each opinion is true to a certain extent. For if we consider the adequate causes of filiation, we must needs say that there are two filiations in respect of the twofold nativity. But if we consider the subject of filiation, which can only be the eternal suppositum, then no other than the eternal filiation in Christ is a real relation. Nevertheless, He has the relation of Son in regard to His Mother, because it is implied in the relation of motherhood to Christ. Thus God is called Lord by a relation which is implied in the real relation by which the creature is subject to God. And although lordship is not a real relation in God, yet is He really Lord through the real subjection of the creature to Him. In the same way Christ is really the Son of the Virgin Mother through the real relation of her motherhood to Christ.