Thomas Aquinas, Summa Theologiae

Objection 1: It would seem that the union of the two natures in Christ is not the greatest of all unions. For what is united falls short of the unity of what is one, since what is united is by participation, but one is by essence. Now in created things there are some that are simply one, as is shown especially in unity itself, which is the principle of number. Therefore the union of which we are speaking does not imply the greatest of all unions.

Objection 2: Further, the greater the distance between things united, the less the union. Now, the things united by this union are most distant---namely, the Divine and human natures; for they are infinitely apart. Therefore their union is the least of all.

Objection 3: Further, from union there results one. But from the union of soul and body in us there arises what is one in person and nature; whereas from the union of the Divine and human nature there results what is one in person only. Therefore the union of soul and body is greater than that of the Divine and human natures; and hence the union of which we speak does not imply the greatest unity.

On the contrary, Augustine says (De Trin. i, 10) that "man is in the Son of God, more than the Son in the Father." But the Son is in the Father by unity of essence, and man is in the Son by the union of the Incarnation. Therefore the union of the Incarnation is greater than the unity of the Divine Essence, which nevertheless is the greatest union; and thus the union of the Incarnation implies the greatest unity.

I answer that, Union implies the joining of several in some one thing. Therefore the union of the Incarnation may be taken in two ways: first, in regard to the things united; secondly, in regard to that in which they are united. And in this regard this union has a pre-eminence over other unions; for the unity of the Divine Person, in which the two natures are united, is the greatest. But it has no pre-eminence in regard to the things united.

Reply to Objection 1: The unity of the Divine Person is greater than numerical unity, which is the principle of number. For the unity of a Divine Person is an uncreated and self-subsisting unity, not received into another by participation. Also, it is complete in itself, having in itself whatever pertains to the nature of unity; and therefore it is not compatible with the nature of a part, as in numerical unity, which is a part of number, and which is shared in by the things numbered. And hence in this respect the union of the Incarnation is higher than numerical unity by reason of the unity of the Divine Person, and not by reason of the human nature, which is not the unity of the Divine Person, but is united to it.

Reply to Objection 2: This reason regards the things united, and not the Person in Whom the union takes place.

Reply to Objection 3: The unity of the Divine Person is greater than the unity of person and nature in us; and hence the union of the Incarnation is greater than the union of soul and body in us.

And because what is urged in the argument "on the contrary" rests upon what is untrue---namely, that the union of the Incarnation is greater than the unity of the Divine Persons in Essence---we must say to the authority of Augustine that the human nature is not more in the Son of God than the Son of God in the Father, but much less. But the man in some respects is more in the Son than the Son in the Father---namely, inasmuch as the same suppositum is signified when I say "man," meaning Christ, and when I say "Son of God"; whereas it is not the same suppositum of Father and Son.

Articulus 9

Argumentum 1

Ad nonum sic proceditur. Videtur quod unio duarum naturarum non sit maxima unionum. Unitum enim deficit in ratione unitatis ab eo quod est unum, eo quod unitum dicitur per participationem, unum autem per essentiam. Sed in rebus creatis aliquid dicitur esse simpliciter unum, sicut praecipue patet de ipsa unitate quae est principium numeri. Ergo huiusmodi unio de qua loquimur, non importat maximam unitatem.

Argumentum 2

Praeterea, quanto ea quae uniuntur magis distant, tanto minor est unio. Sed ea quae secundum hanc unionem uniuntur, maxime distant, scilicet natura divina et humana, distant enim in infinitum.

Ergo huiusmodi est minima unio.

Argumentum 3

Praeterea, per unionem aliquid fit unum. Sed ex unione animae et corporis in nobis fit aliquid unum in persona et natura, ex unione autem divinae et humanae naturae fit aliquid unum solum in persona. Ergo maior est unio animae ad corpus quam divinae naturae ad humanam. Et sic unio de qua loquimur, non importat maximam unitatem.

Sed Contra

Sed contra est quod Augustinus dicit, in I de Trin., quod homo potius est in filio quam filius in patre.

Filius autem est in patre per unitatem essentiae, homo autem est in filio per unionem incarnationis. Ergo maior est unio incarnationis quam unitas divinae essentiae. Quae tamen est maxima unitatum. Et sic, per consequens, unio incarnationis importat maximam unitatem.

Corpus

Respondeo dicendum quod unio importat coniunctionem aliquorum in aliquo uno. Potest ergo unio incarnationis dupliciter accipi, uno modo, ex parte eorum quae coniunguntur; et alio modo, ex parte eius in quo coniunguntur. Et ex hac parte huiusmodi unio habet praeeminentiam inter alias uniones, nam unitas personae divinae, in qua uniuntur duae naturae, est maxima. Non autem habet praeeminentiam ex parte eorum quae coniunguntur.

Ad 1

Ad primum ergo dicendum quod unitas personae divinae est maior quam unitas numeralis, quae scilicet est principium numeri. Nam unitas divinae personae est unitas per se subsistens, non recepta in aliquo per participationem, est etiam in se completa, habens in se quidquid pertinet ad rationem unitatis. Et ideo non competit sibi ratio partis, sicut unitati numerali, quae est pars numeri, et quae participatur in rebus numeratis. Et ideo quantum ad hoc unio incarnationis praeeminet unitati numerali, ratione scilicet unitatis personae. Non autem ratione naturae humanae, quae non est ipsa unitas personae divinae, sed est ei unita.

Ad 2

Ad secundum dicendum quod ratio illa procedit ex parte coniunctorum, non autem ex parte personae in qua est facta unio.

Ad 3

Ad tertium dicendum quod unitas divinae personae est maior unitas quam unitas et personae et naturae in nobis. Et ideo unio incarnationis est maior quam unio animae et corporis in nobis.

Ad 4

Quia vero id quod in contrarium obiicitur falsum supponit, scilicet quod maior sit unio incarnationis quam unitas personarum divinarum in essentia, dicendum est ad auctoritatem Augustini quod humana natura non est magis in filio dei quam filius dei in patre sed multo minus, sed ipse homo, quantum ad aliquid, est magis in filio quam filius in patre; inquantum scilicet idem supponitur in hoc quod dico homo, prout sumitur pro christo, et in hoc quod dico, filius dei; non autem idem est suppositum patris et filii.