Commentaria in Summam contra Gentiles, Liber IV

The sixth point that St. Thomas premises ¹ is that the formation of Christ's body, although it is common to the whole Trinity, is nevertheless fittingly attributed to the Holy Spirit.

I. This is proved, first, from the fittingness with respect to the Incarnation of the Word. The Incarnation of the Word of God is like the vocal expression of our word. But this is made through our spirit, by which the voice of our word is formed. Therefore, the formation of His flesh is likewise fittingly said to be made through the Spirit of the Son of God.

The major premise is proved. For just as our word, conceived in the mind, is invisible, while uttered externally by the voice it becomes sensible: so the Word of God, according to eternal generation, exists invisibly in the heart of the Father, but through the Incarnation has been made sensible to us.

Second, it is proved from the fittingness with respect to human generation. The active power which is in human semen, drawing to itself the matter which has been from the mother ², works through the spirit, because it is founded in the spirit: on account of whose retention the semen must be foamy and white. Therefore, etc.

Concerning the matter of this argument, enough has been said in the preceding passages: namely, in this book, ch. xlv, and in Book II, ch. lvii and lxxxix.

Third, it is proved from the fittingness with respect to the moving cause on the part of God. The moving cause of the Incarnation of the Word could be none other than the love of God for man, whose nature He willed to join to Himself in the unity of the person. But in the divine, it is the Holy Spirit who proceeds as love. Therefore, etc.

Fourth, it is proved from the cause lying on the part of human nature. In Sacred Scripture every grace is wont to be attributed to the Holy Spirit: for what is given freely seems to be conferred out of the love of the giver. But no greater grace has been conferred upon man than that he should be united to God in person. Therefore, etc.

II. For the evidence of this conclusion, it must be considered, from the doctrine of St. Thomas in the Third Book, d. iv, q. 1, a. 1 ³; and it is the mind of Dionysius, ch. II ⁴ of the Div. Nom., that every operation of God upon a creature belongs indeed to the whole Trinity, but nevertheless each one is more appropriable to this person than to that one, according as in it the attribute which is appropriated to that person is more manifested. Hence, since goodness is appropriated to the Holy Spirit, those works in which the divine goodness is chiefly manifested are fittingly attributed to the Holy Spirit. Of this number, moreover, is the assumption of human nature into the unity of the divine person: since this is above every debt and condition of human nature. Therefore it was well said in the conclusion that, although the formation of Christ's body is common to the whole Trinity, it is nevertheless fittingly attributed to the Holy Spirit.

It must also be observed that St. Thomas makes mention only of the formation of Christ's body for this reason, that, as is said in the Tertia, q. xxxii, a. 1 ⁵, the work of conception, which is common to the three persons efficiently, is attributed to the single persons according to some particular mode. For it is attributed to the Father as regards the authority which He has with respect to the Son, who through such a conception assumed human nature to Himself; to the Son, moreover, it is attributed as regards the assumption of the flesh itself; but to the Holy Spirit, as regards the formation itself of the assumed body.