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However, the first exposition, which is taken as explaining what preceded, is better.160 Then when he says, who are born not from blood, he shows the way in which so great a fruit is conferred on men. For since he had said that the fruit of the light’s coming is the power given to men to become the sons of God, then to forestall the supposition that they are born through a material generation he says, not from blood. And although the word “blood” (sanguis) has no plural in Latin, but does in Greek, the translator [from Greek into Latin] ignored a rule of grammar in order to teach the truth more perfectly. So he does not say, “from blood,” in the Latin manner, but “from bloods” (ex sanguinibus). This indicates whatever is generated from blood, serving as the matter in carnal generation. According to the Philosopher [On the Generation of Animals, 1, c 18, 726a26-8], “semen is a residue derived from useful nourishment in its final form.” So “blood” indicates either the seed of the male or the menses of the female.
The cause moving to the carnal act is the will of those coming together, the man and the woman. For although the act of the generative power as such is not subject to the will, the preliminaries to it are subject to the will. So he says, nor from the desires of the flesh, referring to the woman; nor from man’s willing it, as from an efficient cause; but from God. It is as though he were saying: They became sons of God, not carnally, but spiritually.
According to Augustine, “flesh” is taken here for the woman, because as the flesh obeys the spirit, so woman should obey man. Adam (Gn 2:23) said of the woman, “This, at last, is bone of my bones.” And note, according to Augustine, that just as the possessions of a household are wasted away if the woman rules and the man is subject, so a man is wasted away when the flesh rules the spirit. For this reason the Apostle says, “We are not debtors to the flesh, so that we should live according to the flesh” (Rom 8:12). Concerning the manner of this carnal generation, we read, “In the womb of my mother I was molded into flesh” (Wis 7:1).
161 Or, we might say that the moving force to carnal generation is twofold: the intellectual appetite on the one hand, that is, the will; and on the other hand, the sense appetite, which is concupiscence. So, to indicate the material cause he says, not from blood. To indicate the efficient cause, in respect to concupiscence, he says, nor from the desires of the flesh [ex voluntate carnis, literally, “from the will of the flesh”], even though the concupiscence of the flesh is improperly called a “will” in the sense of Galatians (5:17), “The flesh lusts against the spirit.” Finally, to indicate the intellectual appetite he says, nor from man’s willing it. So, the generation of the sons of God is not carnal but spiritual, because they were born from God. “Every one who is born from God conquers the world” (1 Jn 5:4).
162 Note, however, that this preposition de (“of,” or “from”), always signifies a material cause as well as an efficient and even a consubstantial cause. Thus we say a blacksmith makes a knife de ferro (“from” iron), and a father generates his son de seipso (“from” himself), because something of his concurs somehow in begetting. But the preposition a (“by”) always signifies a moving cause. The preposition ex (“from,” or “by”)—[in the sense of “out of” or “by reason of”]—is taken as something common, since it implies an efficient as well as a material cause, although not a consubstantial cause.
Consequently, since only the Son of God, who is the Word, is “of” (de) the substance of the Father and indeed is one substance with the Father, while the saints, who are adopted sons, are not of his substance, the Evangelist uses the preposition ex, saying of others that they are born from God (ex Deo), but of the natural Son, he says that he is born of the Father (de Patre).
163 Note also that in the light of our last exposition of carnal generation, we can discern the difference between carnal and spiritual generation. For since the former is from blood, it is carnal; but the latter, because it is not from blood, is spiritual. “What is born from flesh is itself flesh; and what is born from Spirit is itself spirit” (below 3:6). Again, because material generation is from the desires of the flesh, i.e., from concupiscence, it is unclean and begets children who are sinners: “We were by nature children of wrath” as it says in Ephesians (2:3). Again, because the former is from man’s willing it, that is, from man, it makes children of men; but the latter, because it is from God, makes children of God.
164 But if he intends to refer his statement, he gave them power, to baptism, in virtue of which we are reborn as sons of God, we can detect in his words the order of baptism: that is, the first thing required is faith, as shown in the case of catechumens, who must first be instructed about the faith so that they may believe in his name; then through baptism they are reborn, not carnally froin blood, but spiritually from God."-
Excerpt from Aquinas commentary on the Gospel of John (This is from his commentary on John 1:13 "Who are born, not of blood, nor of the will of the flesh, nor of the will of man, but of God qui non ex sanguinibus, neque ex voluntate carnis, neque ex voluntate viri, sed ex Deo nati sunt."