Intrinsic Nature of Affinity According to the Common Opinion of Doctors

To n. 1002.

As consanguinity is a natural bond from communion of blood, so affinity is a natural bond from the commingling of seeds.

St. Thomas, 4 Sent., d. 41, q. 1, a. 1, qc. 4 ad 2: "Man and woman become one flesh through the mingling of seeds. Hence, however much someone invade the enclosures of modesty, unless commingling of seeds follows, affinity is not contracted from this." "It must be said," says the same St. Thomas, Supplement, q. 55, a. 3 ad 3, "that intercourse against nature does not have commingling of seeds, which can be a cause of generation, and therefore from such intercourse affinity is not caused."

St. Bonaventure, book 4 Sent., d. 41, q. 1, response: "Affinity is generally contracted through every coitus in which there is commingling of seeds. And the reason for this is that it is not contracted except by reason of union; but then they are said to become one flesh when, through the commingling of seeds, the consummation of that act is made."

Pyrrhus Corradus: "Through the conjugal bond, with the power for intercourse, husband and wife are united in the notion of one principle for generation, and thence affinity arises between them... It is called affinity only when man and woman are made one and the same flesh; whence the blood of one is said to touch the blood of the other." The blood of one, the blood of the other: certainly not the blood of the man, but the seed, which by a more modest word was called blood by the ancients; and accordingly neither the blood of the woman, but the seed, or ovule. Dispens. Apost., book 8, c. 7, nn. 18, 19, 20.

And by this same expression of blood and commingling of blood affinity is explained in C. Lex Divinae 18, can. 27, q. 2. Commingling of blood, that is, of the seed of each, man and woman, since commingling is not made when only one emits seed.

But still more clearly the same Pyrrhus Corradus, Praxis Dispensationum Apostolicarum, book 7, c. 1, n. 18: "for causing affinity the concurrence of the seed of each is required."

Perez, De Matrimonio, disp. 31, s. 1, nn. 1, 2, says: "Affinity arises from this alone, that the blood of the man, that is, seed, which by a more modest name is called blood because it is proximately made from blood, and the menstrual blood of the woman, or her seed, are commingled within the womb of the woman herself... Without this commingling, those who are otherwise joined do not become one flesh... to become one flesh or one blood is to become a principle of itself sufficient for one flesh and offspring."

The Salmanticenses, Cursus Theologiae Moralis, De Matrimonio, tract. 9, c. 4, punct. 1, nn. 5-6, say: "By that intercourse, and not another, marriage is consummated, by which the spouses become one flesh and affinity results with the consanguineous relatives; but even if the man penetrates the enclosure of modesty, unless the commingling of the seed of each follows, neither does affinity follow, nor do the spouses become one flesh. Therefore unless the seed-emission of the woman is joined with the seed-emission of the man within the vessel, marriage is not consummated."

St. Alphonsus in his Moral Work takes up almost the whole moral doctrine of the ancients. Now in book 6, n. 1075, he treats the question whether, for affinity to be contracted, it is required that intercourse be consummated on the part of both, man and woman, and he responds: "It is certain that the seed-emission of the man is required; whether, however, the seed-emission of the woman is also required, Pontius, Palaus, etc., deny... The reason is because, according to the opinion of very many, female seed is not necessary for generation. But the second opinion, also probable, affirms it, and this is held by St. Bonaventure, Innocent, etc.... The reason is both because, according to the opinion of many others, the seed of the woman also is required for generation, and because (and this reason presses more strongly) affinity is not contracted unless those having intercourse become one flesh; but they do not become one flesh unless the seeds are commingled. And this opinion is greatly strengthened from the authority of St. Thomas," etc. Hence St. Alphonsus, ibid., n. 918, says that this latter opinion is to be held in practice.