Commentaria in Leviticum
[In his reply to] Augustine’s questions, chapter 6, [Gregory says]: “A certain earthly law in the Roman commonwealth permits that a brother and sister, or the son and daughter of two brothers german, or of two sisters, be joined together; but we have learned by experience that offspring does not usually grow up from such a marriage.”
8. THOU SHALT NOT UNCOVER THE NAKEDNESS OF THY FATHER’S WIFE—as if to say: Thou shalt not have to do with thy stepmother.
FOR IT IS THE NAKEDNESS OF THY FATHER.—The Hebrew is, for it is the nakedness of thy father, as if to say: Thy father has laid her bare and known her: hence it is altogether unseemly and dishonourable that thou shouldst know her.
9. THE NAKEDNESS OF THY SISTER BY THY FATHER (who has the same father as thou, but another mother), OR BY THY MOTHER (who has the same mother as thou, but another father), BORN AT HOME OR ABROAD, THOU SHALT NOT UNCOVER.—“Born at home” means born of lawful marriage and a wife; for the wife alone belongs lawfully to the husband’s house: hence “born abroad” is she who was born of a concubine or mistress. So Rudolph and Abulensis. But you will take the words more simply as they sound; for lest anyone think that only marriage with a sister born at home was forbidden, the law added “or born abroad”: such is, for instance, she whom the mother had borne to a former husband, and with whom she had come into the house when she married this second husband, as if to say: A uterine sister, even born abroad and outside the house, no one shall wed. So St. Augustine, whose also is this noble saying, book 15 [On the City of God]: “The more ancient the intermarriage of sisters and brothers was under compelling necessity, the more condemnable it became afterwards, religion forbidding.” So Amnon expiated by death his incest with his sister Thamar, 2 Kings 13:29.
10. THE NAKEDNESS OF THY SON’S DAUGHTER, OR OF THY DAUGHTER’S DAUGHTER THOU SHALT NOT UNCOVER, BECAUSE IT IS THY OWN NAKEDNESS—because, namely, the granddaughter descends from thee in a direct line, and is therefore reckoned as one with thee, so that if thou uncover her nakedness thou dost uncover both thine own and hers; and if thou abuse her unseemly parts, thou dost act as unworthily as if thou abusedst thine own. So Abulensis and Oleaster.
11. THE NAKEDNESS OF THY FATHER’S WIFE’S DAUGHTER, WHOM SHE BORE TO THY FATHER, WHO IS THY SISTER, THOU SHALT NOT UNCOVER—as if to say: The daughter of thy stepmother, who is to thee as a sister by the father, thou shalt not wed nor know.
12. THOU SHALT NOT UNCOVER THE NAKEDNESS OF THY FATHER’S SISTER (as if to say: thou shalt not wed nor know thy aunt), BECAUSE SHE IS THY FATHER’S FLESH—because, namely, she is of thy father’s blood. The Hebrew is, because she is the remainder of thy father: for the father and the aunt were cut off from one parent and one flesh, of which one part the parent has, the other his sister, who is the aunt of the nephew; as if to say: Because the aunt is nearest akin to thy father, so that she seems to be one flesh with him: hence it is not fitting that thou uncover her nakedness, as it is not fitting that thou uncover the nakedness of thy father. Abulensis thinks that here marriage of an uncle with a niece is equally forbidden, as that of an aunt with a nephew is forbidden: for the degree of consanguinity is the same in both. But Cajetan more rightly thinks that it is not forbidden, because it is not expressed, as all the rest are, which are here so exactly and minutely described by Moses. Hence there is an example of such a marriage in Othoniel and Axa, Judges 1:13.
You will ask why God forbade marriage with an aunt rather than with an uncle?
I answer: The reason is that since the husband is the head of the wife, if the nephew married his aunt, she would have to be subject to the nephew; but this is unseemly: whereas in the other case the niece is subject to the uncle, which is more seemly.
[13. Thou shalt not uncover the nakedness of thy mother’s sister...]; 14. NOR SHALT THOU APPROACH TO HIS WIFE, that is, thou shalt not wed or know his wife, even when he himself is dead. Hence Moses adds in explanation:
NOR SHALT THOU APPROACH TO HIS WIFE.—The preceding degrees hitherto forbidden were of consanguinity; here begin the degrees of affinity formerly forbidden, which spring from carnal intercourse, as the former of consanguinity are derived from the same common origin, namely from the same father, grandfather or great-grandfather.
15. THOU SHALT NOT UNCOVER THE NAKEDNESS OF THY DAUGHTER-IN-LAW—thou shalt not have to do with, nor be joined in marriage to, thy son’s wife, even when he is dead.
NEITHER SHALT THOU UNCOVER HER SHAME—that is, the nakedness, namely of the daughter-in-law, not of the son, as Abulensis will have it. It is plain from the Hebrew.
16. THOU SHALT NOT UNCOVER THE NAKEDNESS OF THY BROTHER’S WIFE: BECAUSE IT IS THE NAKEDNESS OF THY BROTHER.—Except if the brother die without children: for then not only may, but must, the brother of the dead man take her to wife, to raise up seed and children to his brother, as is commanded in Deuteronomy 25:5. Here, therefore, it is only forbidden that a brother wed his brother’s wife, if children survive of her, or if she departed from her brother while he was still living, by divorce. So St. Augustine, question 58. Hence it is plain that Henry VIII, king of England, wrongly wished by this law to repudiate his wife Catherine, as though the marriage contracted with her were void, because she had previously been married to Arthur, Henry’s brother; for Arthur had had no offspring by Catherine, and so by the law of Deuteronomy 25:5 Henry ought to have wed Catherine, to raise up seed by her to his brother Arthur. Henry pretended that this law belongs to the law of nature, and so binds Christians even now: whether this be true I shall discuss at verse 18. Wrongly also Tertullian, in his book On Monogamy, chapter 7, understands by “brother” here any Jew, or one not a foreigner, as if the law here forbade the wife of a dead Jew to be taken by any other Jew, and enjoined monogamy on him; for it is certain that “brother” is here taken properly: for so are properly taken father, mother, sister and the other names of kinship which are here recounted.
17. THOU SHALT NOT UNCOVER THE NAKEDNESS OF THY WIFE AND OF HER DAUGHTER.
Thou shalt not contract marriage with the mother and her daughter, that is, thy step-daughter, so as to have both as wife, whether together or successively.
HER SON’S DAUGHTER, AND HER DAUGHTER’S DAUGHTER THOU SHALT NOT TAKE TO UNCOVER HER SHAME (in Hebrew, nakedness), BECAUSE THEY ARE HER FLESH (because, namely, they are joined by blood most nearly to thy wife, and therefore it is altogether unseemly that thou shouldst uncover their nakedness), AND SUCH COPULATION IS INCEST.—The Hebrew is, it is wickedness; the Septuagint, it is impiety; the Chaldee, it is a counsel of sinners.
Further, incest is the abuse of blood relations or relations by affinity, and is therefore a distinct species of lust, for a threefold reason, says St. Thomas, question 154, article 9: “First, because naturally man owes a certain honour to parents and kindred, so much so that among the ancients, as Valerius Maximus relates, it was not lawful for a son to bathe together with his father, nor for them to see one another naked; secondly, because persons joined by blood must needs converse with one another, and so would continually have an occasion of lust and grow too soft; thirdly, because by this the multiplication of friends would be hindered, as St. Augustine teaches, book 15 On the City of God, chapter 16. To which Aristotle adds, fourthly, in book 2 of the Politics, that since man naturally loves one who is of his blood, if there were added the love which comes from venereal union, there would be excessive ardour of love, and a very great incentive to lust, which is repugnant to chastity.”
Hence incest is said to be, as it were, “not chaste” (non castus); though others derive it from cestus, the girdle with which the wife, when conjugal faith was given to her, was girded by the husband, or rather ungirded, as Phyllis says in Ovid in her epistle to Demophoon:
To whom my virginity was offered under ill-omened birds, and my chaste girdle was loosed by a treacherous hand. Incest, then, is unlawful intercourse, to which the girdle, the token of lawful marriage, cannot be applied because of consanguinity.
Finally, St. Augustine, and it is found in 32, question 7, chapter “Adulterii”: “The evil of adultery,” he says, “surpasses fornication, but is surpassed by incest. For it is worse to lie with one’s mother than with another man’s wife.”