(8) The nakedness of thy father's wife.--Whilst the former prohibition refers to the son's own mother, this law is directed against illicit commerce with his stepmother. Here we have an instance where the phrase "to uncover the nakedness" denotes both illicit commerce and incestuous marriage. Accordingly the administrators of the law during the second Temple defined it as follows; a man's father's wife is for ever prohibited, whether she be simply betrothed or married to his father, whether she be divorced or not, whether she be a widow or not; all connection with her on the part of the father's son is forbidden. If he lie with her while her husband is alive, he is doubly guilty, first, because she is near of kin, and secondly, because she is another man's wife. This, therefore, includes the sin of Reuben with Bilhah, his father's concubine (Genesis 35:22), and of Absalom with the wives of his father (2Samuel 16:20-23; 1Kings 2:17), which was not incestuous marriage but adultery, since their husbands were alive and the wives were not divorced from them, as well as the sin practised among some of the Christians in Corinth, which consisted in sons actually marrying their divorced stepmothers in the lifetime of their fathers, and which the Apostle denounced with such severity (1Corinthians 5:1-4). Among the ancient Arabs, marriages with stepmothers were common, and to this day among some tribes in Africa, when a father is unable through advanced age to attend to his young wives, he voluntarily gives them over to his eldest son. The Koran, however, like the Mosaic law, proscribes these marriages (Koran, 4:27).

18:1-30 Unlawful marriages and fleshly lusts. - Here is a law against all conformity to the corrupt usages of the heathen. Also laws against incest, against brutal lusts, and barbarous idolatries; and the enforcement of these laws from the ruin of the Canaanites. God here gives moral precepts. Close and constant adherence to God's ordinances is the most effectual preservative from gross sin. The grace of God only will secure us; that grace is to be expected only in the use of the means of grace. Nor does He ever leave any to their hearts' lusts, till they have left him and his services.The nakedness of thy father's wife shalt thou not uncover,.... That is, who is indeed a man's father's wife, but not his own mother, but a stepmother or mother-in-law; or otherwise this law would coincide with the former; a man lying with such an one is accursed by the law, Deuteronomy 27:23; such an incestuous copulation was that of Reuben with Bilhah, and Absalom with his father's concubines or secondary wives, and such an incestuous marriage was that of the Corinthians, 1 Corinthians 5:1; and of Antiochus Soter, king of Syria, with Stratonice his mother-in-law (c): and even it was criminal to do this after a father's death, as Jarchi interprets it; and though she was only betrothed, and not married, and the father dead after such betrothing; as Gersom; nay, though she was divorced by the father, yet was not lawful for the son to have, no, not after his death:

it is thy father's nakedness; being espoused to him, and so one flesh with him; and the son and father being one flesh, such a mixture must be unlawful; and since then the nakedness of a mother-in-law is the father's, then surely that of an own mother's must be so likewise, which confirms a sense given of it in Leviticus 18:7, Cicero (d) exclaims against such marriages as incredible and unheard of, as instances of unbridled lust and singular impudence.

(c) Vid. Julian. in Misopogon, p. 72, &c. (d) Orat. 14. pro A. Cluentio Avito.