The next thirteen verses contain the law of incest, or the prohibited degrees of marriage. The positive law of marriage, as implanted in the human heart, would be simply that any man of full age might marry any woman of full age, provided that both parties were willing. But this liberty is at once controlled by a number of restrictions, the main purpose of which is to prevent incest, which, however much one nation may come to be indifferent to one form of it, and another to another, is yet abhorrent to the feelings and principles of mankind. The Hebrew restrictive law is contained in one verse. None of you shall approach to any that is near of kin to him to uncover their nakedness: I am the Lord. All that follows (Leviticus 18:9-18) is simply an amplification and an explanation of the words, near of kin to him. These words would be literally rendered, flesh of his flesh, or less probably, remainder of his flesh. They certainly include within the compass of their meaning those that are near by affinity, as much as those that are near by consanguinity. This is proved by the instances given below, where no difference is drawn between blood relations and relations by marriage, the latter being supposed to become the former, in consequence of the marriage that has taken place. Nearness of kin is generally counted by "degrees;" but, unfortunately, this word is itself ambiguous, for it is used in different senses by canonists and by civilians. So far as the direct line is concerned, the same method of calculation is observed by the canon and by the civil law. There is one degree from the son to the mother, two degrees to the grandmother; one degree from the father to the daughter, two degrees to the granddaughter. But this is not so with the collateral lines. A brother and sister, for example, are regarded by the canon law as in the first degree of kinship, because there is only one step to the father, in whom their blood meets; but the civil lawyers consider them as being in the second degree, because, as they calculate, there is one step from the brother to the father, and a second from the father to the sister. An aunt is, according to the canonists, in the second degree of propinquity, because there are two steps from her nephew to his grandfather, who is likewise her father, in whom their blood unites; but, according to the civilian's calculation, there are three steps, namely, from her nephew to his grandfather, two steps, and a third from that grandfather to his daughter the aunt; and therefore the aunt and nephew are in the third degree of propinquity. The case of an uncle and niece is exactly the same as that of a nephew and aunt. On the same principle, according to the canonists, first cousins are in the second degree of kinship; according to the civilians, in the fourth. Propinquity by affinity is calculated in just the same way; so that the brother's wife is in the same degree of relationship as the brother, and wife's sister as the sister by blood. In the code before us, confirmed by that in Deuteronomy, marriage is forbidden with the following blood relations: mother (verse 7), daughter (verse 17), sister (verse 9; Le Deuteronomy 20:17; Deuteronomy 27:22), granddaughter (verse 10), aunt (verses 12, 13; Le Deuteronomy 20:19); and with the following relations by affinity: mother-in-law (verse 17; Deuteronomy 20:14; Deuteronomy 27:23), daughter-in-law (verse 15; Le Deuteronomy 20:12), brother's wife (verse 16; Le 20:21), stepmother (verse 8; Deuteronomy 20:11; Deuteronomy 22:30; see Genesis 49:4; 1 Corinthians 5:1), stepdaughter and step-granddaughter (verse 17), uncle's wife, or aunt by marriage (verse 14; Le Deuteronomy 20:20); putting aside for the present the question of who is meant by a wife to her sister, in verse 18. In these lists, according to the canonists' method of reckoning, the mother, the daughter, and the sister are related in the first degree of consanguinity; the wife's mother, the wife's daughter, the stepmother, the daughter-in-law, the brother's wife, are related in the first degree of affinity. The granddaughter and the aunt are in the second degree of consanguinity; the wife's granddaughter and the uncle's wife in the second degree of affinity. According to the civilians' reckoning, the following would be the degrees of propinquity:—The mother and the daughter would be in the first degree of consanguinity; the wife's mother, the wife's daughter, the stepmother, the daughter-in-law, would be in the first degree of affinity. The sister and the granddaughter would be in the second degree of consanguinity; the brother's wife and the wife's granddaughter would be in the second degree of affinity. The aunt by blood would be in the third degree of consanguinity, and the uncle's wife, or aunt by marriage, would be in the third degree of affinity. The wife's sister, with regard to whom it is questioned whether she is referred to or not in verse 18, is in the first degree of affinity (a man's wife being regarded as himself) according to the canonists' reckoning, and in the second according to the civilians'. There is no mention made in the code of the grandmother, the niece, and the cousin-german. All of these are in the second degree of consanguinity according to the canon law; and according to the civil law, the grandmother would be in the second degree, the niece in the third, and the cousin-german in the fourth. It may reasonably be supposed that by the expression, None of you shall approach to any that is near of kin to him, to uncover their nakedness, intercourse is forbidden between all those who are related by consanguinity or affinity in the first and second degrees according to the canonists' reckoning (except cousins-german, whose case is considered below); in the first, second, and third degrees 'according to the civilians' method of calculating; whether they are mentioned by name in the list or not. It is only by implication, not by direct injunction, that marriage even with a daughter is forbidden (verse 17).
Incest with a stepmother is placed next after that with a mother. On account of the unity caused by marriage ("they shall be one flesh," Genesis 2:24), the stepmother's nakedness is the father's nakedness. The tie of affinity is thus declared to be similar in its effects to the tie of consanguinity. Reuben's sin, by which he forfeited his birthright, is connected with this offense, but is of a more heinous character, as his father was alive at the time of his transgression (Genesis 49:4). It is one of the sins which Ezekiel enumerates as those which brought the judgment of God on Israel (Ezekiel 22:10). "That one should have his father's wife" is declared by St. Paul to be "such fornication as is not named among the Gentiles," and to call for the excommunication of the offender (1 Corinthians 5:1-5). Adonijah's marriage with Abishag, so strongly resented by Solomon on political grounds, is not denounced as morally reprehensible, probably because Abishag was not the wife of David in such a way as to cause the marriage with his son to be abominable in the eye of the law (cf. 1 Kings 1:4 with Amos 2:7). Absalom's" going in unto his father's concubines" was regarded as the final act which made reconciliation with his father impossible (2 Samuel 16:22; 2 Samuel 20:3). The history of the Church has shown that marriage with the stepmother has had to be again and again prohibited by Council after Council (see Smith and Cheetham's 'Dictionary of Antiquities,' s.v. 'Prohibited Degrees').