Such prohibited relationships are commonly referred to as incest or incestuous, though that term does not appear in the biblical and rabbinic sources.
[4] As with the case of a man's own daughter, the shortness of the list in Leviticus 20, and especially of that in Deuteronomy, are explained by classical Jewish scholarship as being due to the obviousness of the missing prohibitions.
[5][6] Apart from the case of a man marrying his daughter, the list in Leviticus 18 roughly produces the same rules as were followed in early (pre-Islamic) Arabic culture.
Thus in several prominent cases in the Torah, the incest rules are ignored in favour of marriage to a close relative; Jacob is described as having married his first wife's sister.
[7][8][9] Some secular Biblical scholars have instead proposed that forbidding incest with a daughter was originally in the list, but was then accidentally left out from the copy on which modern versions of the text ultimately depend, due to a mistake by the scribe.
[24] On the other hand, those relationships which were prohibited due to qualifying as seconds, and so forth, were regarded as wicked, but still valid;[23] while they might have pressured such a couple to divorce, any children of the union were still seen as legitimate.
[25] Thus Jacob ben Meir deliberately wrecked a wedding, stopping the marriage and spoiling the banquet and celebrations, because the man would have married his father-in-law's wife.
[5] In the 11th century, two Karaite reformists rejected the principle that a marriage was a true and full union, instead arguing that the only relationships that should be forbidden were those analogous to those in the biblical prohibitions.