Sefer HaChinuch ספר החינוך

To not reveal the nakedness of the mother: That a son not reveal the nakedness of the mother, as it is stated (Leviticus 18:7), “it is your mother, do not reveal her nakedness.” About the reason for relatives, Rambam, may his memory be blessed, wrote (Guide for the Perplexed 3:49) that the matter is because the Torah distanced sexuality from people besides that which is needed for being fruitful and multiplying, or for the commandment. Therefore, the Torah forbade [close] relatives to us — as stumbling is found with them more, since they are always found (proximate). And Ramban, may his memory be blessed, said (Ramban on Leviticus 18:6) that this reason is very weak — that the Torah should make one liable for excision for one of these because of their being always found near him; and on the other hand, permit a man to marry one hundred women — or a thousand — if he wants. [Rather,] he, may his memory be blessed, said — and this is his language: “But according to logic, there is a matter of a secret from the foundations of creation that is implanted in the soul, and it is included in the ‘secret of intercalation’ (a play on words that can also mean conception), that we have already hinted to.” To here [are his words]. And I have seen that Rambam, may his memory be blessed, wrote another reason about the matter (Guide for the Perplexed 3:49): From the angle of the simple understanding — according to my opinion — did he say [it is] because the Torah distanced [it], that he should not be brazen-faced to have intercourse with the woman that he is to treat with honor. And he pushes [it] to explain most of [the sexual prohibitions] from this angle, meaning to say on account of the trait of shame — all as it appears in his book; the matter would be [too] lengthy if we came to write it all.

From the laws of the commandment is that which they, may their memory be blessed, said (Sanhedrin 54a) that the prohibition of the mother is even if she was raped by his father, as behold she is his mother in any event. And so [too,] that which they said (Sanhedrin 53a) that one who has intercourse with his mother that is the wife of his father — meaning that she was not raped by him — is liable for two, on account of his mother and on account of the wife of his father. And there is no distinction in this whether he had intercourse in his father’s lifetime or after his death, as she is always called the wife of his father, and [so] he is liable for two, as we said. And so [too,] that which our Rabbis, may their memory be blessed, forbade (Yevamot 21a) as a fence of this prohibition of the mother, the mother of his mother — meaning to say, his grandmother — and likewise the mother of his grandmother, going way back even to a thousand generations if it was possible to see them all; they are all forbidden by the words of the [Rabbis]. And so [too] they forbade the mother of his father — meaning to say, his grandmother from the father’s side — as a fence for the mother, and likewise the mother of his grandmother, going way back. And so [too,] they forbade just the mother of the father of his father, and just the mother of the father of his mother only, as a fence of this [prohibition] of the mother. And there are some (Rashi on Yevamot 21a s.v. dalet nashim) that explained that these are also forbidden way back. And if you ask why the prohibition extends way back with the first ones — according to the first opinion — more than with these last two; the reason is because the first ones approach the prohibition of the mother which is the main prohibition, as behold they have a forbidden grandmother, which is the mother of the mother or the mother of the father. And therefore is it fitting to decree about them even going way back. But with these last two, they do not have a forbidden grandmother. And since the matter does not approach the mothers which is the main prohibition, they, may their memory be blessed saw (said) that it is enough that we decree with these two alone. And “the words of the Sages are grace.” And these that are forbidden rabbinically are called secondary ones — meaning to say, they are secondary to those sexual prohibitions that were prohibited by the Torah.

And this prohibition is practiced in every place and at all times. And one who transgresses this and lays with his mother volitionally is liable for excision, and is stoned if there are witnesses; inadvertently, he is liable to bring a fixed sin-offering at the time of the [Temple]. And if he laid with one of the secondary ones, he is struck with lashes of rebellion. And this is from the sexual prohibitions that all people of the world were prohibited more generally. But there is a distinction between Israel and the rest of the nations regarding the secondary ones; as Israel made a fence regarding the secondary ones, but [with] the rest of the nations, only the mother alone is forbidden to them.