The mishna explains: A prohibition resulting from a mitzva is referring to secondary forbidden relatives, e.g., one’s son’s daughter-in-law, who are forbidden by rabbinic law. The Sages instituted this prohibition in order to distance people from incest. The Gemara explains: And why do the Sages call it a prohibition resulting from a mitzva? Because it is a mitzva to listen to the words of the Sages.

The mishna continues: A prohibition stemming from sanctity is referring to the marriage of a widow to a High Priest, or to the marriage of a divorcée or of a woman who has performed ḥalitza [ḥalutza], to an ordinary priest. The Gemara explains: And why do the Sages call it a prohibition stemming from sanctity? As it is written with regard to the priests: “They shall be sanctified to their God” (Leviticus 21:6).

And it is taught in a baraita with regard to this mishna: Rabbi Yehuda switches the terms; he refers to the marriage of a divorcée or of a ḥalutza to a priest as a prohibition resulting from a mitzva, and to the marriage of secondary relatives as a prohibition stemming from sanctity.

The Gemara infers: He switches the terms but does not dispute the halakha; but even Rabbi Yehuda agrees that if a priest marries a ḥalutza and then dies, she requires ḥalitza. And if it enters your mind that Rabbi Yehuda holds in accordance with the opinion of Rabbi Akiva, this poses a difficulty; after all, those liable for violating prohibitions, according to the opinion of Rabbi Akiva, are considered like those liable to receive karet, and those liable to receive karet are not capable of performing ḥalitza and levirate marriage, as the mitzva of levirate marriage does not apply to such relationships.

The Gemara resolves the difficulty: Rabbi Yehuda does hold in accordance with the opinion of Rabbi Akiva, and as for his comment with regard to the definitions of a prohibition resulting from a mitzva and a prohibition stemming from sanctity, he merely stated his opinion in accordance with the statement of the first tanna; but he himself does not hold accordingly.

§ When Rav Yitzḥak came from Eretz Yisrael to Babylonia, he taught a baraita as we learned in the mishna: Rabbi Yehuda says: One who engages in intercourse with his mother who is also his father’s wife is liable to bring only one sin-offering, due to the fact that she is his mother, whether or not her marriage to his father was legitimate.

The Gemara asks: And what is the reason for this? Abaye says: It is because the verse states: “The nakedness of your father and the nakedness of your mother you shall not uncover: She is your mother; you shall not uncover her nakedness” (Leviticus 18:7). Rabbi Yehuda derives from this verse that you shall render him liable due to the fact that she is his mother, but you shall not render him liable to bring another sin-offering due to the fact that she is his father’s wife.

The Gemara asks: If that is so, it should be derived from the verse: “The nakedness of your father’s wife you shall not uncover; it is your father’s nakedness” (Leviticus 18:8), that you shall render him liable due to the fact that she is his father’s wife, but you shall not render him liable due to the fact that she is his mother.

But if so, with regard to one’s mother who is his father’s wife, it is found that the verse stands, i.e., is written, here, with regard to intercourse with one’s mother, and excludes liability due to the fact that she is his father’s wife, and stands there, with regard to intercourse with one’s father’s wife, and excludes liability due to the fact that she is his mother. The result now is that for his mother who is not his father’s wife he is rendered liable, and for his father’s wife who is not his mother he is rendered liable, but for his mother who is his father’s wife he is not rendered liable at all, which seems absurd.

And furthermore, according to the Rabbis too, who disagree with Rabbi Yehuda and hold that one who engages in intercourse with his mother who is married to his father is liable to bring two sin-offerings, isn’t the verse: “She is your mother,” written; i.e., how do they explain this verse? Rather, that phrase is necessary for the halakha derived from it by Rav Sheisha, son of Rav Idi, as the Gemara will soon explain (54a). Therefore, according to Rabbi Yehuda as well, that phrase is necessary for the halakha derived by Rav Sheisha, son of Rav Idi, and Rabbi Yehuda’s opinion in the mishna cannot be derived from it.

Rather, Rav Aḥa, son of Rav Ika, says that there is a different source for Rabbi Yehuda’s opinion: The verse states: “You shall not uncover her nakedness [ervatah],” in the singular form, from which it is derived that you shall render him liable for one forbidden sexual relationship [erva], but you shall not render him liable for two types of forbidden sexual relationships.

The Gemara asks: If that is so, then from the verse: “You shall not uncover the nakedness of your daughter-in-law; she is your son’s wife, you shall not uncover her nakedness” (Leviticus 18:15), should it also be derived that you shall render him liable for one forbidden sexual relationship, but you shall not render him liable for two types of forbidden sexual relationships?

But didn’t we learn in the mishna: One who engages in intercourse with his daughter-in-law is liable to bring two sin-offerings for his intercourse with her: One due to the prohibition against engaging in intercourse with one’s daughter-in-law, and one due to the prohibition against engaging in intercourse with a married woman; the former liability applies both during his son’s lifetime and after his son’s death. And Rabbi Yehuda does not dispute this halakha.

Rather, it must be explained that since the daughter-in-law has one body, although engaging in intercourse with her involves two prohibitions, the term “her nakedness [ervatah],” in the singular, is written. Here too, with regard to one’s father’s wife, since she has one body, although there are two prohibitions, the term “her nakedness [ervatah]” is written.

Rather, Rava says that there is an alternative source for Rabbi Yehuda’s opinion: Rabbi Yehuda holds that concerning the meaning of the phrase “the nakedness of your father,” this is referring to your father’s wife. And he derives this interpretation by means of a verbal analogy from the verse: “And the man who lies with his father’s wife, he has uncovered his father’s nakedness” (Leviticus 20:11).

And the Torah indicates that she is forbidden whether she is his father’s wife who is his mother or whether she is his father’s wife who is not his mother. From where is it derived that one who engages in intercourse with his mother who is not his father’s wife is liable? The verse states subsequently: “The nakedness of your mother you shall not uncover.” And from the ensuing phrase: “She is your mother,” it is derived that in a case where his mother is also his father’s wife, you shall render him liable due to the fact that she is his mother, but you shall not render him liable due to the fact that she is his father’s wife.