QUESTION: The Mishnah lists women who are permitted to their husbands but prohibited to their Yevamim, and women who are prohibited to their husbands but permitted to their Yevamim. An example of a woman who is prohibited to her husband but permitted to her Yavam (her husband's brother) is a woman who is a Mamzeres who married a Yisrael (who is not a Mamzer) whose brother is a Mamzer. She is prohibited to her husband but permitted to his brother when she falls to Yibum.

The Gemara earlier in Yevamos (11b) discusses a case of "Machzir Gerushaso," in which the woman's husband dies and she falls to Yibum. The Gemara derives through a Kal v'Chomer that she remains prohibited to her husband's brother and may not do Yibum with him (and perhaps her Tzarah may also not do Yibum): since the woman is prohibited to her husband to whom she was once permitted, certainly she is prohibited to her husband's brother to whom she was always prohibited (when there was no Chiyuv of Yibum) because of the Isur of "Eshes Ach."

Why does the Gemara here not apply the same Kal v'Chomer? The Kal v'Chomer should teach that a Mamzeres who married a Yisrael should be prohibited to do Yibum with her husband's brother, even though a Mamzer is allowed to marry a Mamzeres.

ANSWER: The Gemara in Sotah (6a) asks this question. The Gemara explains that the Kal v'Chomer applies only when the wife becomes prohibited to her husband after they were married (as the Girsa of the BACH (11b, #2) explicitly states). In a case in which she was prohibited to him before the marriage, his brother is not more prohibited to her than her husband is. Hence, the Kal v'Chomer does not teach that the brother becomes prohibited from marrying her.

The Gemara, however, mentions the case of a Petzu'a Daka who dies childless, whose wife is permitted to do Yibum with his brother. What is the Halachah in a case in which the husband becomes a Petzu'a Daka after he and his wife were already married? The Gemara implies that the Halachah in all cases of Petzu'a Daka is the same -- the wife is permitted to do Yibum with his brother. Why does the Kal v'Chomer not apply to prohibit her to his brother?

(a) TOSFOS (84b, DH Shiyer Petzu'a Daka) explains that the Kal v'Chomer prohibits her to the Yavam only when something happens to her which causes the Isur to take effect (such as in the case of "Machzir Gerushaso," or the case of a Sotah). If the Isur is created as a result of something which happens to her husband, the Kal v'Chomer does not prohibit her to his brother because she is not the one who has become Asur to her husband (and thus she does not become Asur to his brother), but it is her husband who has become Asur to her.

(b) RASHI in Sotah (6a, DH Leika Isura) implies that the Kal v'Chomer prohibits her to the Yavam only according to the opinion that "Nisu'in Rishonim Mapilin," the relationship of Yibum is an extension of the marriage between the deceased husband and the woman. Since an Isur was present during the marriage, the brother may not do Yibum because the marriage from which it stems was one of Isur. In contrast, according to the opinion that does not maintain that "Nisu'in Rishonim Mapilin," the Kal v'Chomer indeed may not apply.

QUESTION: The Gemara explains that in the case of Yibum, the Mitzvas Aseh of Yibum does not override a Lo Ta'aseh (an exception to the general principle of "Aseh Docheh Lo Ta'aseh"). RASHI (v'Ha Kulei) writes that this is derived from the verse, "v'Alsah Yevimto ha'She'arah" (Devarim 25:7).

However, the Gemara earlier (20b) concludes that the law that Yibum does not override a Lo Ta'aseh is not derived from any verse but is only an Isur d'Rabanan. The Rabanan prohibited the act of Yibum where it would involve transgressing a Lo Ta'aseh (the Gezeirah of "Bi'ah Rishonah Atu Bi'ah Sheniyah"). Why, then, does Rashi here write that the law is derived from a verse? (RASHBA, RITVA, REBBI AKIVA EIGER in Gilyon ha'Shas to Sanhedrin 53a)

ANSWER: Rashi maintains that although the Gemara earlier (20b and 61a) concludes that the prohibition to do Yibum in this case is mid'Rabanan, the Gemara in many other places clearly indicates that the reason why the Aseh of Yibum does not override a Lo Ta'aseh is that it is possible to avoid the Lo Ta'aseh altogether by doing Chalitzah instead ("Efshar b'Chalitzah"). (See Insights to Yevamos 20:1 and 20:4.)

However, this still does not explain the words of Rashi here. Rashi does not write that the reason why Yibum does not override a Lo Ta'aseh is because it is possible to do Chalitzah instead. Rashi writes that the Aseh of Yibum does not override a Lo Ta'aseh because of the verse (which the Gemara earlier cites only as a Havah Amina).

Apparently, Rashi understands that the Gemara here (at this stage of the Sugya) maintains that the Aseh of Yibum does not override a Lo Ta'aseh because of the verse which the Gemara earlier mentions (in the Havah Amina). Rashi infers this from the fact that the Gemara mentions the logic that the Tana of the Mishnah may be of the opinion that "Nisu'in Rishonim Mapilin." Accordingly, if the Yevamah was permitted to the Yavam at the time she married the Yavam's brother, she may do Yibum with him even if she has become prohibited to him with a Lo Ta'aseh by the time of Yibum (for example, the Yavam is the Kohen Gadol, and the woman becomes prohibited to him as an Almanah when her husband dies).

TOSFOS questions the Gemara's suggestion that the logic of "Nisu'in Rishonim Mapilin" is a reason to be lenient. The concept of "Nisu'in Rishonim Mapilin" teaches a stringency: if she was prohibited to her husband at any time during the marriage, she remains prohibited at the time of Yibum. The concept is never utilized to teach a leniency, that if she was permitted at the time of the marriage, she should remain permitted at the time of Yibum.

Rashi is also bothered by this question. The reason why a woman may not do Yibum with a Yavam to whom she is prohibited with a Lo Ta'aseh cannot be merely that Chalitzah may be done instead, because this reason would also apply when the Nisu'in (which caused the Yibum) took effect before the Isur. In such a case the logic of "Efshar b'Chalitzah" still applies. Similarly, the reason cannot be the Gezeirah d'Rabanan of "Bi'ah Rishonah Atu Bi'ah Sheniyah," because that Gezeirah still applies in this case.

Therefore, Rashi writes that the Gemara here understands that the reason why the Mitzvah of Yibum does not override a Lo Ta'aseh is the verse, "v'Alsah Yevimto ha'She'arah." The verse teaches that there are some women who may not do Yibum even though they should. The Gemara establishes that they are the women who are prohibited to their Yevamim because of Isurei Lavim. However, if the verse is the only source to exclude some women from Yibum, perhaps it applies only to cases in which the Isur Lav preceded the Nisu'in, and it does not apply where the Isur Lav took effect after the Nisu'in, in which case the Mitzvah of Yibum indeed overrides the Lo Ta'aseh. This is the grounds for the Gemara's Havah Amina that if the woman became prohibited to her husband after the Nisu'in, she remains permitted to the Yavam. (Based on the KEREN ORAH)