1) HOW THE "ASEH" OF YIBUM OVERRIDES THE "LO TA'ASEH" OF "ESHES ACH"
QUESTION: The Gemara earlier (3b) teaches that the source for the rule that the Mitzvah of Yibum does not override the prohibition of Arayos (other than that of "Eshes Ach") is the verse, "Aleha" (Vayikra 18:18). The Gemara there asks why a verse is needed to teach this; why would one have thought that the Mitzvah of Yibum overrides the prohibition of Arayos?
The Gemara commences a lengthy attempt to find a source that, in general, an Aseh overrides a Lo Ta'aseh that has Kares, and thus the verse of "Aleha" is needed to teach that the Aseh of Yibum does not override the Lo Ta'aseh of Arayos (which are punishable with Kares).
The Gemara concludes that an Aseh does not override a Lo Ta'aseh that has Kares. Consequently, the Gemara searches for a reason for why one would have thought that the Mitzvah of Yibum should override the Lo Ta'aseh of Arayos such that a verse is needed to teach that it does not override the Lo Ta'aseh of Arayos.
Why does the Gemara not learn from the case of Yibum that an Aseh does override a Lo Ta'aseh that has Kares? The Torah states that one is permitted to marry his brother's wife in order to fulfill the Mitzvah of Yibum, even though marrying one's brother's wife ("Eshes Ach") is punishable with Kares. The Gemara should learn from there that an Aseh does override a Lo Ta'aseh that has Kares! Consequently, another verse ("Aleha") is needed to teach that in the case of Yibum with any other Ervah, the Aseh does not override the Lo Ta'aseh that has Kares. Why does the Gemara not give this answer?
ANSWERS:
(a) TOSFOS (4a, DH Lo Ta'aseh) explains that nothing can be derived from the verse which teaches that the prohibition of "Eshes Ach" is pushed aside for the Mitzvah of Yibum. Nothing can be learned from there because the Mitzvah of Yibum can never be performed, under any circumstances, without overriding the prohibition of "Eshes Ach." (This is called "Mitzvaso b'Kach" -- the Mitzvas Aseh can be performed only by overriding a Lo Ta'aseh.) The Gemara cannot derive from this case that in all other situations in which an Aseh conflicts with a Lo Ta'aseh that has Kares that the Aseh overrides the Lo Ta'aseh, because in all other cases there are circumstances in which the Aseh can be done without overriding the Lo Ta'aseh.
RAV ELCHANAN WASSERMAN Hy'd (in KOVETZ HE'OROS 9:1) explains that Tosfos means to say that this is a Pircha to the suggestion that the law of "Aseh Docheh Lo Ta'aseh she'Yesh Bo Kares" be learned from Yibum through a Binyan Av. That is, Yibum differs from all other cases of an Aseh which conflicts with a Lo Ta'aseh that has Kares, because in the case of Yibum the Mitzvah can never be done without overriding the Lo Ta'aseh. An example of such a Pircha is found in Menachos (5b).
(b) The YAD RAMAH (Sanhedrin 53b) suggests a different approach. Since Yibum can be done only with an "Eshes Ach," it is clear that the Torah never prohibited "Eshes Ach" in the first place in a situation of Yibum. (That is, the prohibition of "Eshes Ach" is "Hutrah" and not "Dechuyah.") This is different from all other cases in which an Aseh overrides a Lo Ta'aseh, in which the Lo Ta'aseh remains in force but the Aseh is deemed more important and pushes it aside. In the case of Yibum, there is no Lo Ta'aseh of "Eshes Ach" whatsoever. This is why the Gemara cannot derive from the Mitzvah of Yibum that -- in all other cases -- an Aseh overrides a Lo Ta'aseh that has Kares.
That the prohibition of "Eshes Ach" is not present at all ("Hutrah") in a situation of Yibum is clear from a number of laws of Yibum.
For example, the law is that when the surviving brother has lived with the Yevamah once and thereby fulfilled the Mitzvah of Yibum, he is permitted to remain intimate with her ("Bi'ah Sheniyah"), despite the fact that she was once married to his brother ("Eshes Ach"). Clearly, the prohibition of "Eshes Ach" is entirely rescinded in a situation of Yibum. In contrast, the Gemara (20b) says that in a case in which the Yevamah is prohibited to the Yavam by a Lo Ta'aseh, although the Mitzvah of Yibum overrides the Lo Ta'aseh the Yavam must divorce her immediately after the act and he is not permitted to be with her again.
Similarly, the law is that if a Yavam divorces the Yevamah after he performs Yibum with her, he is permitted to remarry her (as long as no other man has married her in the interim). If the Mitzvah of Yibum merely suspends, temporarily, the prohibition of "Eshes Ach," then why is he permitted to remarry her? The prohibition of "Eshes Ach" is present, and no Mitzvah is fulfilled by remarrying her. The Yad Ramah explains that the Torah does not prohibit an "Eshes Ach" at all in a situation of Yibum, and therefore she is completely permitted to him even after the Mitzvah has been fulfilled. (The Gemara itself demonstrates this difference between a prohibition which is suspended only temporarily, "Dechuyah," and a prohibition which is absolutely annulled, "Hutrah." The Gemara (7b) says, "Ho'il v'Ishteri, Ishteri" -- once the prohibition has become permitted, it becomes permitted absolutely so that other prohibitions related to it also become permitted.)
The RAMBAN (in TORAS HA'ADAM, cited by the Kovetz He'oros ibid.), the RASHBA in Kidushin (21b), the RITVA here, and the RA'AVAD in Tamid (end of 27a) also follow this approach. They write with regard to Yibum and other Mitzvos that the annulment of a prohibition which is "Hutrah" is permanent, while the annulment of a prohibition which is "Dechuyah" is only temporary.
The Yad Ramah's understanding seems more logical that Tosfos'. Why does Tosfos consider Yibum a case of an Aseh that overrides a Lo Ta'aseh? If Yibum is permitted only because the Mitzvas Aseh overrides the prohibition of "Eshes Ach," why is the brother permitted to stay married to his deceased brother's wife? Once he has lived with her for the first time, he has fulfilled the Mitzvah of Yibum and there no longer is a Mitzvas Aseh to override the Lo Ta'aseh of "Eshes Ach." He should be required to divorce her, as the Gemara says later (20b)! Moreover, according to Tosfos, why is he permitted to remarry her once he divorces her? The prohibition of "Eshes Ach" should prohibit him from remarrying her, since he will not fulfill any Mitzvah by doing so. (See KEREN ORAH to 3b.)
Apparently, Tosfos understands that it is unacceptable to suggest that the prohibition of "Eshes Ach" is "Hutrah" and does not apply at all to a Yevamah. It is clear from the Gemara that the prohibition does remain, and it is suspended only at the moment the Yavam performs the Mitzvah of Yibum. This is clear from the Gemara later (10b) in which Reish Lakish states that after one of the brothers marries one of the wives of the deceased brother, the other brothers of the Yavam remain prohibited to the other wives of the deceased with an Isur Kares, since the Torah never removed the prohibition of "Eshes Ach" from them.
The opinion of Aba Shaul later in Yevamos (39b) provides further support for the view of Tosfos. Aba Shaul maintains that one who performs the Mitzvah of Yibum with ulterior motives "is considered as though he lives with an Ervah." If the Torah permits this woman to the Yavam and entirely annuls the prohibition of "Eshes Ach," why should the Yavam's motives affect the allowance to marry her? She is not an Ervah at all! (See Insights to Yevamos 40:1.)
For these reasons, Tosfos understands that Yibum is a case of "Aseh Docheh Lo Ta'aseh." The reason why "Bi'ah Sheniyah" is permitted (and the Yavam is not required to divorce her immediately after the first act), and the reason why he may remarry the Yevamah after he divorces her, is because the Torah extends the allowance to marry the Yevamah after the initial suspension of the prohibition through the Mitzvah of Yibum (see 8b, and TOSFOS DH Melamed). While the initial act is one of "Dechiyah," afterwards the Torah removes the prohibition of "Eshes Ach" entirely so that the Yavam may raise a family with his brother's wife.
The other Rishonim apparently disagree and maintain that the prohibition of "Eshes Ach" indeed is removed from all of the other brothers at the moment their brother dies and Zikah begins. (These issues are addressed further in Insights to Yevamos 10:1, 20:3, 35:2, and 41:1.) (M. KORNFELD)

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