1) THE LOGIC BEHIND A TZARAH'S EXEMPTION FROM YIBUM
QUESTION: Rav Asi states that the Tzarah of an Aylonis is exempt from Yibum. The Torah teaches, "ha'Bechor Asher Teled..." -- "The firstborn son whom she bears will perpetuate the name of the dead brother" (Devarim 25:6), which excludes Yibum with an Aylonis who cannot bear children. Since she is exempt from Yibum she retains the prohibition of "Eshes Ach" and -- as an Ervah to the surviving brothers -- she exempts her Tzarah from Yibum.
Rava and Rebbi Yochanan disagree with Rav Asi's ruling. They conclude that the Tzarah of an Aylonis is not exempt from Yibum. The Gemara says that even if an additional prohibition of Ervah prohibits the Aylonis to the brothers, she still does not exempt her Tzarah from Yibum; an Aylonis does not fall to Yibum at all, for the Torah exempts her from Yibum entirely. RASHI (12b, DH Hilchesa) and TOSFOS (8a, DH Trei) explain that despite the fact that the Tzarah must perform Yibum, from the perspective of the Aylonis she is considered as though she is married to an outsider and not to a brother who died childless and whose wives fall to Yibum.
The case of an Aylonis seems similar to the case of "Eshes Achiv she'Lo Hayah b'Olamo." In both cases, the Torah exempts the woman from Yibum ("Asher Teled" in the case of an Aylonis, and "Yeshvu... Yachdav" in the case of an "Eshes Achiv she'Lo Hayah b'Olamo"). However, the Mishnah (2a, 17a) clearly states that the latter does exempt her Tzarah from Yibum, even though the Torah specifically exempts the woman herself! Why does an Aylonis not exempt her Tzarah from Yibum, even though the verse exempts the Aylonis herself? What is the difference between the two cases?
ANSWER: Indeed, the case of an Aylonis and the case of an "Eshes Achiv she'Lo Hayah b'Olamo" are the same. The only difference between the two cases is that the case of an Aylonis is one of "Nefilah Rishonah." That is, the woman is an Aylonis at the time her husband dies, and the verse specifically exempts her from Yibum for that reason. She does not fall to Yibum at all, and therefore her Tzarah does not have the status of a "Tzaras Ervah." (It is obvious that the woman is still an Aylonis when she is subject to Yibum, because an Aylonis' status never changes.)
The Mishnah, however, is discussing a case of "Nefilah Sheniyah" of an "Eshes Achiv she'Lo Hayah b'Olamo." That is, the woman fell to Yibum in the past, before a brother was born to her husband. At that time, another brother performed Yibum with her. Now, her second husband (the second brother) dies, and again she is subject to Yibum. She is exempt from Yibum, but not because of the specific exemption of "Eshes Achiv she'Lo Hayah b'Olamo," since the two brothers (the second and the third) were alive at the same time. Rather, she is exempt from Yibum because she was an "Eshes Achiv she'Lo Hayah b'Olamo" the first time she fell to Yibum, and therefore this time she is prohibited because of "Eshes Ach she'Lo b'Makom Mitzvah." Since the Torah does not specifically exempt her from Yibum this time, but she is exempt because she is an Ervah, she exempts her Tzarah from Yibum.
(It is obvious that the Mishnah refers to a woman who once was an "Eshes Achiv she'Lo Hayah b'Olamo" but is no longer an "Eshes Achiv she'Lo Hayah b'Olamo" the second time she is subject to Yibum. The Mishnah discusses the status of her Tzarah, who is not an "Eshes Achiv she'Lo Hayah b'Olamo." When a brother is born after his childless brother dies, all of the Yevamos are equally exempt from Yibum due to the exemption of "Eshes Achiv she'Lo Hayah b'Olamo.")
The reason for this distinction is as follows. The Halachah that a woman who is an Ervah to the surviving brother exempts her Tzarah from Yibum has a logical basis. The logic is that Kidushin, the bond of marriage, cannot take effect with an Ervah (Kidushin 67b). The bond of Yibum, called Zikah, is similar to Kidushin in that it forms a connection between the Yevamah and the Yavam. If the Yevamah is an Ervah to the Yavam, then just as the bond of Kidushin cannot take effect between the two, so, too, the bond of Zikah also cannot take effect. The Mishnah (41a; see also TOSFOS to 16a, DH Bnei) says that for this reason if a woman falls to Yibum and then one of the surviving brothers performs Kidushin with her sister, rendering the Yevamah an "Achos Ishah" to him (the sister of his wife), he removes the Zikah for Yibum between the Yevamah and that brother.
The Rishonim themselves make this point with regard to performing Yibum with a woman to whom one is prohibited because of a Lav. The Gemara (9a) says that according to Rebbi Akiva who maintains that Kidushin does not take effect with a woman who is prohibited to the man with a Lav, a woman who is prohibited with a Lav does not perform Yibum or Chalitzah (and exempts her Tzarah from Yibum). TOSFOS (9a) asks that there is no source in the Torah that Chayavei Lavim are exempt from Yibum. The Torah teaches only that Chayavei Kares are exempt from Yibum (through the Hekesh of Rebbi Yonah (8a) which compares all Chayavei Kares to "Achos Ishah," or, according to Rava, through the principle of "Ein Aseh Docheh Lo Ta'aseh she'Yesh Bo Kares"). Therefore, a woman who is prohibited only with a Lav should be required to do Yibum even according to Rebbi Akiva!
The Rishonim (see RAMBAN, RASHBA, RITVA ibid.; see also RAMBAN to 20b and Insights there) answer that the exemption from Yibum for an Ervah who is prohibited by Kares shows that the reason is not merely that the Torah exempts specific types of prohibited women from Yibum, but rather that in any situation where Kidushin cannot take effect, Zikah also cannot take effect. If Zikah cannot take effect, there is no Mitzvah of Yibum (and the Isur of "Eshes Ach" remains). Therefore, according to Rebbi Akiva who says that the Kidushin does not take effect for Chayavei Lavim, the verse which teaches that Chayavei Kares are exempt from Yibum also teaches that Chayavei Lavim are exempt, because the same logic applies to both: since the Kidushin does not take effect, Zikah also does not take effect. (Although Rava asserts that no verse is used to teach that Chayavei Kares cannot perform Yibum, he says that only according to the Rabanan. According to Rebbi Akiva, the verse is used to teach that a woman with whom Kidushin does not take effect does not fall to Yibum, whether she is prohibited by an Isur Kares or only by a Lav. From this verse he derives that Chayavei Lav cannot perform Yibum.)
This approach explains why an Ervah exempts not only herself but also her Tzarah from Yibum. When a woman who is an Ervah to the brother falls to Yibum, she breaks, or disrupts, the Zikah. However, she breaks not only her own Zikah-connection to him, but she breaks the entire Zikah to that brother, so that all of her Tzaros are also exempt from Yibum. This is because there cannot be a "half" Zikah; once part of the Zikah (her own) is broken, the entire Zikah of the house to that brother is also broken.
This manner of breaking of the Zikah functions only when the Ervah was fit to do Yibum but her Ervah-status prevented the Yibum and "broke" the Zikah. In contrast, in the case of an Aylonis, her status of "Eshes Achiv she'Lo Hayah b'Olamo" does not "break" any Zikah. There was no cause for her to have Zikah to the brothers in the first place because the Torah specifically exempts her from the laws of Yibum. Since her status as an Ervah does not break her Zikah (since there is none), it also does not break the Tzarah's Zikah, and thus the Tzarah remains obligated to do Yibum.

12b----------------------------------------12b

2) MARITAL RELATIONS WITH A "MOCH"
OPINIONS: Rav Bivi quotes a Beraisa in which Rebbi Meir says that there are three women who may use a Moch during marital relations: a Ketanah (minor), Me'uberes (a pregnant woman), and Menikah (a nursing mother). The Gemara explains that a Ketanah is any girl between the ages of eleven and twelve. Before the age of eleven, however, she may not use a Moch. The Chachamim disagree with Rebbi Meir and rule that no women should use a Moch; rather, they should have relations in the normal manner and Hash-m will protect them ("Shomer Pesa'im Hash-m").
What is the Halachah in situations other than the three situations mentioned in the Gemara (and in the three situations according to the Chachamim)? Is the use of a Moch permitted?
(a) RABEINU TAM (cited by TOSFOS) writes that a woman is always permitted to insert a Moch after the act, because no prohibition is involved in such an act. The man transgresses no prohibition when the woman inserts a Moch after the act; he is no different from a man who lives with a woman who cannot have children (such as an older woman) -- as long as the act is done in the normal manner, he may live with her. The woman also transgresses no prohibition when she inserts the Moch, because women have no prohibition against destroying Zera. When Rebbi Meir says that three women may use a Moch, he means that they are obligated to do so, to protect themselves from danger. The Chachamim who disagree maintain that the women are not in severe enough danger to obligate them to use a Moch. They are permitted not to use one, but they may use one if they choose to do so.
(b) RASHI (here and in Kesuvos 39a) writes that these three women are permitted (but not obligated) to use a Moch according to Rebbi Meir. This implies that the Chachamim maintain that they are prohibited from using a Moch. TOSFOS and other Rishonim seem to understand that Rashi refers to a Moch which a woman inserts after the act, as Rabeinu Tam explains. Rashi is saying that under normal circumstances such a Moch is prohibited. Apparently, Rashi maintains that the woman is also prohibited from destroying Zera.
The Rishonim question Rashi's explanation that when Rebbi Meir says "three women use a Moch," he means that they are only permitted, but not obligated, to use a Moch. If Rebbi Meir merely permits, but does not obligate, the women to use a Moch, then when he says that a girl below the age of eleven should have relations in the normal manner, he must mean that she is prohibited from using a Moch. The Gemara, however, says that a girl below the age of eleven cannot become pregnant. Why, then, should she be prohibited from using a Moch? There is no destruction of Zera involved if she uses a Moch, because the Zera would not have produced children in any case!
Rashi in Nidah (45a) addresses this question when he writes that the statement "three women use a Moch" means that "it is beneficial for them to use a Moch." Rashi means that the words "Meshamshos b'Moch" mean only that the use of a Moch benefits them in some important way. Since it benefits them in some way, its use is permitted in these cases, even though the Moch is usually prohibited. This implies that for a minor under the age of eleven and for all other women, a Moch provides no benefit. Since it provides them with no benefit, for some women the Moch is prohibited (for all other women), and for others its use is not recommended (for a girl under the age of eleven). Accordingly, Rashi's words in Nidah complement his words here. (RITVA)
REBBI AKIVA EIGER (Teshuvos #71, 72) concludes that this is the majority opinion among the Rishonim.
(c) TOSFOS (in Kesuvos 39a) and the TOSFOS HA'ROSH understand that Rashi refers to a Moch which is inserted before the act of relations, as the simple reading of Rashi implies. These three women are permitted to use a Moch, while all others are prohibited. Why may a woman insert a Moch before the act? Inserting the Moch before the act causes the man to actively destroy his Zera by releasing it into a Moch, which is forbidden mid'Oraisa! Why is he permitted to have relations when the woman is using a Moch? He should be prohibited from performing the act with a Ketanah (or Me'uberes or Menikah) any not be permitted to perform the act with a Moch.
TOSFOS (in Kesuvos) explains that one is permitted to use a Moch because of the Mitzvah of Onah. That is, since the man fulfills the Mitzvah of Onah, he is not considered to be destroying the Zera in vain, even if the woman uses a Moch. Tosfos proves this from the fact that a man is permitted to have relations with an older woman even though he knows that the Zera will be destroyed and will not be productive.
The other Rishonim (cited in (b) above) do not accept Tosfos' comparison between relations while using a Moch and relations with an older woman. They maintain (as Tosfos here writes) that the reason why relations with an older woman is permitted even though the Zera will not be productive is that the act is done in the normal manner. The Torah does not place a limitation on the age of the woman one may marry, and thus the normal manner of relations is never considered an act of Hashchasas Zera, even when the Zera will not be productive because of natural reasons. In contrast, when a woman inserts a Moch before the act, the act then is not done in the normal manner because the Zera is prevented from reaching the womb, and therefore it is prohibited.
(The dispute between this opinion and the previous one apparently depends on the definition of "destruction of Zera" ("Hashchasas Zera"): According to the second opinion, Hashchasas Zera is defined as any act which causes the Zera not to be used for any positive purpose (such as to procreate or to fulfill the Mitzvah of Onah). According to the first opinion, Hashchasas Zera is defined as any act which prevents Zera specifically from procreating.)

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