1) A WOMAN WITH TWO SONS: A KOHEN AND A YISRAEL
QUESTION: The Mishnah states that when a Bas Yisrael was married to a Kohen and bore a son, after her husband's death she may continue to eat Terumah by virtue of her son who is a Kohen. If she later marries a Yisrael (and thus loses her entitlement to eat Terumah) and bears a son with him, after the Yisrael's death her Kohen son no longer entitles her to eat Terumah.
The Mishnah continues and says that when a Bas Kohen marries a Yisrael and bears a son with him, after the Yisrael's death she may not eat Terumah because of her son who is a Yisrael. If she later marries a Kohen (and thus becomes entitled to eat Terumah) and bears a son with him, even after the Kohen's death she may continue to eat Terumah.
Why may the woman not eat Terumah in the first case, in which she has a son from a Kohen and a son from a Yisrael, while she may eat Terumah in the second case, in which she also has a son from a Kohen and a son from a Yisrael?
ANSWERS:
(a) In his first explanation, the ARUCH LA'NER writes that when the woman has two sons, a Kohen and a Yisrael, their respective powers to grant or revoke the mother's right to eat Terumah "annul" each other as it were, and the woman is left with her "default" status. Hence, when the woman is a Bas Yisrael she has no grounds on which to eat Terumah. When the woman is a Bas Kohen, she may eat Terumah by virtue of the fact that she is a Bas Kohen.
(b) Alternatively, the Aruch la'Ner suggests that the woman's allowance to eat Terumah depends on the order of her marriages. In the first case, the woman was married to the Kohen first, and when she later marries a Yisrael she loses her right to eat Terumah. Since she has a son from the Yisrael, she may never regain the right to eat Terumah. In the second case, the woman was married to the Yisrael. She later married a Kohen and acquired the right to eat Terumah. Since she has a son from the Kohen, after the Kohen's death she retains the right to eat Terumah.
Two cases demonstrate the basic differences between the two explanations of the Aruch la'Ner:
1. A Bas Kohen married a Kohen first and then a Yisrael, and she had sons from both. According to the first explanation, she may eat Terumah because whenever she has children from both a Kohen and a Yisrael her status reverts back to that of her father's family (in this case, the Kohen). According to the second explanation, she may no longer eat Terumah since she did not have that right during her most recent marriage.
2. A Bas Yisrael first married a Yisrael and then a Kohen, and she had sons from both. According to the first explanation, she may not eat Terumah because the conflicting powers of the two sons annul each other. According to the second explanation, she may eat Terumah because she was entitled to eat Terumah during her most recent marriage.
The ruling of the RAMBAM provides support for the Aruch la'Ner's second explanation. The Rambam (Hilchos Terumos 6:19) writes, "If a Bas Yisrael married a Yisrael first and had a son from him, and then she married a Kohen... and the Kohen died and left her with a son from him, she may eat Terumah because of the son." The Rambam clearly emphasizes that it is the order of the marriages which determines whether she may eat Terumah.
On the other hand, the words of RASHI in Kerisus support the Aruch la'Ner's first explanation, that the power of a son from a Kohen (or even a living husband who is a Kohen) and the power of a son from a Yisrael annul each other. Rashi there (7a, DH v'Einah Meshalemes) writes that "if the husband (Yisrael) of a Bas Kohen dies and then she marries a Kohen, she may eat Terumah." Why must the daughter of a Kohen remarry a Kohen in order to eat Terumah? Even if she remains single she should be permitted to eat Terumah! Perhaps Rashi means that if she remarries a Kohen, she may eat Terumah even if she had a son from the Yisrael. Rashi's statement that only a Bas Kohen may eat Terumah when she has a son from a Kohen, even though she also has a son from a Yisrael, proves that a woman's right to Terumah depends on her "default" status and not necessarily on whether she was married to a Kohen most recently. (See also Insights to Kerisus 7:1.)

87b----------------------------------------87b

2) THE TORAH'S WAYS ARE WAYS OF PLEASANTNESS
OPINIONS: The Gemara teaches that a woman does not fall to Yibum when her husband's children die after her husband dies. Although her deceased husband now has no living offspring, the Torah exempts her from the obligation of Yibum because "Deracheha Darchei No'am" -- the ways of the Torah are ways of pleasantness, and it would not be pleasant for a woman who was exempt from Yibum at the time of her husband's death to become obligated to do Yibum when the reason for her exemption is unexpectedly removed.
In what way would requiring her to do Yibum if all of her husband's children die constitute a "lack of pleasantness"?
The Rishonim offer three explanations for why requiring her to do Yibum under such circumstances would constitute a lack of pleasantness.
(a) RASHI explains that if the woman would be obligated to do Chalitzah after she remarried another man (based on her initial exemption from Yibum), her act of Chalitzah with the brother of her first husband would cause her to become repulsive in the eyes of her present husband.
(According to this explanation, on what grounds is a woman exempt from Yibum when she does not remarry before the death of the children of her deceased husband? The answer is that according to Rashi, the principle of "Deracheha Darchei No'am" teaches that the verses which discuss the Mitzvah of Yibum apply only when the husband has no children at the time of his death. Consequently, even when the husband's children die before the woman remarries, she is exempt from Chalitzah and Yibum since her husband had children at the time of his death.)
(b) The RITVA explains that if the woman would be required to leave her new husband (with a divorce) and perform Yibum or Chalitzah, this would be a lack of pleasantness. (Rav Aharon Yaffen, in his footnotes to the Ritva, writes that the RAMBAN and RASHBA (17b) seem to give the same explanation.)
Why does the Ritva assume that the woman would have to be divorced from her present husband in order to perform Yibum or Chalitzah? Why should she not be able to perform Chalitzah while she is married? Rav Aharon Yaffen explains that a divorce would be necessary in order to perform Chalitzah, because an act of Chalitzah is valid only in a situation in which Yibum is an option. Yibum is not an option when the woman is married. (The same situation exists in the case of a Me'uberes (35b-36a); since she cannot do Yibum, her Chalitzah is ineffective.)
However, the Gemara later (92b) implies that a woman who is married may perform Chalitzah. How does the Ritva understand the Gemara there?
Perhaps the Ritva means that her husband must divorce her until Chalitzah is performed simply because he is prohibited to live with a woman who is a Yevamah until she does Chalitzah. (Also, the Yavam might prefer to perform Yibum, and the present husband has no right to withhold from the Yavam the option of performing the Mitzvah of Yibum.)
(c) TOSFOS (2a, DH v'Achos Ishto) explains that it is not pleasant for a woman who was permitted to remarry to suddenly become obligated to perform Yibum.
Moreover, even if the obligation of Yibum is still upon her but her Zikah to one of the brothers has been removed (that is, he became invalidated from performing Yibum and Chalitzah), it is not "ways of pleasantness" for the Zikah of that brother to be reinstated (since the woman assumed that she no longer had any connection to him; see Insights to Yevamos 30:1).
3) REQUIRING AN "ESHES ISH" TO DO YIBUM
QUESTION: The Gemara teaches that a woman does not fall to Yibum when her husband's children die after her husband dies. Although her deceased husband now has no living offspring, the Torah exempts her from the obligation of Yibum because "Deracheha Darchei No'am" -- the ways of the Torah are ways of pleasantness, and it would not be pleasant for a woman who was exempt from Yibum at the time of her husband's death to become obligated to do Yibum when the reason for her exemption is unexpectedly removed.
RASHI explains that if the woman would be obligated to do Chalitzah after she remarried another man (based on her initial exemption from Yibum), her act of Chalitzah with the brother of her first husband would cause her to become repulsive in the eyes of her present husband. (See previous Insight.)
If the woman remarries before her husband's children die, why should any obligation of Yibum take effect at the time the children die? At that point, she is forbidden as an Ervah ("Eshes Ish") to the Yavam. Every Ervah is exempt from Yibum and Chalitzah, as the Gemara earlier (3b) derives from a Mah ha'Tzad or Hekesh (see Tosfos there, DH Mah Achos Ishah; the Gemara in Kidushin (68a) explicitly includes "Eshes Ish" in Rebbi Yonah's "Hekesh of the Arayos"). The exemption of an Ervah from Yibum should apply to the Ervah of "Eshes Ish" as well. (NODA B'YEHUDAH EH 2:150; see also REBBI AKIVA EIGER to 16a.)
ANSWERS:
(a) The NODA B'YEHUDAH and REBBI AKIVA EIGER answer based on the words of TOSFOS (16a, DH Bnei Tzaros). Tosfos proves from the Gemara (16a) that when a woman who is a Shomeres Yavam marries another man and becomes an "Eshes Ish," the obligation of Yibum does not depart. This is in contrast to the law in the case of a woman who becomes an "Achos Ishah" after she falls to Yibum (i.e. the Yavam marries her sister), in which case the obligation of Yibum does depart. Why is an "Eshes Ish" different?
Tosfos gives two explanations:
1. Since the prohibition of "Eshes Ish" is removable through the act of divorce, it is considered "in our hands" to remove the prohibition, and therefore the obligation of Yibum remains. In contrast, the prohibition of "Achos Ishah" is not "in our hands" to remove, since it can be removed only through the death of her sister. Therefore, when the Shomeres Yavam becomes forbidden as "Achos Ishah," the Zikah for Yibum departs entirely.
2. Tosfos writes in the name of the RI that perhaps the verse, "Lo Siheyeh Eshes ha'Mes ha'Chutzah l'Ish Zar" (Devarim 25:5), teaches that even if the Shomeres Yavam performs an act of Kidushin ("Siheyeh" is related to the word "Havayah," a reference to Kidushin) with another man, the Zikah for Yibum remains and she must divorce her husband and do Yibum with the Yavam.
The Acharonim (ACHIEZER #2, CHAZON ISH 134:5, KOVETZ HE'OROS 4:6) point out that this explanation does not suffice to answer the question. Tosfos (10a, DH l'Olam) explicitly writes that an "Eshes Ish" can exempt her Tzarah from Yibum (in a unique case in which one of the Tzaros is forbidden to the Yavam because she is an "Eshes Ish" in relation to him, due to a condition in the divorce from a previous husband which excluded that Yavam from the Gerushin's allowance for her to marry all other men). Tosfos clearly differentiates between a woman who was an "Eshes Ish" at the time she fell to Yibum and a woman who became an "Eshes Ish" only after she fell to Yibum. A woman who was an "Eshes Ish" at the time of the Nefilah certainly is exempt from Yibum (as derived from the Hekesh to "Achos Ishah"). In contrast, when Tosfos (on 16a) writes that a married woman's status as an Ervah does not exempt her from Yibum (in contrast to the Ervah of "Achos Ishah" which does exempt her from Yibum), he refers only to when she became an "Eshes Ish" after she fell to Yibum. In such a case, the verse of "Ne'esarah" does not apply to prohibit her forever (for the reasons Tosfos gives), and she may become permitted to do Yibum later (through the death of her husband or through divorce).
However, in the case of a woman who already remarried before the death of the children of her former, deceased husband, the answers of Tosfos should not apply. Rather, she should be exempt from Yibum because she is an "Eshes Ish"! (The Noda b'Yehudah himself addresses this problem, but others refute his arguments. See Kovetz He'oros 6:8).
(b) RAV AHARON YAFFEN discusses the Noda b'Yehudah's question at great length in his notes to the Ritva on Yevamos (Mosad ha'Rav Kook, volume I, Milu'im 3:5). He suggests that perhaps Rashi maintains that at the moment of the death of her former husband's last child, the woman falls to Yibum retroactively from the time of her husband's death. If Rashi maintains that, at the moment the child dies, the woman falls to Yibum retroactively from the time of her husband's death (and not merely from the moment of the child's death), it emerges that her new marriage occurred after the Zikah for Yibum took effect. Consequently, this case is comparable to the case discussed by Tosfos (16a), and the logic of Tosfos suffices to explain why she is not exempt from Yibum because of her present marriage.
However, the words of Rashi (DH v'Lo Na'aseh) do not imply that she falls to Yibum retroactively, but rather that the Zikah takes effect at the moment her deceased husband has no children ("u'Ven Ein Lo").
(c) The CHAZON ISH (134:5) writes that if not for the words of Tosfos (16a), he would offer another answer for why a woman's present marriage does not exempt her from Yibum, based on simple logic:
The Zikah to Yibum gives the Yevamah a status of "acquired" to the Yavam, like a Kinyan. The Kidushin which another man performs with her not only renders her an Ervah to the Yavam and prevents her from doing Yibum, but it causes her to become a Kinyan of the man who did the Kidushin. However, it is not reasonable to suggest that his act of Kidushin creates a new Kinyan and is able to remove the Kinyan which the Yavam already has with her. Rather, it is more logical that the Zikah of the Yavam remains and is not superseded by the new husband's Kinyan. (A logical argument carries more weight than even a Hekesh. See Insights to Yevamos 3:2.)
Perhaps this logic may be applied to the case of the Gemara here as well. Although the Zikah should take effect only when the husband's child dies (according to the Gemara's Havah Amina that Zikah exists in such a case), the cause for the Zikah certainly is nothing other than the death of her first husband. It is not logical that a Kinyan she makes later, after the death of her husband, should prevent her from falling to Yibum when the child dies, because the cause of the Zikah preceded that Kinyan. Although an "Eshes Ish" is also an Ervah, her status of Ervah is only a result of a Kinyan. Since that Kinyan cannot override the Kinyan of Zikah which preceded it, her status as an Ervah does not prevent the obligation of Yibum from taking effect.
This logic is sound even according to the Rashba and Ritva (2a), who (like Tosfos on 10a) accept the possibility that the Ervah of "Eshes Ish" can cause, in theory, a woman and her Tzarah to become exempt from Yibum. The case in which they allow the Ervah of "Eshes Ish" to exempt the woman from Yibum is one in which the Ervah of "Eshes Ish" preceded the marriage of the deceased husband (whose death caused the Zikah). That case is not comparable to the case of the Gemara here in which the Ervah of "Eshes Ish" followed the death of the deceased husband.
Tosfos (16a), who does not accept this reasoning and gives different explanations for why the Ervah of "Eshes Ish" (which occurs after a woman falls to Yibum) does not exempt a woman from Yibum, follows his own reasoning as expressed elsewhere. Tosfos gives a different explanation for the exemption of "Deracheha Darchei No'am" (see previous Insight) which is unrelated to whether Zikah persists after a woman marries someone else.