Gemara

Version #1 - Question: May a woman (whose husband had no children) do Yibum based on one witness who said that her husband died?

If the reason why we believe one witness to permit a woman to remarry is because (if he is alive) this is Avid l'Igluyei (prone to become known), and people do not lie about such things, here also, he would not lie!

If it is because Daika u'Minseva (she investigates well before remarrying), here, sometimes she desires the Yavam and she will not check well!

Answer (Rav Sheshes - Mishnah): If they told her that her son died before her husband died, and she did Yibum, and later they told her the husband died first, she must leave her husband.

The case is, one witness told her the first time, and two contradicted him. If not for the latter witnesses, one witness is believed!

Version #2 - Question: May a Yevamah marry l'Shuk (a stranger) if one witness testified that her Yavam died?

If one witness permits an Eshes Ish because this is Avid l'Igluyei, here also, he would not lie!

If he is believed because Daika u'Minseva, here she might not check well if she hates the Yavam!

Answer #1(Rav Sheshes - Mishnah #1): A woman's husband and son went overseas. They told her that her husband died and then her son died. She remarried. Later, they told her that the son died first (so she needed to do Yibum or Chalitzah). She must leave her husband. Children she had from him, both the first and last, are Mamzerim.

We cannot say that two witnesses told her the first time, and two witnesses later contradicted them. We would not believe the second witnesses more than the first. Also, the child would be only a Safek Mamzer! Rather, one witness told her the first time, and afterwards two contradicted him.

Inference: Had he not been contradicted, he would have been believed!

Rejection: We can say like Rav Acha bar Minyomi said (elsewhere), that two witnesses testified, and later they were found to be Zomemim (they lied).

Answer #2 (Rav Mordechai - Mishnah #2): A woman is not believed to say 'My Yavam died' to marry l'Shuk, nor to say 'My sister died' to marry her sister's husband.

Inference: She is not believed, but one witness would be believed!

Rejection (Rav Ashi - Seifa): A man is not believed to say 'My brother died' so he will do Yibum, nor to say 'My wife died' to marry her sister.

If you make the above inference, you should also infer here that he is not believed, but one witness is!

Objection: Granted, Chachamim were lenient to believe one witness to permit a woman to avoid Igun (a woman who may not marry). This does not apply to marrying one's wife's sister!

Answer: Rather, Mishnah #2 is like R. Akiva. Since he holds that a child of Chayavei Lavin is a Mamzer, we would have thought that a Yevamah is concerned for her children and will check well before remarrying, so she is believed. The Mishnah teaches that this is not so. She is concerned only for her own ruin, not for her children's ruin.

Answer #3 (Rava): A Kal va'Chomer teaches that one witness is believed to permit a Yevamah l'Shuk. One witness permits Chayavei Kerisus (Eshes Ish), all the more so he permits Chayavei Lavin!

Objection (a Chacham): A woman's credibility refutes your reasoning! She is believed for Chayavei Kerisus (to say that her husband died), but not for Chayavei Lavin (to say that her Yavam died)!

This is because sometimes she hates the Yavam, and she will not check well. The same applies when one witness says that the Yavam died!

Rishonim

Rif: In the second version Rav Sheshes and Rava resolved that one witness is believed to permit a Yevamah l'Shuk. Our Rebbeyim say that even though the Gemara dispelled the proofs, these were mere Dichuyim.

Rebuttal (Rosh 10:4): How can we be lenient? The Dichuy of Rava is proper! If we are concerned lest she testify falsely b'Mezid because she hates the Yavam, all the more so we must be concerned lest she not check well! Even though we are not concerned lest love of the Yavam deter her from checking properly, Tosfos explains that hatred of haters is more intense than love of lovers. Also, it is more common that she hates the Yavam. Also, the desire to fall to a Yavam she loves is not so great, for he might do Chalitzah, and in any case she could find other men as good or better. She dreads falling to one she hates, for she must suffer under him or be an Agunah if he does not want to do Chalitzah. Rav Acha said that the Mishnah discusses Edim Zomemim to be Docheh Rav Sheshes' proof. This is a proper Dichuy. We could have used it also to refute the proof in the first version. We did not, for the second version is sure that one witness is believed to permit Yibum. The Gemara uses it to reject the proof to permit a Yevamah l'Shuk, for it is reasonable that one witness is not believed for this.

Rambam (Hilchos Yibum 3:5): One witness is believed to testify that a Yavam died. Even a slave or woman or Nochri l'Fi Tumo can testify, just like they are believed to permit an Eshes Ish.

Question (Lechem Mishneh): We cannot rely on Daika u'Minseva here, for she is not believed to permit herself l'Shuk! If one witness is believed, it is because this is Avid l'Igluyei. Also, if the Gemara resolved here that one witness is believed due to Avid l'Igluyei, why does the Rambam rule (Hilchos Gerushin 13:1,19) that the questions about one witness in war or when she was quarreling with him were not resolved?

Answer (Lechem Mishneh): The Rambam explains like Tosfos (93b DH Ed), that the question is which is the primary reason why one witness is believed. If Daika u'Minseva is primary, we require that she check well. If Avid l'Igluyei is primary, it suffices if she will check minimally. The Gemara says that both are needed; perhaps in truth Avid l'Igluyei is primary. Nevertheless, a Yevamah is not believed to permit herself l'Shuk, for we also need Avid l'Igluyei, and this applies to witnesses. One witness is believed to permit a Yevamah l'Shuk, for she checks well. She does not check well in war or when they were quarreling. The Gemara did not resolve these cases because it did not resolve which reason is primary.

Rambam (11): Even though a woman is believed to say 'my husband died' in order to do Yibum or remarry, a Yevamah is not believed to say 'my Yavam died' in order to marry l'Shuk. Because it is only an Isur Lav (if her Yavam is really alive), it is light in her eyes.

Tosfos (92b DH Aval): Rav Ashi established the Mishnah to be R. Akiva. It is no Chidush according to Chachamim. Rav Ashi permits a Yevamah to do Yibum even after she married l'Shuk. Since she will not be fined, obviously she is not believed to permit herself l'Shuk!

Poskim

Shulchan Aruch (EH 158:1): A woman is not believed to say 'my Yavam died' in order to marry l'Shuk. Therefore, if she went overseas with her husband and Yavam and returned and said that both died, in either order, she is not believed.

Shulchan Aruch (3): One witness is believed to say that her Yavam died to permit her to marry l'Shuk, or to say that her husband died and then her son died.

Rema: Some say that one witness is not believed to say that the Yavam died to permit a Yevamah l'Shuk.

Beis Yosef (DH v'Rabeinu): R. Yerucham says that most Poskim say that one witness is not believed; only the Rif permits. This is wrong! The Rambam also permits. The Ba'al ha'Ma'or and Rashba are even more lenient. The Ra'avad permits, he argues only in a different case. Even the Rosh did not say that he is not believed, just that it is difficult to be lenient. The majority opinion is lenient; we follow it.

Bach (DH Emnam): It seems that the Rema is stringent not to rely on one witness when it is possible to get full testimony, but he agrees that we rely on the lenient opinions to avoid Igun if a second witness cannot be found.

Beis Shmuel (2): Mahari Mintz says that nowadays that we do not do Yibum, she herself is believed to say that her Yavam died. All the more so the Rosh would believe one witness nowadays. This is not necessarily true.

Hagahos Ma'ase Nisim (6, in Shulchan Aruch Rosh Pinah): Mahari Mintz is lenient because nowadays there is no concern lest she hates the Yavam. The Beis Shmuel is skeptical because perhaps we do not distinguish. Alternatively, since the Isur l'Shuk is a mere Lav, perhaps she does not check well.