Gemara

(Mishnah): If one witness said that he died, and two say that he did not, even if she got married, she must leave;

If two witnesses say that he died and one says that he did not, even if she did not yet marry, she may marry.

Question: What is the Chidush (in the Reisha)? Obviously, we do not believe one witness against two!

Answer: The Mishnah discusses Pasul witnesses, like R. Nechemyah taught:

(Beraisa - R. Nechemyah): Wherever the Torah believed one witness, we follow the majority. Two women contradicting one man are like two men against one man.

Version #1: (The Mishnah discusses two women who contradicted one man.)

Version #2: If a valid witness came at first, even 100 women are like only one man (and she would remain permitted);

The Mishnah discusses when the first witness was a woman.

We must explain R. Nechemyah to say that wherever the Torah believed one witness, we follow the majority. Two women against one woman are like two men against one man;

Two women against one man are like one man against one man.

(Mishnah): If two witnesses say that he died ...

Question: What does this teach?

It need not teach about invalid witnesses, that we follow the majority like R. Nechemyah. The Reisha taught this!

Answer: Indeed, it teaches this. One might have thought that we follow the majority only to be stringent, but not to be lenient. The Seifa teaches that this is not so.

Kesuvos 22b (R. Yochanan): If two witnesses say that Leah's husband died and two say that he did not die, she may not remarry. If she remarried, she need not leave. If two say that she was divorced and two say that she was not divorced, she may not remarry. If she remarried, she must leave.

Question: Why is the law different in the two cases?

Answer (Abaye): Really, only one witness says that she was widowed or divorced and one contradicts him. Chachamim believe one witness who says that a man died to permit his wife to remarry, like Ula taught;

(Ula): Wherever the Torah believes one witness, he is believed like two.

23b (Beraisa): If a woman says 'I am Teme'ah but my friend (who was also captured) is Tehorah', she is believed;

If she says 'I am Tehorah and my friend is Teme'ah', she is not believed.

(Rav Papa): In the Beraisa there are witnesses that they were captured, and one witness who contradicts her claims of who is Tehorah;

She admitted that she is forbidden (one is believed to forbid himself). Her testimony permits her friend.

Rishonim

The Rif and Rosh (15:9) bring the Gemara verbatim.

Rosh: In the Seifa, even if the witness who says that he did not die came before we permitted her, we permit her. However, if a Kosher witness said that he died and later two Pesulim said that he did not, she is permitted only if Beis Din permitted her before the Pesulim testified. The latter explanation of the Mishnah is primary.

Question (Korban Nesan'el 3): When two women contradict one man, why may she remain married? One who has Bi'ah with her brings an Asham Taluy, like when there are two contradictory pairs of Kosher witnesses! It is difficult to say that she married the witness, for this is forbidden when a lone witness permits her! However, we must say so. In a Teshuvah the Rosh says that b'Di'eved, she need not leave in this case.

Nimukei Yosef (DH Piska): If she was permitted by a Pasul witness, even if a Kosher witness later contradicted him, the first was already believed like two. The Gemara about captives (Kesuvos 23b) supports this. Others say that when a Kosher witness comes later she must leave. We cannot learn from captives, for Chachamim were lenient about captives. The Rambam holds that in such a case she loses her Heter, and if she married she must leave. The Ramban holds that if she testified for herself she loses her Heter, but not if another woman testified for her.

Question (Ran Kesuvos 10b DH Garsinan): When a captured woman says 'I am Tehorah and my friend is Teme'ah' and one witness contradicts her, why do we permit her friend? We believe one witness like two only when we permitted her before the other witness came. Since one witness is believed here, a woman is like a man and she can contradict him!

Answer #1 (Ran): Indeed, we permitted her based on the witness before her friend testified. Even though the Gemara connotes otherwise, this is for parallel structure to the Reisha.

Answer #2: Even if she testified first her friend is permitted. We are more lenient about a captive. The Rambam (Isurei Bi'ah 18:17) permits when a woman permits and a man contradicts her, without specifying that this is only if he came after we permitted her.

Rambam (Hilchos Gerushin 12:19,20): If one witness said that he died, and two say that he did not, even if she got married, she must leave. This is when the two were like the one. I.e., all were men. Alternatively, she remarried based on her own testimony or that of a woman, and two women contradicted her. But if a Kosher witness said that he died and many women or Pesulim say that he did not, it is like half-half. If she married one of her witnesses and she says that she knows that her husband died, she does not leave.

Magid Mishneh: The Rambam rules like the latter explanation. He explains that the man and the two women came together. If the man came first, the Gemara says that this is like half-half. It seems that the Rambam explains like Rashi, that she does not leave her Heter. However, the Rashba and Ramban say that the Rambam holds that she loses her Heter. If she was permitted through two women and one man contradicted them, it seems that the Rambam holds that she remains permitted.

Nimukei Yosef (ibid.): The Gemara did not say that when two Pesulim contradict a Kosher witness that she must leave. The Rambam inferred that if she married her witness she need not leave. The Ramban holds that when two contradictory witnesses come together, they do not help at all. He learns from Kesuvos 22b. To explain why she need not leave when two witnesses contradict each other the Gemara needed to say that one was believed first, like Ula. If she need not leave even when the witnesses come together, the Gemara would have taught this Chidush. The Gemara did not say that she must leave because it discusses only permitted marriage.

Rebuttal (Gra EH 17:121): This is no source. Kesuvos 22b discussed also divorce. Had the Gemara said that we discuss when the witnesses came together, this would imply that had the permitting witness come first she would be permitted even regarding divorce! Rather, the Ramban argues because he holds (Yevamos 25b DH Kanas) that if Reuven testified alone that Leah was widowed or he brought her Get and she married him, he must divorce her (even without contradictory testimony, lest people say that he testified falsely in order to marry her). The Rambam holds like Rashi, who permits her to remain married.

Nimukei Yosef (ibid.): The Yerushalmi permits her to stay. Surely, this is when they came together. If not, she may marry l'Chatchilah, like Ula taught! This supports the Rambam. The Ramban was forced to say that really, the permitting witness came first, and the Yerushalmi disagrees with Ula.

Poskim

Shulchan Aruch (EH 17:37): If one witness said that Leah's husband died and Beis Din permitted her and one came and said that he did not die, she may not remarry. If she did she must leave, because it is a Safek.

Taz (50): If she remarried before the latter witness came she need not leave.

Beis Shmuel (111): Tosfos, the Rambam and the Tur all say that if the permitting witness came first, she need not leave.

Shulchan Aruch (ibid.): If Leah married the first witness and it is clear to her that he died, she does not leave. Some say that she must leave. If two came and said that he did not die, even if she remarried all agree that she must leave.

Chelkas Mechokek (68): Rashi says that if she is sure that if he was alive he would have come, she would not bring Asham Taluy. This is called 'it is clear to her that he died'. The Ran requires that she saw that he died.

Rema: This is when Leah is silent. If she also says that he died and the contradictory witnesses are women, this is like two pairs of contradictory witnesses. The husband she married is not believed about himself (to be like a second witness).

Chelkas Mechokek (70): The Rema permits when Leah and another woman said that her husband died before Leah remarried. This is like Rashi (Yevamos 88b DH v'Ibo'is. Hagahos Ma'asei Nisim (2) questions this.)

Chelkas Mechokek (71): She is believed because she will suffer fines if her first husband returns. Her new husband would not be fined. However, if a Kosher witness permitted her, he may keep her. He is not considered partial!

Gra (122): The correct text of the Rema should say 'her husband is not believed about himself', i.e. if a man comes and claims that he is her old husband (and we do not recognize him - Yevamos 88b).

Beis Shmuel (115): When two pairs of Kosher witnesses contradict each other, she must leave unless she married one of the permitting witnesses. When two pairs of Pasul witnesses contradict each other she need not leave.

Shulchan Aruch (ibid.): This is when the one witness is like the two. But if a Kosher witness said that he died and many women or Pesulim say that he did not, it is like half-half.

Rema: This is only if they came together. If Beis Din permitted her based on the witness before the Pesulim testified, she keeps her Heter.

Shulchan Aruch (ibid.): If she married one of her witnesses and she says that she knows that her husband died, she does not leave.

Chelkas Mechokek (73): The Shulchan Aruch said above that some do not permit her to remain married to the witness even when there are two contradictory witnesses, all the more when two oppose the witness!