Gemara

Mishnah: The following are disqualified (from judging or testifying): diceplayers, those who lend on interest, Mafrichei Yonim (people who bet on bird racing, or use birds to take others' birds), people who sell Shemitah produce, and slaves.

These people cannot give any testimony that a woman cannot give.

Gemara - Inference: These people can give testimony that a woman can give.

Rav Ashi: This teaches that a thief mid'Rabanan is Kosher for Edus Ishah (to testify alone that a man died to permit his widow to remarry. In Yevamos this teaching is attributed to Rav Menasheh.)

Yevamos 122a - Mishnah: It became established to permit (accept Edus Ishah) based on Ed mi'Pi Ed, slaves and women.

25a - Mishnah: If a man says 'Leah's husband died', 'I killed him' or 'we killed him', he may not marry her;

R. Yehudah says, if he says 'I killed him', she may not marry. If he says 'We killed him', she may marry.

Question: But Rav Yosef taught that if Shimon says 'Ploni forcibly had homosexual relations with me', Shimon may join a second witness to condemn Ploni to death;

If he says 'I willingly had relations with Ploni', Shimon is a Rasha - " Al Tashes Yadcha Im Rasha Lihyos Ed Chamas" (he is Pasul for testimony).

Suggestion: Perhaps Edus Ishah (testimony to permit a widow to remarry) is different, for Chachamim were lenient.

Objection: Rav Menasheh taught that a thief mid'Rabanan is Kosher for Edus Ishah, but a thief mid'Oraisa is not. Must we say that Rav Menasheh holds like R. Yehudah (unlike Chachamim)?!

Answer: Rav Menasheh can hold like Chachamim. She may marry because of Rava's law.

Rava: A person cannot establish himself to be a Rasha (and disqualify himself from testifying) because he is related to himself (relatives cannot testify).

Suggestion: Rav Yosef holds like R. Yehudah.

Rejection: Rav Yosef can hold like Chachamim. He says that we are more lenient regarding Edus Ishah than in general; Rav Menashe holds like R. Yehudah!

Rishonim

Rif and Rosh (Yevamos 46b and 16:10): A thief mid'Oraisa is Pasul for Edus Ishah, but a thief mid'Rabanan is Kosher.

Nimukei Yosef (7a, DH v'Ein (6b)): Rava disqualifies only Ed Chamas (a robber) because he is evil to Hash-m and people. He agrees that one who transgresses for Hana'ah, e.g. he eats Neveilos to satisfy his appetite, is Pasul. Perhaps he is being paid to testify falsely!

Nimukei Yosef (Sanhedrin 5b DH Shevu'as): The Halachah follows Abaye, who disqualifies even a Rasha not of Chamas. However, the Pesul is due to "Al Tashes Yadcha Im Rasha Lihyos Ed Chamas"; it applies to testimony. Such a Rasha is Kosher for Isurim, which do not require testimony.

Minchas Chinuch (37:15 DH ba'Meh): The Rambam (Edus 11:7), Shulchan Aruch (YD 2:5 and 119:7) and Poskim do not distinguish like the Nimukei Yosef. Rather, even a Rasha of Chamas is believed about Isurim regarding others. We are not concerned lest he is being paid for what he says. Even a Rasha not of Chamas is not believed about Edus Ishah.

Rif and Rosh (ibid.): A witness who is Pasul mid'Oraisa is no worse than a Nochri, and what he says l'Fi Tumo (unaware of the consequences) is Kosher for Edus Ishah.

Question: Why did the Gemara (25a) seek to refute Rav Menasheh from the Mishnah? Perhaps the case is, he spoke l'Fi Tumo!

Answer #1 (Nimukei Yosef Yevamos 7a, DH Gazlan): In the Reisha of that Mishnah he intended to testify. Presumably, the same applies to the Seifa.

Answer #2 (Korban Nesan'el 7): If he spoke l'Fi Tumo, R. Yehudah would agree that he is believed.

Question (Korban Nesan'el 8): We do not believe what a Nochri wrote l'Fi Tumo. It is not clear whether or not someone Pasul mid'Oraisa is like a Nochri in this respect.

Rambam (Hilchos Gerushin 12:17): If mid'Oraisa someone is Pasul for Edus due to an Aveirah, he is Pasul for Edus Ishah. If he spoke l'Fi Tumo, he is no worse than a Nochri, and he is believed. If mid'Rabanan he is Pasul for Edus, he is Kosher for Edus Ishah.

Question: How we can believe one witness, even a Pasul, even a Nochri l'Fi Tumo, even Ed mi'Pi Ed, without interrogation?

Answer #1 (Rambam Hilchos Gerushin 13:29): The Torah requires two witnesses only for things that can be known only through witnesses, e.g. a loan or a murder. The Torah is not particular about something we can know without this witness, and which he cannot defend himself if he is lying, e.g. that Ploni is dead. It is farfetched that someone would testify falsely about this. Therefore Chachamim were lenient, so Benos Yisrael will not be Agunos (unable to remarry).

Answer #2 (Nimukei Yosef 28a DH v'Yesh Lismo'ah citing Ro'oh): Certainly the testimony is Emes. Since we are so stringent if her husband returns after she remarries (13 fines apply to her), surely she checks very well, so (if he is alive) the matter is prone to become known. Such testimony is valid mid'Oraisa.

Note: Both of these answers hold that the witness is extra careful because he knows that if he is wrong, surely this will be revealed. This does not apply to l'Fi Tumo, even if the witness is Kosher. The Gemara explicitly allows even a Nochri l'Fi Tumo (121b)! Perhaps since there was a reason to believe a Kosher witness mid'Oraisa, Chachamim enacted even in cases when this reason does not apply.

Poskim

Shulchan Aruch (EH 17:3): If a man went overseas and a witness came and testified that he died, his wife is permitted. The witness can even be a slave, Shifchah, woman or relative. If mid'Rabanan the witness is Pasul for Edus, he is Kosher for this Edus, but not if he is Pasul mid'Oraisa.

Question (Chelkas Mechokek 9): Slaves are assumed to be rampant transgressors. If a slave is found to be a thief, the sale is valid, because most or perhaps all slaves are thieves (Bava Basra 92b-93a). The Tur holds that slaves are not reliable for Shechitah. If so, why is a slave better than one who is mid'Oraisa Pasul for Edus?

Answer #1 (Chelkas Mechokek 9): Even though a slave is Muchzak to be a sinner, perhaps a Yisrael Ba'al Aveirah is worse.

Taz (2): A Yisrael Ba'al Aveirah definitely lies. This is not true about a slave.

Answer #2 (Beis Shmuel): The Bach (DH Ishah) says that we allow slaves that are known to be Kosher.

Rebuttal (Hagahos Baruch Frankel, brought in Shulchan Aruch ha'Shalem): There is no proof that this is the Bach's intent (perhaps he rather says that the Pesul of slaves is not due to Aveiros).

Answer #3 (Hagahos Baruch Frankel): Perhaps slaves are like shepherds, who are Pasul only mid'Rabanan. Mid'Oraisa, each has Chezkas Kashrus until we know otherwise.

Ketzos ha'Choshen (46:17): A Rasha not of Chamas is not suspected to lie. A Gezeiras ha'Kasuv disqualifies him. The Nimukei Yosef (Sanhedrin 5b, brought above) says that he is Kosher for Isurim, e.g. Edus Ishah.

Pischei Teshuvah (18): The Beis Yosef (CM 30) was unsure about this. Why did he not mention it here? We do not find anyone who disagrees! Nevertheless, one must be stringent.

Shulchan Aruch (ibid.): If a Pasul mid'Oraisa spoke l'Fi Tumo, he is Kosher. The same applies to a Nochri or a Yisrael, even if he wantonly transgresses idolatry or the entire Torah.

Beis Yosef (DH u'Mah she'Chosav v'Im): The Ran (Teshuvah 3) says that the Rif and Rambam hold that even a Mumar is believed l'Fi Tumo; the Rashba agrees with them. Some argue with the Rif (and disqualify l'Fi Tumo).

Aruch ha'Shulchan (30,31): If one is suspected of Arayos, we cannot believe what he says l'Fi Tumo. Since his lust overpowers him, perhaps he is scheming! Perhaps the same applies to Apikorsim.

Rema (42:5): One who is suspected to transgress Arayos is Pasul for Edus Ishah, both for Kidushin and divorce.

Chelkas Mechokek (14,15): One who is merely suspected is Pasul mid'Rabanan, so he is Kosher for Edus Ishah (of death). The Rema disqualifies only for Kidushin and divorce. This is unlike Rashi (Sanhedrin 26b DH Le'afukah).

Pischei Teshuvah (17:23): Mar'os ha'Tzov'os says that we cannot infer this from the Rema's words. However, since the Poskim do not discuss one who is suspected regarding Edus Ishah, this shows that he is Kosher.