QUESTION: The Mishnah states that originally ("Mishnah Rishonah"), the testimony of a woman who said that she had become defiled (through adultery) was accepted, and based on that testimony she was prohibited to her husband. The Chachamim later decreed that her testimony in this matter is not accepted. They made this decree in order to prevent women from using this claim as a ruse for obtaining a divorce.
The RAN asks that if m'Ikar ha'Din the woman's testimony is accepted as the Mishnah Rishonah ruled, why were the later Chachamim permitted to disqualify her testimony and allow the couple to continue living together? She is forbidden mid'Oraisa to her husband!
(a) The RAN writes that since her testimony is believed mid'Oraisa, the only way the Chachamim could permit her to remain married is by applying the principle of "Afke'inu Rabanan l'Kidushin Minei" -- the Chachamim utilized their authority to uproot the original Kidushin (retroactively) and, consequently, she was not married at the time of her infidelity. Accordingly, although the woman's testimony is believed, she does not become forbidden to her husband as a result because "Afke'inhu" effectively makes her unmarried at the time of her illicit act.
This principle, however, applies only to remove the Isur of Eshes Ish and permit the woman to her husband. If she was married to a Kohen and she sinned with a man, such as a Nochri, with whom relations renders her unfit to marry a Kohen, she will remain forbidden to her husband even if "Afke'inhu" is applied. Annulling the Kidushin retroactively merely puts her illicit before she became an Eshes Ish; it does not rectify the fact that she lived with a Nochri and thereby became disqualified from marrying any Kohen.
(b) The RAN cites the view of the RASHBA and TOSFOS who explain that although the woman is believed, the Chachamim allow her to remain married to her husband because of the principle, "Yesh Ko'ach b'Yad Chachamim la'Akor Davar Min ha'Torah" -- the Chachamim are empowered to override a Halachah of the Torah where they deem necessary. The Ran asks that this principle applies only when the Halachah will be transgressed passively ("Shev v'Al Ta'aseh") but not in situations such as this one in which the law is actively transgressed. In this case, if the husband continues to live with his wife he actively transgresses the prohibition ("Kum v'Aseh"). The other Rishonim answer that in cases of great necessity the Chachamim are empowered to override a Halachah of the Torah even with "Kum v'Aseh."
(c) The RAN cites another opinion which states that in truth the woman's testimony was never accepted. Since she is obligated ("Meshubedes") to her husband, she is not believed to prohibit herself upon him and thereby nullify her obligation. The reason why the Mishnah Rishonah maintained that she is believed is that it is unlikely that she would lie about such an embarrassing matter and thus she presumably is telling the truth. However, when it became known that women were willing to suffer the embarrassment in order to obtain a divorce, the Chachamim repealed their original Takanah and no longer accepted her testimony.
It emerges that there is a Machlokes Rishonim about the intent of the original Halachah of the Mishnah Rishonah, which maintained that the woman is believed. According to one opinion, the Mishnah Rishonah maintained that the woman is believed mid'Oraisa. According to another opinion, the Mishnah Rishonah maintained that the woman is believed only mid'Rabanan.
According to the first opinion, why is she believed mid'Oraisa? A general rule states that "Ein Davar sheb'Ervah Pachos mi'Shenayim" -- any matter which involves a prohibited relationship requires the testimony of two witnesses. Accordingly, two witnesses should be required to prohibit the woman to her husband. The answer is that her testimony is accepted based on the principle of "Shavyah a'Nafshah Chatichah d'Isura," which states that one is believed to testify about a matter of Isur insofar it relates to herself. Testimony which is "Shavyah a'Nafshah Chatichah d'Isura" overrides even the testimony of valid witnesses. Therefore, the woman's testimony effectively prohibits her from living with her husband even though the husband has no reason to believe her; her testimony is accepted only as it relates to her, but not as it relates to her husband. The reason the husband is forced to divorce her is in order not to force her to transgress an Isur.
The RASHBA writes that although, according to the Mishnah Acharonah, the Chachamim do not accept the woman's testimony, if the husband says that he believes her testimony he is required to divorce her, because by making such a claim the husband invokes the principle of "Shavyei a'Nafshei Chatichah d'Isura." The REMA (EH 178:9), however, writes that the rationale of the Mishnah Acharonah not to believe the woman -- that the Chachamim fear that she set her eyes on another man and therefore wants a divorce from her husband -- applies even to the husband, whom the Chachamim suspect of similar intent. Therefore, in practice (nowadays, after the Cherem of Rabeinu Gershom prohibited polygamy), even if the man says that he believes the woman, he is suspected of impure intent and thus he and his wife may continue living together. The YAM SHEL SHLOMO disputes this ruling on the grounds that since the woman originated the statement concerning her adultery, there is no reason to suspect the husband of impure intent.