Eduyos, Chapter 3, Mishnah 6. Before us is the last of the disputes between Rabbi Dosa and the Sages, and here too Rabbi Dosa is the lenient one. The subject under discussion is a woman connected to the priesthood - whether because her father is a kohen or because her husband is a kohen.
Eating Terumah in the Kohen's Household:
A woman married to a kohen, even if her father is not a kohen, becomes part of his household and is permitted to eat Terumah by virtue of being a member of his household - just as his children eat Terumah as part of his household, and so too his slaves and his animals. It emerges that even though she is not from a priestly family and is not the daughter of a kohen, she nevertheless eats Terumah.
The Prohibition of a Zonah and the Status of a Captive Woman:
A kohen may not marry a woman who has the halachic status of a zonah, and he may not remain married to her either. This includes a woman who was raped by a non-Jew. From here the question arises: a woman who was seized and held captive by non-Jews - do we assume that she was violated?
The view of the Sages, which is also the halachah in practice, is that we assume the worst, by rabbinic decree. She therefore takes on the status of a zonah and cannot remain married to her husband the kohen; and if she is the daughter of a kohen, she may no longer eat Terumah - even though we do not know with certainty what happened.
The Approach of Rabbi Dosa:
Rabbi Dosa, in keeping with his lenient approach that was not accepted as halachah, holds that since we do not know what happened, we do not assume the worst, and she is still permitted to eat Terumah.
The Approach of the Sages - 'The Mouth That Forbade Is the Mouth That Permitted':
The wording of the Mishnah presents the status of the captive woman as being in question, but one must be careful not to be misled: it is not in question at all, and the basic answer is that she does not eat. The only case in which the ruling changes is where the source of our knowledge that the woman was held captive is the woman herself - she is the one who told us about it.
In such a case the following rule applies: if she said, "I was taken captive, but I was not violated" - "hapeh she'asar hu hapeh shehitir" - the mouth that forbade is the mouth that permitted. The very same source of information from which we learned to forbid her is the source that permits her. Since we have nothing before us but her own testimony, if we accept it, we believe everything she says equally: just as we believe her that she was taken captive, so too we believe her that she was not violated, and this woman continues to eat Terumah.
But as a rule a captive woman does not eat, and certainly when we know from other witnesses that she was taken captive - even if she insists that she was not violated, by rabbinic decree we assume that she was, and she is forbidden to eat Terumah. This is the view of the Sages and this is the halachah in practice.
The words of the Mishnah:
"HaShevuyah ocheles biTerumah, divrei Rabbi Dosa" - a woman who was taken captive may eat Terumah, these are the words of Rabbi Dosa. We do not assume the worst; rather, we leave her in her presumed state of fitness, and therefore she continues to eat Terumah.
"VaChachamim omrim: yeish shevuyah ocheles v'yeish shevuyah she'einah ocheles" - and the Sages say: there is a captive woman who eats and there is a captive woman who does not eat. It depends, as the Mishnah goes on to explain.
"Keitzad? Ha'ishah she'amrah nishbeisi u't'horah ani - ocheles, shehapeh she'asar hu hapeh shehitir" - how so? A woman who said, "I was taken captive, but I am pure" - she eats, because the mouth that forbade is the mouth that permitted. She herself is the one who reports that she was held captive by gentiles, and she herself adds that nothing happened to her. The very same mouth from which the prohibition arose is the mouth that permits.
"V'im yeish eidim shenishbeis, v'hi omeres t'horah ani - einah ocheles" - and if there are witnesses that she was taken captive, and she says, "I am pure" - she does not eat. When witnesses testify that she was taken captive, that information exists independently of her own statement, and there is no longer any basis for the reasoning of "the mouth that forbade is the mouth that permitted." Therefore, even though she insists that she is pure, she does not eat Terumah, because by rabbinic law we assume the worst - that she was violated, as the gentiles would typically do to a captive woman.
In summary: In this Mishnah, Rabbi Dosa and the Sages disagree about the law of a captive woman. Rabbi Dosa does not assume the worst and permits her to eat Terumah, while the Sages distinguish between a captive woman who is herself the source of the information - about whom we say "the mouth that forbade is the mouth that permitted," and she eats - and a captive woman whose captivity is established by witnesses, who does not eat even though she denies that anything happened, because by rabbinic law we assume that she became tamei. The halachah follows the opinion of the Sages.