This mishnah deals with a woman who is suspected of having had relations with a man whose identity is unknown to us. The question is whether we must be concerned that this man was not a Jew of fit lineage, which would disqualify her and disqualify her child as well. This is the subject of our mishnah and the one that follows.
The case in the mishnah:
She was seen "medaberes im adam echad ba'shuk" - speaking with a certain man in the marketplace. Although the mishnah uses the language of speaking, the intention is seclusion: she was secluded with that man, and therefore we are concerned that they had relations. It is worth noting that some texts do not read "ba'shuk" (in the marketplace), since that does not fit a situation of seclusion.
They said to her: "mah tivo shel ish zeh?" - who is this man? And she answered: "ish peloni, ve'kohen hu" - he is so-and-so, and he is a kohen. She does not necessarily mean an actual kohen, but rather that he was a man of fit lineage, a proper Jew.
The dispute among the Tannaim:
Rabbi Yosi and Rabbi Elazar: She is believed. Once she has said that the man was of fit lineage, she is not disqualified and she is permitted to marry a kohen. Their reasoning is her presumption of fitness: until now she was known as a woman of fit standing, permitted to a kohen, and when we stand in a case of doubt, we leave her in her prior presumed status. She is therefore believed when she says that he was fit, and she remains as fit as she was until this moment.
Rabbi Yehoshua: He disagrees and says "ein anu chayim al piha" - we do not conduct ourselves according to her word. Rather, we must assume that she had relations with a nasin, one of the Gibeonites, whom it is forbidden to marry even after they converted, unless she brings proof for her claim.
Rabbi Yehoshua's reasoning:
Even though she had a presumption of fitness, the very fact that she went and secluded herself with an unfamiliar man constitutes a flaw in that presumption, a re'usah. The flaw arises from her having done something that indicates that matters are not as they should be, and once the presumption has been flawed we can no longer leave her in it. We must therefore assume the worst: that she indeed had relations with a man forbidden to her, and from now on her status is that of a zonah and she is forbidden to marry a kohen.
In summary: The Tannaim disagree about a woman who was secluded with an unknown man and claimed that he was of fit lineage. According to Rabbi Yosi and Rabbi Elazar she is believed on the strength of her presumption of fitness, while according to Rabbi Yehoshua her presumption was flawed by the act of seclusion itself, and therefore we do not follow her word unless she brings proof, and she is presumed to have had relations with a nasin and is forbidden to a kohen.
In the next mishnah we will continue to deal with the question of the suspected woman's credibility and with determining her fitness and the fitness of her child.