Niddah, Chapter 9, Mishnah 4. This chapter deals with kesamim, bloodstains whose origin is unknown, and with the circumstances in which a woman may attribute a stain she has found to some source other than her own body and thereby remain tehorah. Our mishnah applies that principle to garments and seats that were used by more than one woman.
The mishnah opens: "Hish'ilah chalukah," a woman lent out her tunic, "lenachris o leniddah," to a non-Jewish woman or to one who was already in the state of niddah. Afterward the owner took the garment back, put it on herself, and noticed blood on it. In theory the blood might have come from her own body in the interval during which she was wearing it. Even so, the halachah is "harei zo tolah bah": she is entitled to trace the stain to the woman who had borrowed the garment.
Why is this permitted? Because such an explanation is genuinely reasonable. A non-Jewish woman pays no heed to whether she leaves blood behind on clothing, and a woman in her niddah state is bleeding at that very time in any event. Given how likely it is that the borrower produced the stain, the owner may assign it to her and remains tehorah herself.
The mishnah moves on to a case in which no such explanation is available. "Shalosh nashim shelavshu chaluk echad": a single tunic was worn in turn by three different women. "O sheyashvu al safsal echad": alternatively, the three of them took their places one after another upon the same bench. "Venimtza alav dam": once all three had used the garment or the bench, blood was discovered on it, with nothing to show whose body it came from.
Here the ruling is "kullan temei'os," all three are temei'os. In such a case each woman must be choshesh, must reckon with the possibility that she herself was the source of the blood. Since no one of them can point to a source she is entitled to rely on, each one must treat herself as temei'ah.
The mishnah then limits this ruling in one particular setting. "Yashvu al safsal shel even": if the women had seated themselves upon a stone bench, "o al itztava shel merchatz," or upon the stone shelf of a bathhouse, then "Rabbi Nechemya metaher": in Rabbi Nechemya's opinion they stay tehoros.
Rabbi Nechemya's reasoning is spelled out in the mishnah itself: "kol davar she'eino mekabel tumah," whatever is incapable of becoming tamei, "eino mekabel kesamim," stands outside the halachos of kesamim as well. Stone utensils and stone surfaces never receive tumah at all; accordingly, blood found upon a stone bench has no power to make the women who sat on it temei'os.