Niddah, Chapter 9, Mishnah 5. Much of this chapter is occupied with blood that turns up somewhere and the question of whom, if anyone, we may hold responsible for it. Here the Mishnah presents a case of shared sleeping quarters, where several women could equally be the source, and asks how each of them is to be treated.
Here is the case. "Shalosh nashim", three women, "shehayu yesheinos bemitah achas", who were asleep together in a single bed. Afterward, "venimtza dam tachas achas meihen": underneath one of the three, blood was discovered. The halachah in such a case is "kulan temeios", that every one of them is given the status of a temeiah.
Why should the other two be implicated at all, when the blood was lying beneath a particular woman? Because people do not lie perfectly still while they sleep. Over the course of a night a woman shifts and turns and may well have been lying, at some earlier point, exactly where the blood was later found. We therefore do not simply pin the blood on the woman above whose place it happened to be discovered; each of the three is a genuine possibility, and each is treated as temeiah.
When one of them examines herself
"Badkah achas meihen venimtzes temeiah", one of the three performed an internal examination and found that she was in fact temeiah, that there was uterine blood inside her. In that case, "hee temeiah ushtayim tehoros": the one who examined herself is temeiah, and the other two remain tehoros. The Mishnah adds "vesolos zo bezo", the women are cleared through one another.
The logic is straightforward. Once it has been established that one of the three was actively a niddah at the time, the blood on the bed has a ready and obvious source. The other two are then entitled to attribute the stain to her, and they need not examine themselves at all in order to remain tehoros. One woman's discovery resolves the doubt for her companions.
Women who are not expected to see blood
The Mishnah then closes off an avenue one might have thought to use: "ve'im lo hayu re'uyos liros, ro'in osan ke'ilu hen re'uyos", if some of these women were not at a stage of life at which seeing blood is expected, we nevertheless regard them as though they were.
Consider a woman who is past menopause, or a girl too young to have begun seeing blood, or a woman who is currently pregnant or nursing. In each of these situations blood is not the ordinary expectation. One might therefore have argued that the stain should be laid at the door of the woman for whom bleeding is normal, leaving the others tehoros. The Mishnah rules otherwise. In this circumstance that consideration does not help her. All three are viewed as equally liable to have been the source, and for this purpose they stand on precisely the same footing as one another.
The lesson of the mishnah, then, is that shared space creates shared doubt. Where three women slept side by side, the blood belongs to all of them until something concrete, such as an examination that identifies its true source, settles the question.