Niddah, Chapter 9, Mishnah 3. This chapter is occupied with kesamim, bloodstains that appear somewhere and leave a woman uncertain whether the blood came from her own body at all. The recurring question is whether she may point to another possible source and remain tehorah. Our mishnah brings such a case, where a man and a woman have both used the same vessel.
The mishnah describes "ish ve'ishah," a man and a woman, who "asu tzarcheihem" (relieved themselves) into a single "sefel," a bowl that collected what came from each of them. Then, "venimtza dam al hamayim": blood turned up in the water that had gathered there. Because both of them had used that one bowl, its origin cannot be established; the blood may have come from either body.
On this the Tannaim differ. "Rabbi Yose metaher": Rabbi Yose rules that the woman remains tehorah. "VeRabbi Shimon metamei": Rabbi Shimon rules that she is temei'ah.
Rabbi Yose's reasoning rests on the second person who used the basin. It is possible that the blood came from the man, and where such a possibility exists we are entitled to rely on it. Since the woman need not assume that the blood is hers, she is tehorah.
Rabbi Shimon rejects that reliance: "she'ein derech ha'ish lehotzi dam." It is quite unusual for a man's body to discharge blood, while a woman's body discharges blood regularly. Therefore, he says, "ella shechezkas damim min ha'ishah": the presumption regarding blood is that it comes from the woman. Once that is the presumption, the mere fact that a man also used the vessel is not enough to remove her from suspicion, and she must treat herself as temei'ah.