Niddah, Chapter 7, Mishnah 4. This chapter is occupied with the laws of kesamim, bloodstains discovered on a garment. When a stain is found and we do not know whose garment it is or where the blood came from, the question becomes whether we must treat it as blood of a niddah, which carries tumah, or whether we may assume it came from a source that involves no tumah at all. Our mishnah applies that question to garments that turn up in particular places, and the outcome depends on who the local population is and how careful they are.
The mishnah's opening ruling: "Kol haksamim haba'im miRekem tehorin." Every stained garment brought from Rekem is tahor. Rekem lay along the eastern edge of Eretz Yisroel, and its inhabitants were mixed, part Yidden and part gentiles. Nevertheless, a garment with a stain that arrives from there is attributed to a gentile woman, and her blood does not make the garment tamei.
What allows that presumption? Jewish women were scrupulous about their stained garments. A woman who noticed a stain would put the garment away and keep it out of circulation, precisely so that it would not travel about and carry tumah to whoever handled it. Consequently, a stained garment found loose in the world is unlikely to have come from a Jewish woman. If it originated in a mixed settlement such as Rekem, the reasonable conclusion is that its owner was a non-Jewess, and the garment is treated as tahor.
Rabbi Yehuda takes the opposite position. He rules "metamei": such a garment is tamei. His reason is given in two words, "mipnei shehein geirim veto'im," the Yidden of that place are converts who err in halacha. They had not been trained in the discipline of removing a stained garment from use, so one cannot rely on the assumption that the Jewish women there are careful. And once that reliance collapses, the garment before us may well have belonged to a Jewish woman, so it carries tumah.
The mishnah continues: "Haba'im mibein hagoyim tehorin." Stained garments that come from among gentiles are tahor. Here there is no complication at all. The garment comes from a gentile population, so the stain is presumed to be that of a non-Jewish woman, and such a stain conveys no tumah to the garment.
Next: "Mibein Yisroel umibein haKusim." What of a stained garment that comes from a place inhabited by both Jewish women and Kusi women? Rabbi Meir holds it is tamei. He does not credit either group with the care of putting stained garments away, so the stain may well be a stain that carries tumah, and the garment must be treated accordingly.
"Vachachomim metaherin, mipnei shelo nechshedu al kismeihen." The Chachomim rule that these garments are tahor, because neither Jewish women nor Kusi women are under suspicion regarding their stains. Both groups take care to keep a stained garment out of circulation. Therefore, when a stained garment is nevertheless found in such a place, the very fact that it is in front of us tells us that it did not come from a careful woman, and we attribute it to a non-Jewish woman. The garment remains tahor.
The thread running through the whole mishnah is a single principle: the fate of an unattributed bloodstain is decided by the reliability of the women in that place. Where care can be assumed, the stain is pushed off onto a source that involves no tumah; where that care cannot be assumed, the stain remains a real halachic concern.