Niddah, Chapter 10, Mishnah 3. This mishnah deals with chazakah, the established status a woman carries from one internal examination to the next. A niddah is expected to make a bedikah toward the end of her seventh day, bein hashemashos, in order to draw a clean line between her days of niddah and what follows. The mishnah asks what we conclude when she omits that final examination, and only later checks herself.
When the day seven check came back clean
"Niddah shebadkah atzmah": a woman in her days of niddah performed a bedikah, "yom shevi'i shacharis", during the early hours of day seven, "umatza'asah tehorah", and the cloth came back clean. "Uvein hashemashos", at dusk of that same day, "lo hifrishah": she made no further check to mark off her niddah period from whatever might follow. "Ule'achar yamim badkah", several days later she checked once more, "umatza'asa temeiah", and this time blood was found.
The mishnah rules: "harei hi bechezkas tehorah". Her standing remains one of purity. We take it that when she immersed she was genuinely tehorah, and the blood she has just discovered is credited to the present moment alone, not read backward as something that had already been there.
A bedikah in the morning that found blood
Now the mirror image: "Badkah atzmah beyom hashvi'i beshacharis", she did her bedikah early on day seven, "umatza'asa temeiah", and the flow was still present. "Uvein hashemashos lo hifrishah", at twilight, once again, no check was made. "Ule'achar zman badkah", sometime later she examined herself, "umatza'asa tehorah", and this bedikah showed her clean.
Here the ruling is "harei zo bechezkas temeiah". The pattern she established is one of tumah, and it carries forward until the moment of that clean bedikah. Consequently, any taharos she handled during the intervening period are retroactively rendered temeios.
"Umetameiah me'es le'es umipkidah lifkidah" - she conveys tumah retroactively as well: back for as much as a twenty four hour period, and back to the time of her previous internal examination.
"Ve'im yeish lah veses, dayah sha'atah": a woman with a fixed cycle is judged from her sighting alone. Nothing before that point is affected, and taharos she handled earlier remain pure.
Rabbi Yehudah and the Chachamim
"Rabbi Yehudah omeir": "kol shelo hifrishah betaharah", any woman who failed to close off her niddah with a clean bedikah, "min haminchah ulema'lah", from the hour of minchah onward, that is, from midday of day seven, "harei zo bechezkas tum'ah", retains her presumption of tumah from that point forward. Rabbi Yehudah is thus arguing with the tanna kamma, who was satisfied with an examination taken early on the seventh day as enough to set up a fresh chazakah of taharah.
The Chachamim swing to the far side: "Vachachamim omrim: afilu bashniyah lenidasah", even where the clean check took place already on day two of her niddah, "umatza'asa tehorah", that bedikah showing her pure, "bein hashemashos lo hifrishah", while on the seventh day she performed no hefsek taharah at all, "ule'achar zman badkah umatza'asa temeiah", and once her niddah week had passed she checked and found blood, the ruling is still "harei zo bechezkas taharah". That one clean examination, taken anywhere within the week of niddah, even as early as its second day, has already fixed her presumption as one of purity.
The gap between the two opinions is thus a wide one. Rabbi Yehudah insists that the hefsek taharah be made from midday of the seventh day and onward, while the Chachamim hold that a clean examination at any time during her days of niddah is sufficient to establish her chazakah of purity.