Niddah, Chapter 7, Mishnah 3. Our mishnah deals with a familiar and unsettling situation: something tamei turns up, and we have no idea how long it has been lying there. How far back does its tumah reach? The mishnah gives one rule for a dead sheretz discovered in a public passageway, and the very same rule for a stain of blood, a kesem, discovered on a garment.
A sheretz found in an alleyway
"Hasheretz shenimtza bamavoi metamei lemafreia." A dead crawling creature is found lying in an alleyway. Since we cannot know when it arrived there, its tumah operates retroactively: whatever passed through that alleyway is treated as tamei.
But this backward reach has a boundary. The mishnah sets the cutoff at the most recent moment when we have positive knowledge that the passageway was free of tumah. Two things can establish such a moment. One is testimony: someone declares that he inspected the spot and found it clear of any dead creature. The other is a known sweeping: we can pinpoint when the alleyway was last cleaned out, "ad sha'as hakibbud." Everything that passed through from that point onward is held to be tamei, while anything earlier remains unaffected.
A kesem found on a garment
"Vechein kesem shenimtza bachaluk metamei lemafreia." The identical rule governs a stain of blood found on a garment. It, too, conveys tumah retroactively, since we have no way of knowing when it appeared.
Here too we trace our way back to the latest moment the garment was verifiably clear. Either the owner attests that he checked the garment over and found no mark on it, or we can identify when it was last washed, "ad sha'as hakibbus."
"Umetamei bein lach bein yaveish." According to the first opinion in the mishnah, the condition of the stain when it is discovered makes no difference to this calculation. Whether the blood is still moist or has already dried, it conveys tumah retroactively all the way back to the last examination or laundering.
The view of Rebbi Shimon
Rebbi Shimon takes issue with this, and the wetness of the stain is exactly where he draws his line. "Hayaveish metamei lemafreia": a dried stain does carry tumah backward, for we may suppose it was fresh when it first appeared and dried out only later, which means it could have been sitting there for a long stretch. A stain still wet, however, gives evidence about itself. Its tumah reaches back no further than the earliest moment at which it could still be found in the wet state we are seeing now.
Applied concretely: should the moisture of the blood show that it can only have arrived a short while ago, Rebbi Shimon confines its retroactive tumah to precisely that brief window and nothing earlier. Such is the ruling in his opinion, and it rests on a straightforward observation: the appearance of the stain itself is testimony about when it came to be there.