Taharos, Chapter 4, Mishnah 4. Our chapter has been occupied with cases of safek tumah, and in particular with tumah that is in motion, where the halachah often treats us generously. Now the Mishnah presents a case in which that generosity does not apply, because the tumah involved is the tumah of ohel. A corpse does not only transmit tumah by touch or by being carried: it spreads tumah through the airspace above and beneath it. And for ohel there is no leniency of motion. Whether the source of tumah is resting or being carried through the air, the ohel it forms is an ohel.
The Mishnah opens: "Kezayis min hameis befi ha'orev", an olive's bulk of a corpse held in the beak of a raven. A kezayis of a meis is enough to create tumas ohel, and here it is airborne, carried off by a bird. Ordinarily the fact that a tumah is in motion would work in our favor. Here it does not help at all, precisely because we are dealing with ohel.
"Safek he'ehil al ha'adam ve'al hakelim birshus hayachid": there is a doubt whether the raven passed directly overhead, so that the kezayis formed an ohel over a person and over vessels standing below, and the incident took place in a reshus hayachid, a private domain. That detail is decisive. A safek tumah that arises in a reshus hayachid is ruled tamei, while the same safek in a reshus harabbim would be ruled tahor. So the private setting is what creates our problem in the first place.
Having established that we are in a reshus hayachid, the ruling splits between the two parties standing underneath. Regarding the person, the safek is tamei. Regarding the vessels, the safek is tahor. The distinction rests on a principle that has been guiding us through these mishnayos: the rule that a safek in a reshus hayachid is tamei applies only where there is someone with da'as lisha'el, an entity capable of being questioned about what happened. A human being can be asked, "Did the bird fly over you or not?", and therefore his doubt is resolved stringently. Vessels have no awareness and cannot be interrogated, ein bahem da'as lisha'el, and so their doubt is resolved leniently and they remain tahor.
From here the Mishnah shifts to a doubt of an entirely different character. Its words are "Hamemalei asarah deli'im", followed by the case of a sheretz turning up in one of them. Imagine a household of an earlier era, with its own well in the courtyard. The owner lets down his bucket, hauls it up, and repeats the labor until ten filled buckets are lined up beside him. Only then does his eye fall on a sheretz, one of the small dead creatures that convey tumah, lying inside one of the ten. It is a deflating discovery, and one would naturally expect that every last bucket must be emptied out and the whole tiring effort begun again from the start.
The Mishnah rules otherwise: "hu tamei vechulan tehorin". Only that one bucket is tamei; the other nine remain tahor. Why not suspect that the sheretz was in a different bucket first and only afterward ended up where we found it, or that all the buckets drew from contaminated water? Because of the rule we encountered earlier in this chapter: "kol hatumos kish'as metzi'asan", every tumah is judged according to the moment it was discovered. The sheretz was found in one bucket, and that is the only bucket we may declare tamei. Even if it is entirely plausible, not at all far-fetched, that it had been elsewhere beforehand, we do not extrapolate backwards.
One might still argue that the sheretz surely came up out of the well, and that the well water itself was therefore tamei, poisoning every bucket drawn from it. That argument fails for a separate reason: water in a well is mechubar lakarka, joined to the ground, and as we will see, what is attached to the ground is not mekabel tumah. So even if we could reconstruct that the sheretz had been floating in the well, the well water would not be rendered tamei by it. The only thing that would render the other nine buckets tamei is proof that the sheretz was actually in them, and we have no such proof.
The closing case sharpens the issue even more. The Mishnah speaks of "Hame'areh mikli lechli", where a sheretz then turns up in the receiving vessel below. Someone empties water out of one utensil into another, and afterward a sheretz is spotted in the lower one. Reason would seem to settle the matter instantly: nothing had been in that lower utensil beforehand, the sole thing poured into it was the water from above, so the sheretz can only have traveled down along with that water, meaning it had been sitting in the upper utensil just moments before. And yet the Mishnah declares "ha'elyon tahor": the vessel on top keeps its taharah. The principle stands as before, kol hatumos kish'as metzi'asan. Tumah is judged in the place and at the time it came to light, and we are not authorized to trace backward to wherever it presumably was earlier.
Two lessons emerge side by side in this Mishnah. Where a corpse forms an ohel, motion offers no escape, and a person standing beneath a doubtful ohel in a private domain is tamei because he could have been asked. But where the question is one of reconstructing a tumah's history, the halachah holds firm: we judge what we found, at the moment we found it, and no further.