Taharos, Chapter 10, Mishnah 7. This chapter has been occupied with the world of the winepress and the olive press: the grapes, the olives, the vats and the pits, and the many ways that tumah and taharah play out in the middle of a working harvest. Our mishnah takes up a very practical mishap. A man is emptying out his wine pit, filling jar after jar with the wine that has collected there, and in the middle of the work he discovers a dead sheretz. What has become tamei, and what has remained tahor?
Where the sheretz turns up
The Mishnah opens: "Hazolef es habor, nimtza sheretz barishonah, kulan temei'os." If one is drawing the wine out of the pit and the sheretz is discovered in the very first jar, all of the jars are tamei. Since the sheretz showed up at the very beginning of the work, we assume that it came from the pit itself, that the pit was tamei, and therefore everything drawn from it afterwards is tamei as well.
"Ba'acharonah, hi temei'ah vechulan tehoros." If, however, the sheretz is discovered in the last jar, only that jar is tamei and all the rest are tahor. Here we have no license to spread the tumah backwards. We are not permitted to invent a history for the sheretz and imagine that it was floating there all along; we assume it arrived only at the end, and the earlier jars keep their taharah.
When does this apply?
"Eimasai? Bizman shehu zolef bechol achas ve'achas." The Mishnah asks when this ruling holds, and answers: when he fills each jar directly, one at a time, straight from the pit. Each jar has its own immediate relationship with the wine of the pit, and so the location of the sheretz tells us something about the pit.
"Aval im haya zolef bemachatz, nimtza sheretz be'achas meihen, hi temei'ah bilvad." But if he was drawing the wine with a machatz, a large ladling vessel from which he then poured into the jars, and the sheretz was found in one of the jars, only that one jar is tamei. Because there is an intermediary step in the process, we attribute the sheretz to the ladling vessel rather than to the pit, and the rest of the jars are unaffected.
Checking and covering
Once again the Mishnah narrows the case: "Eimasai?" Under what circumstances was that last ruling stated? Only where the worker's care was partial. Either he inspected the jars but left them open ("bodek velo mechaseh"), so that although they were clean when the wine began to flow in, a sheretz could have dropped inside afterwards; or he closed them up without ever having looked inside beforehand ("mechaseh velo bodek"), so he has no way to vouch for their condition before the wine arrived. In both scenarios a gap remains in the jar's record, and the sheretz can reasonably be traced to that jar or to the ladle rather than to the pit, so the tumah spreads no further.
"Haya bodek umechaseh": but if he did both, examining each jar and then sealing it so that nothing further could enter, the picture changes entirely. Now, when a sheretz is discovered, there is nowhere else to place the blame. Therefore: "nimtza sheretz bachavis, hakol tamei." If the sheretz is found in a jar, everything is tamei, because since the jars were all inspected and sealed, the only possible source is the pit itself. "Babor, hakol tamei": if it is found in the pit, then of course everything is tamei, both the wine remaining in the pit and everything drawn out of it. "Bamachatz, hakol tamei": and if it is found in the ladling vessel, again everything is tamei, since every jar passed through that vessel.
The lesson of the mishnah is the honest weighing of evidence. We do not spread tumah farther than the facts allow, and we do not shrink it smaller than the facts demand. Where a jar's own history is unknown, the tumah stays with the jar. Where the worker has done his job faithfully and the jars have a clean record, the sheretz itself becomes testimony about the pit, and the whole batch is tamei.