Ohalos, Chapter 5, Mishnah 4. This mishnah continues the case that the chapter has been building: a meis lying on the ground floor of a house, an opening in the floor of the upper story above it, and an earthenware pot belonging to an am ha'aretz set into that opening. The question all along has been what happens to the things sitting in the upper story.
To place this mishnah properly, we need the ruling that preceded it. Beis Shammai and Beis Hillel eventually reached one position ("oso midah"), and the halachah is that ochlin, mashkin and klei cheres in the upper story remain tahor. Why? Because there is no concern that a chaver will come along and borrow such a vessel from an am ha'aretz. An earthenware vessel has no taharah in a mikvah at all, so the chaver knows from the start that it is unusable to him.
Vessels that can be restored to taharah by immersion, however, whether of wood, of metal or of hide, are declared tamei. The worry of the Chachamim ran like this: a chaver might borrow one of them, dip it, and consider the matter settled. What would escape him is that the tumah at issue is tum'as meis, for which immersion alone accomplishes nothing; haza'ah is also required, sprinkling on the third day and again on the seventh with water into which the ashes of the Parah Adumah have been mixed.
The lagin of tahor liquid
Now to our mishnah. The case is a "lagin," a jug fashioned of wood or of metal, "shehu malei mashkin tehorin," filled with liquids that are tahor. The jug is standing in the upper story. Below is the meis, and the opening between the two floors is covered by the earthenware pot of an am ha'aretz.
The ruling opens with "halagin tamei": the jug picks up tumah, and not a mild one but a full seven-day tumah, tum'as shivah. Since a jug of this kind is exactly the sort of vessel a mikvah can purify, the earthenware pot lodged in the opening affords it no protection at all.
And then comes the striking half of the halachah: "veha'mashkin tehorin." The liquid sitting inside that very jug stays tahor. It does not become tamei from the jug that holds it. Only "im pinan," if one empties the liquid out and transfers it "bikli acher," into a second vessel, one that is not earthenware but rather has taharah b'mikvah and therefore is itself tamei, then the halachah is "tem'ein," the liquid becomes tamei. The leniency that the vessel is tamei while its contents remain tahor applies only to the original vessel in which the liquid was lying.
Why the liquid stays tahor
The Bartenura explains the logic. Suppose we did not say this. What, then, would be left of the ruling that ochlin and mashkin in the upper story are tahor? Liquid does not float in mid air. Orange juice has to be sitting in some vessel. And most liquids are kept in vessels that have taharah b'mikvah, metal, wood or hide. So if the Chachamim told us that mashkin upstairs remain tahor, they must have meant even mashkin resting inside a vessel that has itself become tamei. Once the liquid is poured off into a different vessel, that reasoning falls away and we return to the ordinary rules of tumah.
The mefarshim press the point: why is this the only way? The liquid could be sitting in a kli cheres, which does not become tamei here at all, or in a stone vessel, which is not mekabel tumah in the first place. So there is a way for the halachah of tahor mashkin to have meaning even without this chiddush.
The Mishnah Achronah, together with other mefarshim, takes up the difficulty. Their answer is that in real life liquids were not stored in stone utensils or in earthenware, but in the kind of vessels a mikvah can purify. The Bartenura therefore means that the ruling would be left with almost nothing to govern were it not for this teaching: the container is rendered tamei, being immersible, while what it holds, having no route to taharah through immersion, keeps its taharah.
Keep in view throughout that nothing here is Torah tumah. On the level of d'Oraisa, that earthenware vessel plugging the gap does its work and keeps the tumah confined to the lower floor. Everything upstairs is tamei only by rabbinic decree, and a body that enacts a gezeirah draws its own boundaries: the Chachamim extended it to the container and stopped short of its contents.
The woman kneading dough
The mishnah continues: "ha'ishah shehi lashah," a woman standing in the upper story kneading dough "ba'areivah," in an areivah, the large kneading trough, that great shissel we have met a number of times in Mesechtas Keilim, made of wood or of metal.
Here the mishnah teaches a further chiddush. Just as the pot in the opening does not rescue vessels that have taharah b'mikvah, so too it does not rescue people, for a person also has taharah b'mikvah. The reasoning is identical. The concern was that a chaver would borrow the vessel and not realize that tevilah alone is insufficient for tum'as meis. Precisely the same mistake can be made with a person: the chaver may summon someone who had been in that upper story, telling him to immerse and then come handle his food, never suspecting that this man has a far heavier problem, having stood in an ohel hameis. So when there is an earthenware pot of an am ha'aretz set into the opening between the floors, the gezeirah renders tamei both keilim that have taharah b'mikvah and people.
Therefore the halachah in this case, with the meis below and the am ha'aretz's earthenware pot covering the opening, is: "ha'ishah veha'areivah teme'im tum'as shivah." The woman and the trough are both tamei with tum'as shivah, since anything possessing taharah b'mikvah, whether a vessel or a human being, is not shielded in the upper story from the tumah of the meis.
"Veha'batzek tahor." The dough, however, remains tahor, since ochlin and mashkin are spared. "Ve'im pinaso l'kli acher," but if she empties the dough out of the trough and transfers it into another vessel, it is tamei.
Why the mishnah repeats that Beis Hillel retracted
One should not imagine that all this is only the view of Beis Shammai, because the mishnah records "chazru Beis Hillel l'horos k'divrei Beis Shammai," that in the end Beis Hillel reversed themselves and ruled as Beis Shammai did. But why must the mishnah state this a second time?
The mefarshim explain from the Tosefta and from the Gemara in Mesechtas Chagigah. Beis Hillel initially objected specifically to this second stage of the halachah, the very case before us. The Gemara there records Rabi Yehoshua saying that he is genuinely ashamed of the words of Beis Shammai: can it be that a woman kneads in a trough and she and the trough are tamei while the dough is tahor? That a jug is filled with liquid and the jug is tamei while the liquid inside is tahor? But once Beis Shammai laid out their reasoning, Beis Hillel conceded and agreed with them.
So the earlier statement that Beis Hillel came around to Beis Shammai's view referred to the general rule, that ochlin, mashkin and klei cheres in the upper story stay tahor while anything that has tevilah b'mikvah becomes tamei. And the repetition here means that Beis Hillel went further still and accepted even these extreme applications: a metal jug that can be immersed in a mikvah becomes tamei while the liquid it holds stays tahor, and a woman kneading dough, both she and the trough that can be immersed becoming tamei, while the dough itself remains tahor.
An addition from the Bartenura
Consider again the ruling that moving the dough into a second vessel makes it tamei. The Bartenura notes a limitation even short of that: the dough keeps its taharah in the trough only while her hands have remained in it without interruption. Should she withdraw them for a moment and then take up her kneading again, her touch from that moment forward conveys tumah to the dough.
Summing up
Here is the case in summary. Upstairs stands a jug of wood or metal holding tahor liquid; a meis lies on the floor beneath; and the gap between the two levels is stopped up by an earthenware vessel that belongs to an am ha'aretz. The principle established in the previous mishnah is now pushed to its limit: food and drink retain their taharah, while whatever can be immersed in a mikvah does not. The result is that the container turns tamei even though what fills it does not, and only if that liquid is transferred elsewhere does it too become tamei.
The added novelty taught here is that the category of things purified by immersion is not confined to utensils: a human being also attains taharah in a mikvah, and is therefore swept into the decree. That produces the striking law of the woman kneading upstairs. Both she and the trough carry a seven-day tumah, requiring haza'ah on the third and the seventh day with the ashes-water of the Parah Adumah, and staying tamei for no fewer than seven days before their purification can be completed, while the batzek stays tahor. Move that dough into a second vessel and it turns tamei; and even without any transfer, the Bartenura teaches that once she has removed her hands and set them back in, her touch from then on is metamei the dough.