Ohalos, Chapter 15, Mishnah 4. Our Mishnah takes an ordinary house and puts up a partition inside it, and then asks how tumah travels once that partition is in place: does the mechitzah stop the tumah, or does the tumah break through it?
The Mishnah begins: "Bayis shechatzatzo binssarim oi biri'os," a house that someone divided with boards or with hangings. It makes no difference whether the divider is solid wood or a curtain that was hung up.
Two Ways to Divide a House
The word mechitzah usually brings to mind a barrier standing on end between two chambers, reaching upward until it meets the roof above. Our Mishnah teaches that this is merely one of the forms such a divider can take: "ena hachinami, min hatzedadin," it may be erected sideways, so that a person crossing the width of the building would meet first the outer wall, then the first space, then the divider, then the second space, and finally the wall opposite.
But a partition can also run the other way. "Oi min hakoros," or from under the beams: the mechitzah stretches from the wall on the right to the wall on the left, spread out horizontally beneath the ceiling, so that the house now has a lower space and an upper space. Either arrangement is included in our Mishnah, and the halachah that follows applies to both.
Tumah in the House, Keilim Behind the Mechitzah
"Tumah babayis, keilim shebachatzatz tehorim." Where the source of tumah lies within the house, any keilim described as "bachatzatz" stay tahor. Read literally, "keilim shebachatzatz" would describe keilim lodged inside the body of the partition, within its thickness, but the common explanation is that these words refer to keilim standing beyond the divider, and that is the way we will study the Mishnah.
What keeps them tahor? The partition does what a barrier does and prevents the tumah from advancing. There is nothing puzzling here. Picture an undivided house: vessels lying above it on the roof are unaffected, and vessels set down outside its wall are unaffected, since roof and wall stand between them and the tumah within. Precisely the same logic governs our case. Someone has installed a barrier inside the building; the tumah occupies one of the two spaces, the vessels occupy the second, and the barrier stops the tumah from reaching where they stand.
Tumah Behind the Mechitzah, Keilim in the House
The Mishnah continues: "tumah bachatzatz, keilim shebabayis tmei'im." Turn the case around, put the tumah on the far side of the partition, and the keilim in the house become tamei. Here the mechitzah is powerless to prevent the tumah from reaching them.
At this point one is entitled to ask: which side is "the house" and which side is "bachatzatz"? Both sides are, after all, parts of the same building. The answer is the principle we have met many times in this masechta. The area we call "bachatzatz" is a section of the house that has no exit of its own. The only way for anything to leave that space is to pass through the mechitzah, cross the other section, and go out through the doorway.
Once that is clear, both halves of the Mishnah become understandable. Where the tumah lies "babayis," in the portion that opens to the outside through the entrance, keilim beyond the divider are unharmed, for the divider stands in the way and the tumah has no need to pass it. Where the tumah lies "bachatzatz," in the sealed portion possessing no exit of its own, the keilim of the front portion contract tumah, since a partition of this sort cannot confine it. Tumah is never content to stay put: derech tumah latzeis, its nature is to leave, taking whatever route is least obstructed. When the inner space is furnished with neither doorway nor window, the only path outward runs through the divider and across the front of the building, and whatever keli lies along that path becomes tamei.
The Bartenura's Illustration
The Bartenura brings the idea home with a case we know well: an earthenware vessel closed off by a tzamid pasil, a lid fastened tightly around its rim. Should tumah be present in the dwelling, the contents of that sealed keli cheres are shielded from it. Reverse the arrangement, placing the tumah within the sealed vessel, and the surrounding dwelling receives no such shielding, for that tumah has to emerge. Tumah refuses to remain where it is; it presses outward, and no covering will imprison it.
The Third Case: Tumah and Keilim Together Behind the Mechitzah
A third situation is now put before us: "keilim shebachatzatz" sharing their space with the tumah. Until now the two were always apart. First the vessels stood behind the divider while the tumah lay in front; then the tumah stood behind while the vessels lay in front. Here both of them are located in the sealed-off area past the divider, and the ruling reads: "im yesh sham posei'ach tefach, tmei'im, ve'im lav, tehorim."
The decision turns entirely on how much room there is. We instinctively imagine a divider placed at the middle of the room, yet nothing requires that. It may equally be positioned so that no more than a thin sliver of space is left behind it. The Mishnah therefore asks whether that sliver amounts to a posei'ach tefach, a square tefach of open space. If it does, the area carries the status of an ohel, and since the source of tumah and the vessels share that single ohel, the vessels are of course tamei. Lacking a square tefach, the area has no din ohel, and the vessels stay tahor, because in any place where no ohel exists tumah travels upward and downward in a direct line only, never outward to either side.
The outcome is remarkable. Even in this very arrangement the house itself is tamei, since the tumah does make its way forward into the outer section; nevertheless, the cramped area holding the tumah alongside those vessels remains tahor.
Comparing the Cases
A natural question arises here. Think back to the second situation: tumah behind the divider, vessels standing in the outer part of the building, and we ruled those vessels tamei. Suppose the rear space fails to hold a square tefach. Should that not rescue the vessels out front as well?
The answer is that it changes nothing; no rescue is available to them. Those vessels share a single building with the source of tumah, and the divider is incapable of holding it back on their account. The tumah consequently fills the entire structure and defiles all that it contains. Regarding the outer portion our view is of the building in its entirety, since it has already been established that the tumah is bound to emerge, and the building in its entirety sits beneath one roof.
The vessels lying beside the tumah in the rear, within a space that does not amount to a square tefach, are nonetheless saved. Think again about the Mishnah's opening ruling, where tumah in the outer section is halted by the divider and cannot reach what stands behind it. That very divider has here produced a state of affairs in which these vessels are not situated in any ohel whatsoever, so the tumah next to them ascends and descends in a vertical line alone, with no lateral spread at all. The Rishonim offer several different ways of accounting for this precisely, though this is the basic thrust.
Summing Up
Such, then, is our Mishnah. A dwelling has been split by a mechitzah, standing on end across the width of the room or laid out flat beneath the beams, fashioned either from planks or from drapes. Where the tumah lies in the portion containing the entrance, vessels beyond the divider escape tumah, the partition serving as a barrier. Where it lies in the far portion, which can be left only by passing forward through to the front, the vessels of the dwelling contract tumah. And as for vessels sharing that sealed portion with the tumah itself: a posei'ach tefach there renders them tamei, the space being an ohel in its own right; without it, the tumah moves vertically alone, up and down, and the divider rescues those vessels from tumah. The vessels in front are rendered tamei, while the vessels at the back, positioned right alongside the tumah in a space of under one square tefach, come away tahor.
A Word from the Tosfos Yom Tov
Throughout, we have taken "keilim shebachatzatz" to describe vessels situated past the divider. But what would be if they were genuinely lodged inside the divider itself? The Tosfos Yom Tov takes up this question: with tumah present in the dwelling, vessels literally set within the mechitzah keep their taharah only where their space amounts to a posei'ach tefach. The case, then, involves an unusually thick partition, and vessels occupying a full tefach of space inside it are shielded there too, not merely on the other side of it. This is the ruling of the Tosfos Yom Tov.