Ohalos, Chapter 10, Mishnah 5. This chapter is built around the arubah, an opening in a ceiling, and the question of how tumah behaves when a house is pierced from below to above. Our Mishnah takes the case of a house with two such openings, one above the other, and asks what happens when each of them is smaller than a tefach.
The Case
Picture a two story house. There is an arubah in the ceiling of the ground floor, opening into the second floor, and directly above it a second arubah in the ceiling of the second floor, opening out through the roof to the sky. The two openings are lined up one over the other, so precisely that a person standing on the ground floor and looking straight up sees the sky, and if it rains he gets wet where he stands.
In the preceding Mishnah the two arubos each measured a full tefach by a tefach. Here the Mishnah changes one detail and opens with the words "ein ba'arubos pesach tefach," there is not in the arubos an opening of a tefach: neither the lower one nor the upper one has a square tefach of open space. That single difference will eventually reshape the halachah, because tumah does not travel from one ohel to another through a gap smaller than a tefach.
Tumah under the ceiling
The Mishnah opens with tumah "babayis," inside the house: a meis lying on the lower floor, or a kezayis of flesh from one, resting in the area that the ceiling genuinely covers. Any keli sitting on that floor beneath the solid ceiling contracts tumah, since the ceiling acts as an ohel and carries the tumah across everything it shelters.
But a keli lying "k'neged ha'arubos," in the open column of the arubos, in the line where the rain would fall, is tahor. That keli has nothing above it. There is no ohel over it, and without an ohel the tumah has no way to reach it.
Tumah Standing Directly Beneath the Openings
Now flip the case. The source of tumah rests inside the house, but it sits "k'neged ha'arubos," positioned right below the two openings, along the vertical path that leads out to the open sky. The ruling is "habayis tahor": the house stays tahor. No keli anywhere inside picks up tumah, neither one under the solid ceiling nor one under the opening, and the story above is equally unaffected. Why? Because nothing whatsoever roofs over the meis. Tumah with no covering above it has nothing to collect it and push it out to the sides, so the house is untouched.
Up to this point the halachah has not yet parted company with the case of openings a tefach wide. The difference begins when somebody covers an arubah.
Covering an arubah when the tumah is under the ceiling
The Mishnah goes on: the tumah lies "babayis," beneath the solid ceiling of the lower story, and "nosan," someone set over one of the two openings an object that stops it up, "bein davar shemekabel tumah bein davar she'eino mekabel tumah," meaning the object may be of a kind that can contract tumah or of a kind that cannot, and "bein milma'alah bein milmatah," so that he may have placed it on the higher opening, the one leading out through the roof, or on the lower one, the one joining the two stories. Whichever of these possibilities occurred, the halachah is "ein tamei ela hatachton": tumah is confined to the bottom level. Keilim on the ground floor turn tamei, and those upstairs remain tahor.
Once an arubah is stopped up, the house below is a sealed ohel, and the tumah inside it spreads to everything under that ceiling. But it stops there. Note carefully that this is so even when the object he laid down is mekabel tumah, which never has the power to hold tumah back. It makes no difference here, because even with nothing at all covering the opening the tumah would still not rise, since the opening is smaller than a tefach. Tumah spreads from one ohel into another only through a pesach tefach, and the only exception is tumah lying directly beneath the gap, which will come through even a gap smaller than a tefach.
One might ask about the upper arubah: if he sealed the very top opening, does the whole structure not become a single ohel, so that everything in it should be tamei? That reasoning holds only where the openings are a tefach wide, which would join the floors into one enclosure. Without a pesach tefach the floors stay separate, and unless the tumah sits right under the opening it has no route upward.
Sealing an Opening When the Tumah Sits Below the Shaft
Now the Mishnah turns to the sharper case: the tumah is lying "k'neged ha'arubos," exactly in the straight line beneath the openings, and he covered an arubah. Here everything depends on what he used to cover it.
Should he cover either opening with "davar shemekabel tumah," an item capable of contracting tumah, "bein milma'alah ubein milmatah," that is, the higher opening that leads from the upper story out to the sky, or the lower one that joins the two stories, the verdict is "hakol tamei." All the keilim, on both levels alike, are tamei. The tumah is positioned squarely under the shaft, and tumah directly below a gap ascends even where the gap measures less than a tefach. Nothing here obstructs it, since an object that can itself contract tumah has no power to bar tumah's passage.
If instead the covering was "davar she'eino mekabel tumah," an object incapable of contracting tumah, laid across either of the two openings, whether the top one leading out through the roof or the bottom one connecting the stories, the ruling reverts to "ein tamei ela hatachton." Tumah reaches the keilim of the bottom level alone, and whatever stands above stays tahor.
Goud Achis
Half of this ruling is straightforward. Put the non-receptive object over the bottom opening and it sits squarely across the route the tumah would take upward; an object that cannot contract tumah genuinely holds tumah back, even where the source lies directly under the shaft and would otherwise have risen through a gap narrower than a tefach.
The puzzle is the higher opening. Imagine that he blocked only the top one, the opening leading out through the roof, and left the gap between the stories completely bare. The tumah lies directly beneath both openings, and tumah so situated travels upward even through a gap of less than a tefach. Why, then, should the keilim upstairs escape? By rights the tumah should rise out of the bottom level into the story above and render everything there tamei.
The resolution comes from a halachah l'Moshe miSinai that goes by the name goud achis. Two such principles exist, each the mirror image of the other. Goud asik instructs us to treat something situated below as though it were extended upward; the classic illustration is a sukkah with walls of ten tefachim, where we regard the wall as continuing until it meets the sechach even though the sechach is far higher. Goud achis works in the reverse direction: an object positioned above is treated as though it were drawn straight downward. Anyone who has studied Maseches Eruvin has already encountered these rules there.
Invoking goud achis here yields a remarkable leniency, yet a halachah l'Moshe miSinai is precisely the sort of authority that allows it. The non-receptive covering rests upon the top opening, and we regard it as stretching downward in a straight line so as to close the opening between the stories as well, even though in fact nothing at all lies across that gap. The upshot is this: a kezayis min hameis may be lying exactly beneath the shaft, yet as long as a covering that is not mekabel tumah has been set over one of the two openings, high or low, the tumah stays where it is and does not rise.
Reviewing the Mishnah
To review the case: a pair of arubos, the lower joining the ground level to the story above it and the upper leading from that story out through the roof, aligned so exactly that rain would fall on a man standing below, and neither of them a full tefach across.
- Tumah beneath the solid ceiling of the bottom level: keilim under that ceiling become tamei, while keilim standing in the open shaft remain tahor.
- Tumah directly under the openings, with nothing covering them: nothing in the house is tamei, neither under the ceiling nor under the opening, since no ohel covers the tumah at all.
- Tumah beneath the ceiling with one opening blocked, either opening and either type of object: only the keilim of the bottom level are tamei. The blockage forms an ohel that spreads the tumah below, and the tumah cannot climb higher because the gap is under a tefach. True, an object that is mekabel tumah does not bar tumah, but nothing needs barring here: the narrowness of the gap does the work.
- Tumah directly under the openings, with one of them covered by an object that is mekabel tumah, top or bottom: everything is tamei. Such an object never halts tumah, it only helps it spread, and tumah lying under a gap ascends through even less than a tefach.
- Tumah directly under the openings, with one of them covered by an object that is not mekabel tumah, top or bottom: tumah is limited to the bottom level. Set below, the covering blocks it outright; set above, goud achis draws it down in our reckoning to seal the lower gap too.
The two directions of the rule sit side by side in these halachos. Where the object is mekabel tumah, goud asik works as a stringency: even when it lies over the lower arubah we treat it as drawn upward, so that the tumah spreads throughout. Where the object is not mekabel tumah, goud achis works as a leniency: even when it lies only over the upper arubah we treat it as drawn downward, holding the tumah on the floor where it began.