Ohalos, Chapter 10, Mishnah 4. This chapter deals with an arubah, an opening in a ceiling or a roof, and with the question of what such an opening does to the ohel around it. Our mishnah presents a case of two arubos lined up one directly above the other, and works out where tumah travels and where it stops.
The Case: "Arubos Zu Al Gav Zu"
Imagine a two-story house. The ceiling of the lower level, which is also the floor of the aliyah, has a hole cut into it, and directly overhead, on the same vertical line, the roof itself has a second hole. We are not speaking about a window set into a wall, the sort one peers through to see whether it is snowing outdoors. An arubah is a gap in what covers you from above. Since the pair of openings line up exactly, someone standing on the lower level can raise his eyes and see the open sky. He has no need of a window to learn the weather: should rain be falling, it will land on him.
The mishnah adds: "veyesh bahen poseiach tefach," each of the two openings has a clear space of a tefach by a tefach. That is the measure that matters for the laws of ohel.
Case One: Tumah in the House
The first situation: tumah is present in the house, though not within the shaft of the openings. Say a meis is lying on the lower level, beneath the ceiling. Concerning this the mishnah rules "keneged arubos tohor": a kli positioned within the line of the arubos, in the open shaft that rises from the floor through both holes to the sky above, remains tohor. Neither ceiling nor roof stretches over that kli, so nothing joins it in one ohel with the tumah. A kli resting anywhere else on that level, sharing the covering with the meis, does contract tumah, since the roofing overhead forms a single ohel spanning both the kli and the corpse.
Now a question worth asking: with the tumah downstairs under the ceiling, what is the status of keilim upstairs in the aliyah, under the roof? The answer is that they too remain tohor. Why does the tumah not rise through the lower arubah and fill the upper story? Because that lower arubah has a second arubah directly above it. When one opening sits over another in a straight line, the tumah does not spread sideways into the upper floor; it goes out through the column.
The same is true in reverse. If the meis is in the aliyah, under the roof but not opposite the openings, keilim in the aliyah become tamei, keilim standing in the line of the arubos are tohor, and everything downstairs is tohor. In short, when the tumah lies under a covering, it defiles only what shares that covering with it on that floor, and whatever stands in the path of the arubos is spared.
Case Two: Tumah Opposite the Arubos
The second situation: the tumah is itself "keneged arubos," positioned in the open shaft beneath the two holes. Regarding this the mishnah teaches "habayis tohor," that the house is clean. Nothing in it contracts tumah: not the keilim on the lower level beneath the ceiling, and not those in the aliyah beneath the roof. And a kli placed within the shaft itself, alongside the tumah? It also remains tohor, since no ohel covers either of them. The mishnah's declaration that the house is tohor is comprehensive, and it includes the airspace of the openings themselves.
Case Three: Sealing an Arubah With Something Mekabel Tumah
The third situation begins "hatumah bein babayis bein keneged arubos." It makes no difference whether the tumah is inside the house, on either level beneath its covering, or whether it stands in the shaft opposite the openings. What changes is that someone has taken "davar shehu mekabel tumah," an item susceptible to tumah, and plugged one of the two holes with it, "bein milma'alah bein milmatah," the higher hole or the lower one, either way. The verdict is "hakol tamei": everything contracts tumah.
Let us take it step by step. If he covered the upper arubah, the one between the aliyah and the sky, that is the smaller chiddush. The sky can no longer be seen; the house has become one sealed ohel from the ground floor up, and every kli inside becomes tamei.
The chiddush is the lower arubah. He covered only the opening between the ground floor and the aliyah, and he covered it with an object that is mekabel tumah. Such an object does spread tumah, but it does not block it: a kli that can receive tumah never serves as a chatzitzah. So on the one hand the space under that opening downstairs is now roofed, and the tumah of the lower floor fills it. On the other hand, since the covering cannot hold the tumah back, the tumah rises into the aliyah as well. But the mishnah teaches something larger than that. Because the covering is mekabel tumah, we regard it as though it were placed not on the lower arubah but on the upper one. The entire house, both stories, becomes a single closed ohel, and therefore wherever the tumah happens to be, downstairs or upstairs, everything in the house is tamei.
There are different shittos in exactly how this mechanism works. Some explain it through gud asik, the principle that something below is treated as though it extends straight upward. You may remember it from Maseches Sukkah: a mechitzah ten tefachim high can stand well below the sechach, and as long as the sechach is within twenty amos and in line with the wall, we view the mechitzah as rising straight up, even though in reality there is open air in between through which a bird or an airplane could pass. Here the same idea is applied lechumra: the covered lower arubah is treated as though the covering were up at the roof, and consequently everything under that ohel, including the keilim in the aliyah, becomes tamei.
Case Four: Blocking the Opening With an Object That Cannot Become Tamei
The mishnah then turns the case around. This time the plug placed over the hole is "davar she'eino mekabel tumah," an item incapable of becoming tamei, and once again it is irrelevant which of the two openings received it. An object of this sort genuinely interposes. Hence the ruling "mimenu ulematah tamei, mimenu ulema'alah tohor": whatever lies below the plug is tamei, and whatever lies above it is tohor.
To see it concretely: suppose the lower hole was plugged with such an item while the tumah lies on the ground level. The tumah now fills that entire level, the airspace beneath the plugged opening included, since a covering now spans it, and the keilim there become tamei. At the plug, however, the tumah halts. Every kli throughout the aliyah stays tohor, because the sealing object functions as a chatzitzah and detains the tumah on the level below.
And a kli in the aliyah that is resting directly on the arubah, supported by that very covering? It too is tohor. Once the covering is not mekabel tumah and blocks the tumah from rising, it blocks it entirely, and even the kli sitting right on top of it is spared.
In Sum
Two openings, each a tefach square, aligned one over the other, create an unobstructed shaft running from the floor of the house all the way to the sky. Tumah resting beneath a covering conveys tumah only to what lies under that same covering, while anything positioned in the shaft of the openings escapes, and the tumah does not cross from one level to the other. Tumah situated in the shaft itself conveys nothing at all, having no ohel above it. Plug either opening with an item susceptible to tumah and the house turns into a single ohel in which everything is tamei. Plug either opening with an item not susceptible to tumah and it interposes as a chatzitzah: tamei beneath it, tohor above it.