Ohalos, Chapter 10, Mishnah 2. Once more we find ourselves inside a house whose ceiling is pierced by an arubah, a hatch or skylight that opens straight up toward the heavens. The preceding mishnah dealt with an arubah measuring one tefach in each direction, the shiur that Ohalos employs throughout for an aperture capable of conducting tumah. Our mishnah takes up the alternative: "ein ba'arubah posei'ach tefach," the hole does not reach that measure in both dimensions. Every ruling here flows from that single fact.
Tumah in the house, keilim opposite the arubah
The opening case: a meis, or a portion of a meis that conveys tumah b'ohel, lies inside the dwelling. "Babayis" carries the same sense it bore in the preceding mishnah: the tumah rests beneath the solid ceiling and not beneath the gap. What, then, of a keli set on the ground directly below the arubah? The ruling is "k'neged arubah tahor," whatever is positioned opposite the hole remains tahor.
Why? Because that keli has no roof over it. It sees the sky. One might have objected: the gap over it is smaller than a tefach, so let us treat it as though it were sealed and count that spot as part of the house. That objection is really the principle of lavud, which we know from Maseches Sukkah: a gap of less than three tefachim is halachically regarded as closed. The answer of the Mishnah is that in the laws of tumah we do not apply lavud at all, not even to a gap smaller than a tefach. Small as the arubah is, it is an opening, and a keli sitting under it is simply not under the roof of the house. Therefore tumah in the house does not reach it.
Naturally, the keilim that are under the ceiling itself do become tamei. They are inside the ohel hameis together with the tumah.
Tumah opposite the arubah
Now reverse the picture. The tumah in the house is lined up directly under the arubah, meaning that if a person were to stand over that spot he would be looking at the open sky. Here the Mishnah rules that "habayis tahor," the entire house remains tahor. Not a single keli in the house becomes tamei, neither the keilim under the ceiling nor even the keilim lying under the arubah right beside the tumah, because the meis itself is not under the ohel of the house at all.
Here too one is drawn to argue the reverse. Repeatedly we have learned that an ohel is measured by a tefach, so an aperture falling short of that measure ought to be closed up by lavud, and lavud is itself a halacha l'Moshe miSinai. The Mishnah's reply holds firm: lavud has no place in the halachos of tumah. An opening is an opening.
Placing a foot over the arubah
The Mishnah proceeds: the meis lies beneath the roofed portion, "v'nasan raglov milma'lan," and a man up on the roof rests his foot upon the hole, blocking it. The verdict is "tahor." He has fared far better than the man of the previous mishnah, whose arubah reached a full tefach in each direction and whose foot contracted tumah. Here the foot stays clean, because tumah crosses only through an aperture of the full shiur, one tefach in both dimensions. This is exactly the spot where the distinction between a full tefach and anything narrower reveals itself. Given a tefach, the tumah rises through the hole and takes hold of the foot. Falling short of that measure, it stays where it is.
Notice how finely the Mishnah is splitting things. We are not saying that the small opening counts as sealed: that is why tumah under the arubah does not spread into the house, and why tumah in the house does not reach a keli lying under the arubah. The opening is genuinely an opening. But an opening only conducts tumah if it is a tefach wide. So a man who covers a sub-tefach arubah with his foot remains tahor. Tumah neither enters an ohel nor leaves an ohel through less than a tefach.
Rebbi Meir: when the meis lies directly beneath the opening
The Mishnah now relocates the tumah. It lies inside the dwelling yet "k'neged arubah," right below this narrow opening. Then comes "v'nasan es raglov milma'lan": someone standing above lowers his foot onto the hole. "Rebbi Meir m'tamei," Rebbi Meir rules that he becomes tamei, since his foot has fashioned an ohel spanning the tumah.
How does this differ from the earlier ruling? Earlier the meis rested under the covered part of the dwelling, and to affect the foot above it would have had to climb through the narrow gap, which an aperture short of a tefach does not permit; even Rebbi Meir concedes that such a man escapes tumah. In the present case the meis has no ceiling above it other than the man himself. He is not stationed at the far end of a narrow channel; his foot is the covering spread above the meis. That constitutes a genuine ohel, and so Rebbi Meir declares him tamei.
The Chachamim: which came first
"V'chachamim omrim," the Sages introduce a distinction. "Im tumah kadma es raglov": should the meis have been lying below the opening already, and he then came and roofed it over with his foot, he contracts tumah, for in lowering his foot he was fashioning a covering above tumah. "Im raglov kadma es hatumah": should he have taken up his position above the hole first, and only afterward was the meis brought in beneath him, he stays tahor. His foot had already stopped up the gap, so the dwelling stood closed; and from then on, tumah introduced below has no route up to him through an aperture short of a tefach.
Rebbi Shimon and the two feet
Rebbi Shimon supplies one further scenario: "shtei raglayim zo al gav zo shekadmu es hatumah," two feet stacked one above the next, both of which arrived before the tumah did. Envision it: Reuven strolls across the roof and stops the arubah with his foot. Shimon comes and rests his own foot upon Reuven's. Only then is a meis set down below, opposite the hole. Now "mashach harishon es raglo," Reuven draws his foot out from beneath Shimon's, "v'nimtza raglo shel sheini sham," so that it is Shimon's foot which now seals the opening.
The ruling is "tahor." Shimon escapes tumah, "mipnei shekadma raglo shel rishon es hatumah": the first man's foot had been in place while no tumah yet existed below. Reuven stopped the gap at a time when nothing tamei lay underneath, and from that instant the dwelling counted as a closed ohel, impervious to tumah since the hole falls short of a tefach. When Reuven slid out, the seal was at no point genuinely broken, for the second foot had already assumed the role. The ohel thus persists unbroken as one that antedated the tumah, and Shimon remains tahor.
Do the Chachamim agree here? They do not. They too hold that a foot which precedes the tumah leaves its owner tahor, but they do not view Shimon as having preceded anything. Their question to Shimon is simple: when did your foot begin to serve as the covering of the opening? Not when you stepped onto Reuven's foot, for at that moment you were covering nothing. Your turn begins the instant Reuven pulled his foot away, and by then the tumah was already lying underneath. According to the Chachamim, then, Shimon is tamei.
Reviewing the mishnah
Our arubah throughout is smaller than a tefach. First: if there is tumah under the ceiling of the house, a keli lying opposite the arubah remains tahor, since it sees the sky. We do not invoke lavud to seal a gap of less than a tefach for the laws of tumah. Likewise, if the tumah lies directly under the arubah, the keilim under the house remain tahor.
Now to the point where the small measurement alters the result. The meis rests under the covered area and a man blocks the arubah with his foot. Were the hole a full tefach he would contract tumah; since it falls short of that measure, he stays tahor, for an aperture below the shiur lets tumah neither out nor in.
So we are indeed splitting a hair, and in both directions. The opening is not considered closed, which is why tumah under it does not contaminate the house and tumah in the house does not contaminate keilim under it. Yet for tumah to travel out of the house up to the man's foot, a full tefach is required, and here there is none.
And what if the meis lies not beneath the ceiling but directly under the arubah, and a man covers it with his foot? Rebbi Meir rules tamei, since the issue is no longer whether tumah can squeeze through an aperture; the man himself constitutes the covering above the meis. In practical terms, someone who came to a rav saying "I rested my foot on an arubah narrower than a tefach" would be asked where the meis was lying: under the ceiling, and he is tahor; straight beneath the hole, and by Rebbi Meir's ruling he is tamei.
But the halacha follows the Chachamim, and so the rav's follow-up question is: what was there first? If the tumah was already there and you then covered it, you are tamei. If you covered the opening first and the tumah was brought afterward, you are tahor, because your foot had already turned the house into a sealed ohel, and tumah does not pass through an opening of less than a tefach.
Last comes the stacked case. Reuven relates that he sealed a hole at a time when nothing tamei lay below, and he is reassured: the ohel was shut before any tumah arrived, and a gap short of a tefach carries no tumah. Then Shimon relates that he had stood upon Reuven's foot and Reuven afterward pulled his own away. On this the two views part. The Chachamim rule Shimon tamei: Reuven's withdrawal restarts the reckoning, for only then does Shimon's foot begin to function as the covering, and by that time the meis already lies beneath. Rebbi Shimon rules him tahor, holding that nothing whatsoever has changed. Reuven's foot shut the hole before the tumah appeared, both feet were in position before the tumah appeared, and swapping one for the other is no fresh event. The seal was made and endures, and it predated the tumah, so the tumah underneath never travels up to Shimon's foot through a hole narrower than a tefach.