Ohalos, Chapter 6, Mishnah 1. Before entering the words of the Mishnah itself, we need to review the two opposite jobs that an ohel performs, since our Mishnah rests entirely upon them.
An ohel is a roof or covering spread out above human beings and above keilim. Should a meis be lying in the space beneath such a covering, whatever is found there contracts tumah: every person present and every keli present. Such a person can plead that he never made contact with the tumah and never stood above it, and none of that will save him. Being under one roof together with the tumah is by itself sufficient, and that is what the term tumas ohel refers to.
Yet the very same ohel is also able to work in the reverse direction and stop tumah from spreading. Consider a meis situated inside an ohel while a person or keilim are positioned on the roof above it, or turn the picture around, so that people and keilim stand within while a meis lies on top of the covering. In either arrangement, the covering itself prevents them from contracting tumah.
The result is striking in both directions. A person or a keli can be standing far from the meis, with no contact whatsoever, and still be rendered tamei simply because the same ohel spans them both. And in the other case, a person can be positioned directly above a meis, or directly below it, and remain tahor because a covering intervenes between them. In short, an ohel is capable of carrying tumah outward across everything under it, and it is equally capable of functioning as a chatzitzah, a barrier that cuts the tumah off.
Adam Vekeilim Na'asin Ohalin
With that in place, the Mishnah teaches: "Adam vekeilim na'asin ohalin letamei, aval lo letaher." A human being and a keli are themselves capable of becoming an ohel, but only for the stricter of the two functions. They form an ohel that transmits tumah, and they do not form an ohel that blocks it and leaves things tahor.
The Case of the Nidbach
"Keitzad," asks the Mishnah, and gives us the illustration. "Arba'ah nos'in es hanidbach": four men are carrying a nidbach. The Bartenura explains that a nidbach is "even gedolah urechavah," a large, broad stone. The Tosfos Yom Tov adds a detail that is essential to the picture: this is happening out in the open, with nothing at all above their heads. So imagine an open field, and four men shouldering a wide slab of stone.
"Tumah tachtav": tumah turns out to be present in the space below the slab. The ruling is "keilim shel gavav temeiyin," meaning that whatever keilim happen to be resting on its upper surface contract tumah.
At first glance this seems puzzling. Why should the massive stone not shield the keilim above from the tumah below it? The answer lies in who is holding the slab up. This nidbach is being borne by human beings, and we have already established the principle: people form an ohel that transmits tumah and never an ohel that blocks it. So the covering here works entirely against us. Even a keli that is not situated directly over the tumah beneath is caught, because the stone counts as an ohel and an ohel carries the tumah across its whole span. And when we then ask that the very same ohel should also serve as a barrier, the answer is that this it cannot do, precisely because human beings are carrying it.
The reverse arrangement yields the same ruling. The four men are carrying the nidbach and discover a kezayis min hameis lying on top of it. "Keilim shetachtav temeiyin": the keilim underneath the slab are also tamei. Even keilim that are not lined up directly beneath the kezayis are affected, because for the purpose of transmitting tumah the stone functions as an ohel and spreads it throughout the entire area beneath the slab. But for the purpose of blocking, it gives us nothing.
The View of Rebbi Eliezer
This is the position of the Tanna Kamma. "Rebbi Eliezer metaher." Rebbi Eliezer maintains that the keilim remain tahor in both scenarios alike, whether the tumah is under the great stone and the keilim above it, or the tumah is above and the keilim below. In his view, even a slab supported by human beings serves as an ohel that holds back the tumah. The Bartenura notes, however, that the halacha does not follow Rebbi Eliezer.
When the Stone Rests on Something Else
The Mishnah then presents the contrasting arrangement: "nosan al arba avanim," the nidbach is not being shouldered by men but has been set down upon four stones, "o al davar" that possesses "ruach chaim," that is, upon four living creatures such as cows, provided they are not human beings. Now the ruling flips. "Tumah tachtav, keilim shel gavav tehorin": when tumah is present below the slab, whatever rests above it stays tahor. "Tumah al gavav, keilim shetachtav tehorin": when the tumah is above, whatever lies below it stays tahor. What we have here is an ohel in the full sense of the word, one that operates in both directions, at times spreading tumah and at times, as in our situation, shutting it out.
Reviewing the Mishnah
Let us gather the points together. As a rule, if a person has the misfortune to be beneath an ohel that also covers a meis, it makes no difference how far from the meis he stands: he becomes tamei, because the ohel carries the tumah throughout. And if the meis is beneath the covering while he is above it, or the reverse, the ohel shields him. But that protective function applies only when the ohel is not being held up by human beings.
So to our situation: a broad stone shouldered by four men in an open area. If tumah is present underneath it, every keli lying above contracts tumah, including a keli positioned off to the side and not over the tumah at all. If instead the tumah is discovered above, every keli in the space below contracts tumah as well, since the ohel carries the tumah across the entire width of the stone while providing no protection whatsoever, and all of this because people (or keilim) are what hold it up. Take that support away, resting the slab on stones instead, or on animals, on whatever is not a human being, and the ohel returns to its sealing function: tumah underneath leaves what is above it tahor, and tumah above leaves what is below it tahor.