Ohalos, Chapter 3, Mishnah 7. Our chapter has been occupied with the measurements that give a space its halachic standing in the laws of tumas meis. This mishnah states the fundamental measurement of an ohel and then works it through a case that every house had: the covered drainage ditch that ran beneath the floor.
The measurement of an ohel
The shiur appears in the mishnah's own words. The covering must measure "tefach al tefach," a tefach in each horizontal direction, and beneath it there must be "rum tefach," a tefach of vertical clearance. To picture it: a flat board whose surface is a square tefach, held up a tefach above the earth, leaving an empty pocket of that height underneath.
Such a space accomplishes two things: "meviah es hatumah v'choitzes bifnei hatumah," it carries tumah and it blocks tumah. If a kezayis min hameis is lying in that hollow space together with keilim, the keilim contract tumah, because the covering spreads the tumah through everything beneath it. And if there are keilim resting on top of the board, they remain tahor, because the board has the status of an ohel and therefore serves as a chatzitzah: the tumah does not pass up through it to whatever lies above.
The Bartenura underscores how exact the mishnah is in setting up these dimensions. Suppose a person has a board that is two tefachim long but only half a tefach wide, raised a tefach off the ground. One might argue that the total surface is the same, only distributed differently, and that the extra tefach of length should be mentally transferred to the width. That reasoning is rejected. Nothing may be reassigned or imagined. An ohel exists only when there is genuinely a tefach in the length and a tefach in the width, with a tefach of height. Only then do we say that keilim underneath become tamei when a kezayis meis is present there, and that keilim above remain tahor. The ohel is metamei what is under it and cuts the tumah off from what is over it.
Keitzad: the covered ditch beneath the house
"Keitzad," asks the mishnah, in what manner? Having stated both functions, it now brings a concrete illustration, and its focus is on the second of them, the ohel's power to cut tumah off. The example is a "biv shehu kamur tachas habayis," a vaulted conduit passing beneath the house. The Bartenura explains that a biv is a chafirah: a trench excavated in the earth and then roofed over on top. Functionally this was the household drain of that era, receiving the dirty water of the home and carrying it away into the reshus harabbim.
Case one: a full tefach inside and a full tefach at the outlet
Measure the conduit: "yesh bo posei'ach tefach," it holds an open tefach, a tefach of width, a tefach of length, and a tefach from its floor up to the vaulting above it. "V'yesh biytzi'aso posei'ach tefach," and its mouth, at the place where the water spills out to the reshus harabbim, is likewise a full square tefach.
The ruling is "tumah besocho, habayis tahor." Should a kezayis of a meis be lying inside the conduit, the dwelling overhead remains tahor, and neither the people nor the vessels within the house are affected. Why do we not treat that tumah as though it were in the house? Because halachically it is nowhere near the house. It sits in the drain, and the drain constitutes an ohel in its own right, possessing a hollow of one tefach by one tefach with a tefach of height. Its tumah leaves by way of the conduit's own mouth, which is itself of adequate measure, and it never ascends into the dwelling. This is the plain illustration of "choitzes bifnei hatumah": the ohel formed by the drain rescues the people and the keilim in the home above it.
The opposite direction yields the same result: "tumah babayis, mah shebesocho tahor." With that identical drain, adequate both within and at its mouth, should a corpse or a kezayis of a meis be found in the dwelling, everything resting in the conduit below is untouched. The drain, being its own ohel, holds the household's tumah outside of it. The principle of choitzes bifnei hatumah thus cuts both ways: what is tamei in the conduit cannot rise into the home, and what is tamei in the home cannot descend into the conduit.
A difficulty arises, though. We know that a house with an opening to the outside sends its tumah out through that opening. Why not argue, then, that tumah resting in the drain will make its way out through one of the doorways of the house? To this the mishnah supplies a rule: "shederech hatumah latzeis," tumah travels outward, "v'ein darkah lekanes," it does not travel inward. Tumah in the conduit heads for the conduit's own outlet leading to the reshus harabbim. For it to shift from the trench up into the dwelling there would have to be a pesach of a tefach joining them, and there is none, because the trench runs onward and its opening lies out in the public domain. The identical reasoning settles the reverse case: tumah in the dwelling exits through the dwelling's doorways and windows rather than forcing its way downward, since no opening of a full tefach bridges the two spaces. The drain stands as an independent ohel.
Case two: a tefach inside, but no tefach at the outlet
The mishnah now alters a single fact. "Yesh bo posei'ach tefach," the trench itself has its tefach of height, width and length, "v'ein biytzi'aso posei'ach tefach," but the spot at which the water drains out toward the reshus harabbim does not measure an open tefach. In such a setup the shielding is only half effective.
"Hatumah besocho, habayis tamei." If the tumah is in the biv, the house is not protected, because the tumah has no halachically adequate opening through which to leave. There is an outlet to the reshus harabbim, but it is too small to count. Here we invoke the rule of "soif tumah latzeis," tumah must have a way out, and the only opening large enough is in the house itself, so the tumah rises into the house.
"Tumah babayis, mah shebesocho tahor." When the tumah is up in the dwelling, though, whatever sits in the trench stays tahor. Tumah moves outward, and the dwelling is equipped with doorways and windows of its own of sufficient size. Nothing of a pesach tefach ties the dwelling to the drain, so the household tumah has neither cause nor route to descend. A conduit properly sized within but pinched at its mouth therefore provides partial benefit: it cannot protect the house from tumah lying inside the conduit, since that tumah has no other exit available, yet it does protect vessels in the conduit from tumah lying in the house.
Case three: no tefach inside and no tefach at the outlet
What if the deficiency is not only at the exit, but in the ditch itself, so that it lacks the required tefach of length, tefach of width and tefach of height? "Ein bo posei'ach tefach v'ein biytzi'aso posei'ach tefach." Now the biv gives no help whatsoever.
"Tumah besocho, habayis tamei." Tumah inside the trench renders the house tamei, precisely as it would have even where the interior was adequate and only the mouth was deficient. But now the reverse holds as well: "tumah babayis, mah shebesocho tamei," tumah in the house renders whatever lies in the trench tamei. Lacking a chalal, an inner void, of posei'ach tefach, the drain has no independent standing whatsoever; halachically it is nothing but the ground of the house. Tumah in the house consequently drives downward without interruption, all the way to the tehom. The Bartenura points out that once the internal tefach is absent, the presence or absence of a posei'ach tefach at the outlet makes no practical difference: the trench is not an ohel by itself, it belongs to the house. And Chazal learn from the pasuk "kol asher ba'ohel yitma" that this tumah reaches what lies under the floor too.
Reviewing the three cases
Before the mishnah turns to a new subject, let us gather the three cases. The foundation is the shiur of an ohel: a tefach of length, a tefach of width, and a tefach of height. The setting is a biv, a ditch under the ground with a cover over it.
- Within, the drain has its tefach of length, width and height, and its mouth toward the public domain is open a full square tefach. This is a complete ohel standing on its own, and it functions in both directions: tumah in the trench never rises to the dwelling, nor does tumah in the dwelling descend to the trench.
- Within, the drain has its open tefach, but the mouth leading out to the public domain lacks a tefach. This protects only halfway. Tumah in the trench renders the house tamei, for soif tumah latzeis and the sole adequate exit runs through the house; tumah in the house, however, departs through the house's own doorways and never enters the trench.
- The drain lacks its tefach of void inside, and its mouth too lacks a tefach. Now it is no separate entity but part of the house, an instance of tumah retzutzah, tumah compressed and bursting through. Tumah above reaches the trench, and tumah in the trench reaches the house above.
The Rishonim record numerous shittos regarding the precise mechanics here and by what route the tumah travels between the two areas; our presentation follows the simplest pshat.
Openings that no one made
The mishnah now offers additional examples of a tefach by tefach space, some formed naturally and some by human hands, and points out the difference between them. Until now we discussed a biv, which was clearly constructed: someone dug a channel, covered it, and gave it an outlet to the reshus harabbim. The mishnah now speaks of hollow spaces that arose on their own.
"Echad chor shechararuhu mayim," a cavity hollowed out by water: the runoff kept flowing at one spot until it broke through and carved a passage under the ground. "Oy sheratzim," or creatures dug it out; various sheratzim, a mole for instance, are expert tunnelers. "Oy she'achaltu melachas," or the earth itself crumbled away from its salt content and a hollow passage was left behind. "V'chein marbech shel avanim," and likewise a stacked heap of stones: a person arranged the stones in rows, yet gaps remain between them. The Bartenura pictures one stone resting upon two, with the space between the lower two forming a small tunnel. "V'chein sevar shel koros," and likewise a stack of beams. The stacking is never perfect; there is space between one beam and the next. The beams are lying there waiting for the builder, and quite automatically an ohel of a tefach forms somewhere in the pile.
Rebbi Yehuda: an ohel must be made by a person
"Rebbi Yehuda omer," and his position is that any covering which is not "asui bidei adam," fashioned by human hands, "eino ohel," does not qualify as an ohel. Someone might protest that several of the listed cases do involve human activity. It is true that flowing water, burrowing sheratzim and soil disintegrating from its salt content owe nothing to people; yet whoever heaped up those stones or laid out those beams did create an ohel, however inadvertently. And that is exactly Rebbi Yehuda's point. The fellow never had an ohel in mind and never designed one; the hollow emerged incidentally from however the stones and the beams happened to come to rest. A covering like that is not reckoned as made by a person.
Rebbi Yehuda derives this from a pasuk, through the gezeirah shavah of ohel ohel from the Mishkan: just as the ohel of the Mishkan was made by human hands, so must every ohel be.
"Umodeh bishkifin uvislo'im." Rebbi Yehuda nevertheless concedes in the case of a cavity that water has hollowed out in a rock, and in the case of natural clefts in the rock which form a sort of tunnel of their own. Why does he concede there? Because these are considerably larger. His gezeirah shavah of ohel ohel from the Mishkan, requiring that an ohel be deliberately made, applies to a space that is only a tefach. But where the space exceeds a tefach, even if only by a little, so long as it is a meloi egrof, the fullness of a fist, which is reckoned as more than a tefach, Rebbi Yehuda agrees that it functions as an ohel even though it formed on its own.
The halacha
The Bartenura rules that we do not hold like Rebbi Yehuda, and the Rambam similarly paskens that such a space carries the status of an ohel. It serves as a chatzitzah against tumah, preventing tumah below from climbing into the house, or preventing the tumah of the house from sinking down, according to the measurements involved, and it also conveys tumah to everything under it. That is the din whether it was deliberately built to be an ohel or whether it arose by itself, without any human doing. Lemaiseh, nobody needs to have produced it: once the ohel exists, it exists.
A final thought about the hole that water bores into stone. Recall what Chazal relate about Rebbi Akiva, who at the age of forty was still an ignoramus and found learning almost beyond him. Among the things that set him in motion was the sight of a rock pierced through by dripping water. Splashing water onto a rock will not perforate it; it takes year after year of one drop following another until at last it penetrates. Rebbi Akiva said to himself: however difficult it may be, persist and you too will break through.