Ohalos, Chapter 3, Mishnah 1. Until now the Mishnah has been teaching us which things carry tumah through an ohel. Now a new question is raised: what happens when the required measure of tumah is not sitting in one place, but is split into two pieces? Do the two halves join together to make a shiur, or must the full measure be found in a single spot?
The opening case: a kezayis divided in two
The Mishnah opens with the words "Kol hamitameh b'ohel": anything we have already learned to convey tumah by way of an ohel. The standard illustration is meis flesh in the measure of a kezayis. Should I stand beneath one roof together with such a kezayis, tumah reaches me. Now, however, "shenechleku", that kezayis was split into two equal parts, "vehechnisan l'soch habayis," and the pair of them were carried inside. Our man is now sharing a roof with two chatzei zeisim.
Rebbe Dosa ben Harkinas metaher. He rules that the person is tahor, and so are the keilim and the food in that ohel. His reasoning: the Torah requires a kezayis, and there is no kezayis here. There is half a kezayis, and there is another half a kezayis. The fact that both of them happen to be lying under one roof does not fuse them into a single measure.
"Vachachomim metamin." The Chachomim disagree. True, you need a kezayis, and here you have a kezayis under this roof. Is it not in one spot? What difference does that make? The roof covers both pieces, and between the two of them there is a full shiur for tumas ohel.
The machlokes is broader than tumas meis
This machlokes reaches well beyond the tumah of a meis. Where else might it come up? Consider neveilah, an animal that died on its own or was killed without a valid shechitah. A neveilah transmits tumah when a person touches it (maga) and when he carries it even with no contact (masa), yet it has no tumas ohel at all. A meis is different: its tumah travels along all three routes, maga, masa and ohel.
So imagine someone who is nogea bishnei chatzei zeisim min haneveilah: his right hand rests on one half-zayis of neveilah while his left hand rests on the second half. Or nosei: each hand is lifting one of the two pieces, though his skin never meets the flesh itself. Added together, the two pieces amount to a full kezayis. Rebbe Dosa ben Harkinas rules that he stays tahor, for in his view the shiur has to be gathered in a single place. The Chachomim answer that since the tumah of both halves is arriving by one identical route (maga in the first picture, masa in the second), the halves combine and the man is tamei.
The principle at the heart of the mishnah
So the outline of the argument, which the Mishnah is about to work out through a string of cases, looks like this. The Chachomim admit that a half-shiur delivered by one variety of tumah will not join with a half-shiur delivered by a different variety. A man whose hand rests on one half-zayis while he carries the second piece stays tahor. A man carrying one half-zayis while the second piece reaches him by way of ohel likewise stays tahor. On the other hand, two chatzei zeisim beneath a single roof, or two chatzei zeisim held aloft, or two chatzei zeisim under his two hands, do render him tamei, provided we remain inside one category throughout: maga joins maga, masa joins masa, ohel joins ohel.
Rebbe Dosa ben Harkinas refuses to combine even inside a single category. One hand on a half-zayis and the other hand on a second half-zayis leaves the man tahor. A half-zayis lifted in his right and a half-zayis lifted in his left leaves him tahor. Two chatzei zeisim sharing his roof leave him tahor. Unless a complete kezayis sits together in one spot, he maintains, no tumah has been transmitted.
One more piece of background: when is it ohel and when is it maga?
Before going further, there is something else you need to know. Tumas ohel is not only the case of standing in a room together with a kezayis of a meis or with a full meis. Tumas ohel also applies when a person is leaning over a meis, and it applies when a meis is positioned above a person.
Suppose a person bends over a meis with a full tefach of open air separating his body from it: that is tumas ohel. Should the gap be smaller than a tefach, the halachah views him as having touched the meis, and the tumah is maga. And when the meis is positioned above him? If the piece measures a tefach in each direction, we have tumas ohel; anything smaller counts as though he made contact with it.
That is a lot of information at once. Relax. Even if not every detail settled in, you have enough in hand to follow the Mishnah's cases.
The Mishnah goes on: "U'vemeis." Having dealt with a kezayis of neveilah, we now come back to a meis. Meis flesh in the amount of a kezayis carries tumah along every one of the three routes: contact, carrying with no contact, and ohel, which covers sharing a roof with it, bending over it, or having it positioned overhead. Neveilah reaches a person by maga and masa alone; a meis reaches him by maga, by masa and by ohel.
Case one: "Hanogea bechatzi zayis umaeil al chatzi zayis." His hand rests on a half-zayis of meis flesh, while his body simultaneously overhangs a second half-zayis, the gap between him and that piece being under a tefach. Strictly speaking, this second piece is not functioning as ohel; because the tefach is missing, it becomes maga. He therefore has maga twice over, arriving from two directions: once through his hand and once through the body suspended above.
Case two: "Nogea bechatzi zayis vechatzi zayis maeil olov." One half-zayis is under his hand, while a second half-zayis hangs over his head without measuring a tefach in each direction. Lacking those dimensions, the overhead piece cannot serve as an ohel, so its tumah comes to him as maga. Here too, then, maga is arriving from two sides: from the piece he handles and from the piece just above him, whose nearness turns it into contact.
Case three: "Maeil al shnei chatzei zeisim." He himself is hovering over two chatzei zeisim of basar hameis that are not attached to one another. What is interesting here is that the Mishnah does not need to tell us whether there is a tefach of space or not. Either way works. If there is less than a tefach, both halves reach him as maga. If there is a tefach or more, both reach him as ohel. Either way they match, and that is precisely the point of every case in this list: the source of tumah for both halves is one and the same, whether maga, masa or ohel.
Case four: "Maeil al chatzi zayis vechatzi zayis maeil olov." He is hovering over half a zayis, and half a zayis is above him.
Regarding all four of these pictures the Mishnah records "Rebbe Dosa ben Harkinas metaher." It makes no difference to him that everything here is maga, or that everything here is ohel; the kezayis must be found together in a single spot, and two chatzei zeisim will never merge. "Vachachomim metamin." A pair of half-zeisim reaching him by maga renders him tamei; a pair reaching him by masa renders him tamei; a pair reaching him by ohel are added together and render him tamei. For the Chachomim the man escapes tumah only when one half arrives by maga and the other by masa, or when one arrives by ohel and the other by masa (picture him reaching a hand outside the ohel to pick up the second piece). In such a mixture nothing joins, and each avenue would then need its own complete kezayis. Where both halves belong to a single avenue of tumah, however, they are mitztareif.
Now the mismatched cases
The Mishnah now turns to cases where the two halves arrive through two different types of tumah. "Aval hanogea bechatzi zayis vedavar acher maeil olov ve'al chatzi zayis." His hand is outside the ohel touching half a kezayis, while he himself is under a roof, and in that space there is another half a zayis, with a full tefach of clearance. The first half reaches him as maga; the second as ohel.
Second case: "Hamaeil al chatzi zayis vedavar acher maeil olov ve'al chatzi zayis." He is hovering over half a zayis with no tefach of space, so that half is maga, not ohel; and at the same time something is spread above him and above another half a zayis, with a tefach of clearance there, so that half is ohel.
The Mishnah rules: tahor. In both cases he remains tahor, because one half would have to make him tamei through maga while the other would have to work through ohel, and those two are not combined.
The shittah of Rebbe Meir: ohel is really maga
"Omar Rebbe Meir." Now a striking view enters. Rebbe Meir accepts part of what the Tanna Kamma reported and rejects part of it. What he accepts: the Chachomim do join two halves that share one shem of tumah, maga together with maga, masa together with masa, ohel together with ohel, and the man is tamei.
Where does he break away? To Rebbe Meir, ohel is nothing other than a species of maga. How is that possible, given that maga means catching tumah by touch while ohel means catching it by standing under a shared covering? Rebbe Meir grounds himself in the posuk: the Torah declares that once a meis lies inside a tent, all that is in that tent becomes tamei, which amounts to a statement that the entire space enclosed by the ohel is regarded as being in contact with the meis. This is how the Tosfos Yom Tov presents his reasoning. On Rebbe Meir's account, a meis therefore has just two channels of tumah, maga and masa, with ohel filed underneath maga: a gezeiras hakasuv that presence beneath one covering, with every one of the halachos of ohel attached, ranks as touching.
One might wonder what turns on this. The man ends up tamei in either event, so why should the label matter? The nafka mina is this. Imagine his hand outside a house resting on one half-zayis while the rest of his body stands inside that house alongside a second half-zayis. Read Rebbe Meir's way, the Chachomim would declare him tamei, since what he really has before him is two doses of maga, each one measuring a half-zayis.
Look at the Mishnah's words: "Af b'zeh Rebbe Dosa ben Harkinas metaher vachachomim metamin." Even in those last two cases, where a man touches half a kezayis while sharing an ohel with another half, or hovers over half a kezayis without a tefach while something else covers him and another half, cases in which the Tanna Kamma reported that the Chachomim rule tahor because it is maga plus ohel, Rebbe Meir insists that the Chachomim still rule tamei. Since ohel is maga, even where there is a full tefach you are not dealing with maga and ohel at all; you have maga together with another form of maga that goes by the name ohel.
Rebbe Meir goes on to spell out the Chachomim's position: "hakol tamei," all of it is tamei, "chutz min hamaga im hamasa," apart from maga paired with masa, "vehamasa im ha'ohel," and masa paired with ohel. Everything else combines; those two pairings do not. Even for Rebbe Meir, maga and masa stay separate categories, so a hand on one half-zayis of basar hameis together with the second half being carried without contact will not add up. Masa with ohel is equally unworkable: a man carrying one half-zayis while serving as an ohel over the second piece remains tahor. It is only maga and ohel that turn out to be a single thing.
Zeh haklal
"Zeh haklal." The Mishnah now formulates the governing principle for a pair of half-shiurim. "Kol shehu mishem echad": as long as the tumah comes to the person by one single route, two half-zeisim of maga, or two half-zeisim of masa, or two half-zeisim of ohel, he is tamei. Two halves that arrive mishnei sheimos, from two separate categories, leave him tahor, since distinct sheimos do not combine. All that is left to settle is whether maga and ohel constitute two sheimos, which is the position of the Tanna Kamma, or a single one, which is the position of Rebbe Meir.
In sum: Rebbe Dosa ben Harkinas will not join two halves under any circumstances, not maga with maga, not masa with masa, not ohel with ohel; the full measure has to sit together in one place. The Chachomim do join two halves belonging to one category of tumah, though never halves drawn from different categories. What remains in dispute is how the categories are counted: does ohel stand apart from maga, or are the two one and the same? The Tanna Kamma treats them as separate; Rebbe Meir treats them as identical.