Ohalos, Chapter 6, Mishnah 3. Our Mishnah takes up a wall that stands between the inside of a house and the open air outside it, and asks what happens when tumah is found lodged inside the thickness of that wall itself. Is the tumah reckoned as being inside the house, or as being outside of it? The answer will turn out to depend on which half of the wall's thickness the tumah occupies, and the consequences run in two opposite directions at once.
The Mishnah opens: "Koisel hameshamesh es habayis." The word meshamesh means to serve, in the sense of the shamesh of a shul who attends to the needs of the place. A wall that serves the house is a wall that was deliberately built to be a wall, put up so that a roof could rest upon it and so that it would enclose a dwelling.
The Mishnah specifies this because there is another possibility entirely, which appears further on in Mishnah 6 in the reverse formulation: "Bayis hameshamesh es hakoisel," a house that serves the wall. That refers to a wall that nobody ever built as a wall at all. A person hollowed out a space in a rock formation from one side, and someone hollowed out a space from the other side, and by itself, without anyone intending it, a partition of stone was left standing in the middle between the two hollows. Dig two caves facing each other, and you will have manufactured a wall without meaning to. The halachah of such a wall is discussed in its own place. Our Mishnah here is speaking of the ordinary case: a genuine wall, constructed as a wall in order to serve the house.
Judging the wall in halves
Concerning such a wall the Mishnah rules: "Yidon mechtza lemechtza." The wall is judged by dividing its thickness into two halves, one half assigned to one side and one half assigned to the other. The Mishnah then asks "Keitzad?", give us a concrete case, and proceeds to spell it out.
"Koisel shehu l'avir": the wall under discussion borders on empty air. Two dwellings are not flanking it here; rather, one face of it looks into the house while its opposite face opens onto unroofed space, whether a yard, a garden, a narrow lane or the public street. "V'hatumah b'soicho": somewhere inside the masonry there is a hollow or a fissure, and the tumah is resting there. The entire ruling now hinges on where that hollow sits within the wall's width: closer to the dwelling, or closer to the air outside?
Tumah in the inner half
"Mechetzyoi v'lifnim": if the tumah lies from the midpoint of the wall inward. Suppose the wall is two tefachim thick; if the tumah is within the tefach that faces the interior, it is treated as though it were simply inside the house. Consequently, "habayis tamei," everything in the house becomes tamei.
And someone up on the roof? "V'ha'oimeid milma'alah tahor." He stays tahor. The tumah is enclosed beneath an ohel, and an ohel confines tumah to the space under it, so a person positioned overhead is beyond its reach. The Bartenura points out that this holds even keneged hatumah, when the man is lined up exactly over it. Run an imaginary plumb line down from his feet on the roof and it lands on the tumah itself, and still nothing happens to him: since the tumah counts as lying within the house, and the house functions as an ohel, whoever is above the ohel is unaffected.
Tumah in the outer half
"Mechetzyoi ulechutz": should the tumah be situated from the middle of the wall going outward, in the half that borders the open air, the Mishnah rules "habayis tahor." Nothing in the dwelling contracts tumah, since the tumah is never reckoned as being within it. The other clause, however, now flips: "v'ha'oimeid milma'alah tamei." Whoever stands overhead does become tamei.
You cannot have it both ways, and this is the heart of the Mishnah. An ohel carries an advantage and a disadvantage together. The advantage is that whatever is outside the ohel is shielded from the tumah within it. The disadvantage is that within the ohel, wherever a person or a vessel may be, he becomes tamei. So if the tumah sits in the inner half, it is inside the ohel: the stringency is that the entire house is tamei, and the leniency is that one standing on the roof stays tahor, even directly over the tumah. But if the tumah sits in the outer half, it is not inside an ohel at all: the house is tahor, yet there is no ohel over the tumah to hold it down. It rises straight upward, and whoever stands above it becomes tamei. This is what is called tumah retzutzah, tumah that is pressed in and breaks straight through upward.
Exactly at the midpoint
What if the tumah lies precisely mechtza lemechtza, exactly on the dividing line? The wall is two tefachim thick, and measuring, we find the tumah exactly one tefach in. Here the Mishnah rules that habayis tamei: whatever is in the house becomes tamei.
The status of the man overhead, though, is disputed. "Rebbi Meir metamei": according to Rebbi Meir he is tamei. Where the tumah straddles the dividing line evenly, Rebbi Meir applies the strict outcome to each side at the same time. He treats the tumah as if it were under the ohel, which renders everything within the dwelling tamei; and he simultaneously treats it as if no ohel covered it, so that it forces its way straight upward and defiles the person positioned above.
"V'chachamim metaherin." The Chachamim disagree. Even when the tumah is exactly half and half, they regard it as being inside the house; and once it is inside the house, the house serves as an ohel, and the ohel shields whoever stands on top. The Tiferes Yisroel notes that here too this holds even if he is standing directly above the tumah.
The view of Rebbi Yehudah
"Rebbi Yehudah oimeir: kol hakoisel labayis." In Rebbi Yehudah's opinion the wall in its full width is annexed to the dwelling. Even tumah wedged at the very outer surface, right up against the street, is regarded as lying within the house. What follows is straightforward: the house contracts tumah, while a person above, on the wall itself, remains tahor.
Summary
We have a wall built to serve a house. On one side of it is the interior of the house, and on the other side is a garden, an alleyway or a street. Within the thickness of the wall there is a crack or cavity, and tumah is lying inside it.
- If the tumah is in the inner half, the house is tamei and one standing above is tahor.
- If the tumah is in the outer half, the house is tahor but one standing above is tamei.
- If the tumah is exactly at the midpoint, the house is tamei; regarding one standing above there is a dispute, Rebbi Meir declaring him tamei and the Chachamim declaring him tahor.
- Rebbi Yehudah holds that the whole thickness of the wall is annexed to the house, so that even tumah lying toward the street renders the house tamei, while one standing above remains tahor.