Ohalos, Chapter 11, Mishnah 1. The Mishnah sets a scene for us: a dwelling whose ceiling has split open. Picture yourself indoors, lifting your eyes upward, and seeing a fissure running along the roof above you. This is not some modest hole punched through at one spot, the way one might cut a slot into a pushka to slip coins through, but a break that stretches across the entire ceiling, reaching from one wall of the house to the wall opposite.
The result is that the house has effectively been cut in two. One half of the house contains the doorway; call that the outer half. The other half, past the split, has no doorway of its own at all; call that the inner half. To reach it you walk in the front door, cross the outer half, pass beneath the opening in the roof, and find yourself in the back section.
The Mishnah opens: "habayis shenisdak," a house that became split.
Tumah in the Outer Half
The first case is "tumah bachutz," the tumah lies in the outer section, where the entrance is located. The ruling: "keilim shebifnim tehorin," whatever vessels stand in the rear section remain tahor. Why? Because of a principle that governs these halachos throughout: tumah makes its way outward as it leaves a house, but it does not force itself further in. A meis lying near the entrance has no reason to spread toward the back, so the keilim there are unaffected.
Tumah in the Inner Half
The second case is where things grow more complicated: "tumah bifnim," the corpse lies in the rear section, the one with no entrance of its own. Here the principle of "sofo latzeis" comes into play, that this tumah is bound to be removed. A meis will not stay where it is forever; sooner or later it will be taken out, and whatever stands along the path it will travel is reckoned tamei. Which path is that? Through the front section, since the sole entrance is there.
So the keilim in the outer half are not automatically in the clear. Whether they are actually tamei, however, depends on a dispute recorded in the Mishnah, and the whole question is whether the split in the roof is wide enough to turn one house into two independent ohalim.
Beis Shammai
Beis Shammai hold that the keilim in the outer section are not tahor "ad sheyehei basedek arba tefachim," until the split measures four tefachim across. Four tefachim is the shiur of an opening through which a meis can be removed. Beis Shammai reason as follows: yes, in all likelihood the meis will be carried out through the front door, and ordinarily that would render the outer section tamei. But once there is a break in the ceiling as substantial as four tefachim, we are no longer looking at a single ohel with a crack in it. We have two distinct ohalim. Under those circumstances, even though the inner section has no doorway of its own, we do not say that tumah lying inside automatically renders the outer section tamei, because the meis need not be considered as passing through the front. Anything narrower than four tefachim, though, and the outer keilim become tamei.
Beis Hillel
Beis Hillel take the opposite view. To their mind, tumah does not carry from the inner ohel (the space past the fissure) into the outer one (the space where the entrance lies), and this holds even with a split of "kolshehu," the slightest amount. Four tefachim are not required.
Even so, Beis Hillel do insist on an opening of some genuine, measurable size. Their shiur is the thickness of a plumb line, the weighted cord that a builder lowers alongside a wall to check whether it rises straight. A small ball hangs at the end of the cord; as the ball descends, if it strikes against the wall, the builder knows that the wall leans rather than standing true. That cord together with its weight must be able to fit through the split. Once the break is that size, the two portions count as two separate ohalim, and tumah in the rear leaves the front section tahor.
Rebbi Yosi's Version
Rebbi Yosi transmits a different tradition, "mishum Beis Hillel," in the name of Beis Hillel: "poseiach tefach," the break must be open to the extent of a tefach. On this account, a tefach is the deciding measure. Should the split be smaller than a tefach, tumah in the rear does convey tumah to the front, since the house is still treated as one ohel through which the meis will be carried out. But at a tefach the line is drawn: not only will tumah in the front leave the rear keilim tahor, tumah in the rear will likewise leave the front section tahor, because the structure now counts as two ohalim.
Reviewing the Mishnah
Let us review the entire picture. A house has a fissure crossing its roof from wall to wall. Where the tumah sits in the outer portion, the one containing the entrance, the keilim in the rear portion remain tahor, since tumah exits but does not enter.
If the tumah is in the inner part, the halachah is more complicated, because that tumah will have to be removed, and we assume that everything along its path becomes tamei. Should the outer section therefore become tamei automatically, since the doorway is there and that is the route the meis must take?
- Beis Shammai: the keilim in the outer portion do become tamei, unless the roof's split measures four tefachim.
- Beis Hillel: even "kolshehu" is enough, meaning as thick as a plumb line.
- Rebbi Yosi citing Beis Hillel: a full tefach is needed. Below a tefach, tumah in the rear conveys tumah to the front; at a tefach or beyond, the two portions stand as independent ohalim, so that tumah in the front leaves the rear tahor and tumah in the rear leaves the front tahor as well.