Ohalos, Chapter 9, Mishnah 5. This chapter deals with a keveres, a beehive, that is lying in the doorway of a house, so that part of it is inside and part of it is outside. The question the chapter keeps asking is what happens when a kazayis of a meis is placed in one spot or another: which items become tamei, which remain tahor, and does the tumah pass from the outside into the house or from the house into the keveres.
Up until now, every case in the chapter assumed one particular arrangement: the mouth of the keveres, its opening, was turned toward the outside, and the closed back or bottom of the keveres was turned toward the inside of the house. Our mishnah now turns the keveres around. And it is worth knowing in advance what is coming: the next several mishnayos are going to repeat the earlier cases almost word for word. Not a true chazarah, but very close to one. Everything is restated, and the only reason for restating it is that the direction of the keveres has been reversed, and with that reversal a halachah here and there comes out differently.
The mishnah begins by asking a question about everything that preceded it: "Bameh devarim amurim?" To which situation did all those earlier rulings apply? The answer: "Bizman shepiah lachutz", they were taught only about a keveres whose opening pointed away from the house, out toward the street, with its sealed underside turned toward the interior.
"Hayah piah lifnim", but if the keveres was lying there with its mouth facing inward, and not merely facing inward but actually opening into the house itself, while the back or bottom of the keveres was toward the outside, we have a new set of rulings to consider. In every other respect this is the very same keveres described at the beginning of the chapter: its condition, and the fact that it is not raised a tefach off the ground, are exactly as before. The only thing that has changed is which way it is turned. So we should not expect everything to change along with it.
Consider a kazayis min hameis situated on the outside portion, whether it was placed "nasan tachtav", beneath the keveres, "o al gabav", resting upon its upper surface. In this case the halachah is identical to what we learned about a keveres facing the other way. "Kol sheknegad hakazayis, tachtav v'gabav, tamei": everything positioned in a straight line with that kazayis contracts tumah, and this works in both directions. Should the kazayis lie above with a keili situated directly below it, or should it lie below with a keili situated directly above it, that keili is tamei.
Why does the keveres not shield it? Because the keveres is a keili, and a keili is not an ohel. It has no power to interpose and hold back tumah. What it does have is the ability to protect what is contained within it, and therefore "tocho tahor": whatever is inside the keveres stays tahor, even if it is lying directly over the kazayis, and certainly if the kazayis is underneath.
"V'chol she'eino keneged hakazayis, tahor": anything that is not directly opposite the kazayis is tahor. If the kazayis is underneath the keveres and there is indeed a keili resting on top, but it is shifted a bit to the side and is not in a straight line with the kazayis, that keili remains tahor. And "v'habayis tahor": the house and everything in it remain tahor, since the tumah is outside and does not travel inward. So far, then, nothing at all has shifted from what we would have said before, even though the keveres is now turned the other way.
Agav, a point worth stressing here, and perhaps it deserved more emphasis earlier in the chapter. This keveres is itself tahor, meaning it is not mekabel tumah, because of its size: it holds arba'im se'ah belach v'churayim bayavesh, forty se'ah of liquid or two kor of dry produce, and a vessel of such capacity is not susceptible to tumah. But do not confuse two separate matters. Not being mekabel tumah does not turn it into an ohel. Since it is a functioning beehive, its din remains the din of a keili, and precisely because it is a keili it cannot block tumah from passing through.
The mishnah goes on: "b'socho o babayis, hakol tamei". Should the kazayis be lying within the keveres itself, or anywhere in the house, everything becomes tamei.
Here we finally reach the single ruling that comes out otherwise on account of the reversed direction. Previously, with the opening facing the street, we said that tumah situated indoors rendered only the house tamei, while the keveres and its contents stayed tahor, for all the keveres presented to the house was its solid underside. Now the situation is reversed: the keveres opens directly into the house. Consequently tumah found in the house spreads to everything, the keveres and whatever it holds included, since it enters unobstructed through the opening.
That is the whole chiddush of our mishnah: reverse the keveres, and almost every ruling of the chapter stands exactly as it was. Only where the opening itself does the work, tumah in the house reaching the inside of the keveres, does the change in direction change the halachah.