Ohalos, Chapter 10, Mishnah 1. Until now the chapter divisions of this masechta have introduced us to terms such as caveras. Now the Mishnah hands us a new word to become familiar with: arubah. An arubah is a skylight, an opening cut into the roof of a house. It is not a window with glass in it; it is simply a gap in the ceiling through which one can see straight up to the sky. This chapter works out what happens to tumah, and to the keilim in the house, when a roof has such a gap in it.
The Mishnah begins: "arubah shebesoch habayis veyesh ba poseach tefach", a house whose roof has an arubah in it, and the gap is a tefach wide in each direction. The measurement is not incidental. Halachah recognizes an opening as an opening only once it reaches a square tefach; anything narrower is treated as a crack rather than a genuine breach in the roof.
Tumah under the ceiling
The first case: "tumah babayis", there is tumas meis in the house, and by "in the house" the Mishnah means underneath the solid part of the ceiling, not in the airspace beneath the arubah. The tumah here is a piece of a meis that is metamei b'ohel, meaning a kezayis, an amount large enough to convey tumah to everything sharing its ohel.
The halachah is that "keneged haarubah tahor": any kli lying on the floor of the house directly beneath the opening does not become tamei. Every other kli in the house does become tamei, because tumah that sits beneath a roof rises and fills the whole space that the roof covers. But the spot directly under the arubah has no roof over it at all. A kli standing there is, in halachic terms, standing outdoors, and the tumah spreading through the house never reaches it.
Tumah under the arubah
Now reverse the picture. The tumah itself is lying "keneged haarubah", directly beneath the opening. The Bartenura describes such a spot with a beautiful phrase: the kli there roeh es pnei harakia, it sees the face of the sky. Do keilim have eyes? Of course not. The intention is that if you were to stand where that kli is standing and look upward, you would see the open sky above you, with nothing in between.
In this case the Mishnah rules "habayis tahor". All the keilim resting under the covered part of the ceiling remain tahor, because the tumah beneath the opening has no ohel over it and therefore has nothing through which to spread into the house. And the kli that is itself beneath the arubah alongside the tumah is also tahor, for the very same reason: there is no roof spanning that tumah, so there is no ohel joining the two of them together.
A foot over the opening
The Mishnah goes on to a third scenario, one that applies equally to both of the cases we have seen: "hatumah bein babayis ubein keneged haarubah", it makes no difference now whether the tumah is resting beneath the covered ceiling or beneath the gap. A person is up on the roof, and for reasons of his own he sets his foot down across the arubah, as the Mishnah puts it, "venasan es raglo milmalah". From above, the opening has been closed off.
Says the Mishnah, he has "eirav es hatumah", he has mingled the tumah, joining the two areas into one. The house no longer has an arubah in any meaningful sense. We no longer distinguish between the space beneath the opening and the space beneath the ceiling, because by covering the gap he has stretched a single roof over the entire house. Consequently every kli anywhere in that house becomes tamei, no matter where the tumah is lying. And the man himself becomes tamei as well, since his foot is now the ohel spanning the tumah below.
People talk about a fellow who sticks his foot where it does not belong and brings trouble down on himself. Our mishnah gives that expression a literal application: this man set his foot across an arubah and paid for it, contaminating every kli in the house below and picking up tumah himself in the bargain.
Tumah split between the two areas
The final case: "miktzas tumah babayis umiktzasah keneged haarubah", part of the tumah is under the ceiling and part of it is under the opening. Picture a single kezayis of flesh from a meis, half of it lying beneath the roof and half of it beneath the arubah.
On first thought one might have argued the opposite of what the Mishnah says. Under the ceiling there is no full kezayis, and under the arubah there is no full kezayis either, so perhaps nothing anywhere should be tamei. That is not the halachah. The Mishnah teaches just the reverse: "habayis tamei ukeneged hatumah tamei". The keilim under the roof of the house are tamei, and a kli positioned over the outdoor portion of the tumah beneath the opening is tamei as well.
The Bartenura at the end of our mishnah explains the mechanism in a few words: shehazayis nimshach l'chan ul'chan, the kezayis is drawn out to this side and to that side. Since the required measure of tumah is formed by the half under the roof together with the half under the opening, the whole thing is treated as a single unit. Whoever forms an ohel over any part of it is regarded as forming an ohel over the entire kezayis. It therefore makes no difference whether a kli overhangs the half under the ceiling, or sits anywhere else within the house beneath the roof, or overhangs the half lying beneath the arubah. Wherever you touch this tumah, you have touched all of it, and everything is tamei.
As the sugyos of this perek become more familiar, we will be in a position to explain the workings of this halachah at greater length. At this stage it is enough to hold on to the underlying idea: a share in something makes the whole of it yours. That thought reaches well past the laws of tumah. Choose the right people to attach yourself to, and even a modest portion in the good they accomplish is credited to you in full. The same rule runs in the other direction too, but that is a point we need not dwell on.
Reviewing the mishnah
A house has an arubah in its roof, a gap measuring a tefach in each direction. Should tumah be lying beneath the solid ceiling, all the keilim in the house contract tumah, with one exception: a kli positioned directly under the gap, which has no roof over it at all. Should the tumah instead be lying under the gap, the keilim beneath the ceiling remain tahor, since (in the Bartenura's expression) a kli in that spot sees the sky above it. In either arrangement, however, a man on the roof who lays his foot across the opening and closes it has canceled the arubah entirely: the tumah, wherever it lies, now travels through the whole house, so that every kli becomes tamei, and the man himself with them. And where half a kezayis sits beneath the ceiling and half beneath the gap, the kezayis is stretched across both zones, so a kli anywhere under the ceiling is tamei, as is a kli forming an ohel over nothing more than the half beneath the opening. Standing alone, neither half is a kezayis; united, each half carries the status of the full measure.