Ohalos, Chapter 11, Mishnah 2. Our Mishnah continues the chapter's discussion of an ohel whose covering has an opening running through it, and asks what happens when the structure in question is an achsadra.
The term "achsadra" is one we have already met in this maseches. Picture a sheltered porch: walls enclose three of its sides, the fourth side is left entirely open, and a covering stretches overhead. In our case that covering is damaged. The Mishnah's word is "shenisdeka": a crack has opened up in it, and this crack extends from one end of the covering to the other, beginning at the rear wall and reaching all the way to the open front.
Imagine yourself standing inside with your back against the rear wall. Straight ahead of you there is nothing but open air. Above you there is a roof, but a gap runs down its middle, stretching from where you stand to the front edge.
Two Ohalim
The halachah is "tumah betzad zeh, keilim shebetzad hasheini tehorim": if tumah is lying on one side of the split, the vessels resting on the other side remain tahor. The gap divides the covering, and each half is treated as an ohel in its own right.
It is worth noticing how this differs from the case of a house that was split, where the entrance stood on one side of the split only. There, the question of the width of the opening in the roof was critical. Tumah in the inner portion had to exit somewhere, and its only route out was the doorway at the front. So we were forced to measure: if the gap in the covering was wide enough, the inner section could be treated as an ohel separate from the section by the doorway; if it was not wide enough, the tumah of the inner part would spread and render the front part tamei as well.
In our achsadra the situation is not comparable. The split runs down the center, and neither side has an entrance that the other lacks, because the whole front of the porch is wide open. There is nothing to distinguish the right half from the left half; each opens outward on its own. Therefore the ruling stands plainly: tumah on either side of the split leaves the keilim on the opposite side tahor.
Sealing the Gap From Above
The Mishnah now introduces a change. What if "nosan es raglo o kaneh milmalah": a person sets his foot, or a reed, on top, above the gap, and thereby stops up the space between the two halves of the covering? "Eirev es hatumah": he has joined the two roofs into a single roof. Consequently, tumah on one side will now render the other side tamei.
Placing the Reed Below
And what if he did not place the reed above at all? The Mishnah continues: "nosan es hakaneh ba'aretz." He did not use the reed to physically plug the gap between the two halves of the covering. Instead he laid it on the floor, lined up precisely beneath the opening above. If you were to stand on that reed and look straight upward, you would see the split between the two roofs directly over your head.
The Mishnah's verdict here is "eino meivi es hatumah": the tumah is not conveyed across to the far side. And this holds true "ad sheyagbiha", unless he raises the reed "min ha'aretz posei'ach tefach", a full tefach clear of the floor.
Once the reed lying along the line of the split above is a tefach above the floor, the principle of gud achis comes into play. The covering is now regarded as closed off by the reed. Physically nothing has been sealed and nothing appears different to the eye, but halachah leMoshe miSinai establishes that it functions this way. The result is that the roof counts as continuous, and tumah on one side will spread to the other.
There is in fact a discussion here as to which principle we are invoking: do we view the roof as extending downward, which is gud achis, or the reed as extending upward, which is gud asik? See the Tosfos Yom Tov on this Mishnah, who takes up the question.
Review
Let us gather the pieces together. Our structure is an open-fronted shelter, something comparable to a bus stop: three walls, one side fully open, and overhead a covering with a fissure down the middle. Tumah resting to the right of the fissure has no bearing on keilim resting to its left. But if someone blocks off that gap from above, whether by standing his foot there or by resting a reed across it, the two halves fuse into a single covering and the tumah passes over. Setting a reed on the floor directly beneath the fissure achieves nothing at all, unless the reed is held a tefach clear of the ground, at which point gud achis (or gud asik) takes effect. The two principles differ in their technical mechanics, yet they arrive at the identical conclusion: the reed below and the crack above are viewed as meeting, so that once again both halves of the achsadra share one covering.