Keilim, Chapter 20, Mishnah 5. The preceding mishnah dealt with an object that is joined to the ground and nevertheless remains able to receive tumah. Our mishnah continues along the same line, presenting three more situations in which a kli has been fastened into a structure or into an immovable piece of furniture. The question in each case is the same: has the object surrendered its own identity and become part of the building (or part of the large, unmovable item), in which case it is tahor, or does it still stand on its own as a kli, in which case it can still receive tumah?
A block of wood set into a wall
Our mishnah opens with the "kofet shekeva'o benidbach". A kofet is a heavy wooden block carved into a stool, and here someone has embedded it within a row of stones or bricks in a wall. The case is presented in two forms. First, "keva'o velo bana alav": the block was anchored in the wall to remain there for good, yet nothing was constructed above it, so its surface is left uncovered and a person can still sit down on it. The second form is the mirror image: "bana alav velo keva'o", further courses were laid over the block so that it can no longer serve for sitting, while the block itself was never secured into the wall in any lasting fashion.
In both of these cases the mishnah rules "tamei": the block is still susceptible to tumah. Although it is standing inside a wall, it has not been swallowed up by the wall. In the first case its function as a seat is fully intact, since it is open and available to sit upon. In the second case, even though it can no longer be sat upon, its attachment is only temporary, so it never truly became part of the wall and remains a kli in its own right. Being a seat, it can receive tumah through contact and also tum'at midras.
"Keva'o uvana alav": if he both fixed the block into the wall permanently and built up on top of it, the ruling is "tahor". Here the block has lost its independent status altogether. It is now simply a part of the wall, and since a wall cannot become tamei, neither can this block.
A reed mat fastened to the ceiling beams
Next the mishnah takes up the "mapatz shenatano al gabei hakorot". A mapatz is a mat plaited from reeds, and since people stretch out and lie upon such a mat, it belongs to the category of objects that receive tum'at midras. Here the mat was spread across the wooden beams that carry the roof of a house. Two forms are again presented. In one, the mat was nailed down onto the beams ("keva'o") while the plaster layer of the ceiling, the "ma'aziva", was never applied over it. In the other, the plaster was laid on top of it, but the mat itself was left unnailed.
In both cases the ruling is "tamei", the mat is still susceptible. In each of these situations the mat can be taken down easily, so it is not regarded as having become part of the roof. It retains its identity as a kli made for lying upon, and it can therefore still receive tum'at midras.
The third possibility is that he did both: he fastened the mat to the beams ("keva'o") and also covered it with the ma'aziva. In that case the ruling is "tahor". Prying the mat loose at this point would demand real effort, and it has therefore genuinely merged into the roof. A roof, being attached to the ground, lies beyond the reach of tumah, and this mat now shares that status.
A plate fixed into a chest or cabinet
The closing case concerns a "ke'arah", a dish, which someone has fastened ("shekeva'ah") into a "shidah", a "teivah" or a "migdal", that is, into a bin, a large chest, or a cupboard. The mishnah has in mind furniture of these kinds built on such a scale that it cannot be carried about, and an object that cannot be moved is not subject to tumah in the first place. The issue before us is the standing of the dish attached to it.
"Kederech kabalatah, temei'ah." Should the dish have been set in the orientation in which it actually functions, facing upward so that food can still be placed in it and eaten from it, it remains subject to tumah. Nothing has interfered with its use; it is a dish just as before, and it is therefore not viewed as having dissolved into the piece of furniture holding it.
"Shelo kederech kabalatah, tehorah." Where it was installed in a manner that prevents it from doing its work, turned over on its face, for instance, it is tahor. It no longer counts as a dish in its own right, but merely as one part of that bin, chest or cupboard. Since those large items are themselves beyond the reach of tumah, the dish embedded in them lies beyond it as well.
Across all three cases the mishnah works with a single measure: how thoroughly has the object been absorbed into the structure? Where the attachment is permanent and the original function has been taken away, the kli is gone and only the wall, the roof or the immovable chest remains, and none of these can receive tumah. But as long as either the attachment is loose or the object still performs the work it was made for, it keeps its standing as a kli and remains susceptible to tumah.