Keilim, Chapter 22, Mishnah 8. The subject of this chapter is wooden furniture in the home and the conditions under which such items become susceptible to tumah. Here the mishnah discusses a shiddah, a wooden storage chest. Being a wooden object with a hollow that holds contents, it is included in the laws of tumah without any question. The feature that makes this chest worth a mishnah of its own is its two-piece construction: beneath there is the box proper, and above it sits an elyon, a lid that is itself hollowed out so that it too can contain things. The question the mishnah takes up is what happens when one of those two pieces goes missing, and how the halachah of the remaining piece is affected by the piece it lost.
When only one part remains
The mishnah begins: "shiddah shenital ha'elyon, temei'ah mipnei hatachton" - a chest from which the upper part was removed is still capable of becoming tamei on account of its lower part. This is straightforward. Even with no cover at all, the box below is a functioning receptacle, and a wooden receptacle receives tumah.
Then comes the opposite situation: "nital hatachton, temei'ah mipnei ha'elyon". Here it is the base that has been detached, leaving four walls with the lid still sitting above them, and the mishnah rules that the chest can contract tumah on account of that lid. Because the upper piece was fashioned with a hollow of its own, the object has not stopped being a vessel; items can be deposited in it just as before. So long as the lid stays where it belongs, the chest continues to be susceptible to tumah.
When both are gone: Rebbi Yehudah and the Chachamim
"Nital ha'elyon vehatachton, Rebbi Yehudah metamei mipnei hadapin." In this case neither piece survives: the lid is gone, the floor is gone, and the owner is left holding a square wooden frame open at both ends. According to Rebbi Yehudah the object is still capable of contracting tumah, and the basis for his ruling is hadapin, the planks out of which the frame is built. Turn the frame over onto one of its sides, and while it certainly contains nothing anymore, its planks provide a level surface upon which one can rest objects, precisely the service that a table performs. And a table is an item included in the laws of tumah. Because the frame is capable of doing that job, Rebbi Yehudah continues to regard it as a utensil.
"VehaChachamim metaharin." The Chachamim disagree and declare it tahor. Their reasoning is that this chest was never made or designated to serve as a table. Its purpose from the outset was to be a receptacle, and once that primary purpose has been disabled, a possible secondary use is not enough to keep it in the category of a utensil. The frame may indeed be usable as a surface for setting things down, but because that was never what it was for, it no longer receives tumah going forward.
The stonecutter's seat
The mishnah closes with a separate ruling: "yeshivat hasatat temei'ah midras" - the seat of a stonecutter is subject to tumat midras. A craftsman who carves stone would place a cushion beneath himself so that his long hours of work would be somewhat comfortable. That cushion contracts tumat midras. This is so even though the stone he is working on is not itself an article that receives tumah at all. The cushion's own status does not depend on the stone; it depends on the fact that it was made and used to be sat upon, and an object designated for sitting is subject to midras.
Our mishnah has thus taught two related lessons. First, a two-part chest holds its status as a vessel as long as either the lower body or the upper cover can serve as a receptacle, and when both are gone the Chachamim rule that its incidental usefulness as a surface does not preserve its tumah, because that was never its purpose. Second, in the case of the stonecutter's cushion, the object's purpose is precisely to be sat upon, and so it is subject to tumat midras regardless of the material it rests against.