Keilim, Chapter 7, Mishnah 2. This chapter deals with the kira, the earthenware cooking stove, and with the various pieces that are built onto it or under it. The question that runs through these mishnayot is always the same: when the kira itself becomes tamei, which attached pieces are swept along with it, and which are independent utensils that have their own separate status?
The Mishnah opens: "hadaf shel kira, tahor mishum kira ve'tamei mishum keli kibul." The daf is a shelf built onto the outside wall of the kira, made of the same earthenware as the kira itself. Regarding the kira, this shelf is tahor: it is not treated as a limb of the stove, so if the kira contracts tumah, the shelf is unaffected. It simply has nothing to do with the kira's status.
Still, the shelf is not simply inert clay without any halachic significance. "Ve'tamei mishum keli kibul": it can contract tumah for its own reasons, as a piece of earthenware that functions as a receptacle. A rim encircles the shelf, so it is capable of holding whatever is placed in it, and that gives it the standing of a container. Any earthenware vessel with a hollow becomes tamei once a source of tumah enters the space inside it, and this shelf is no exception.
Next the Mishnah turns to the walls of the shelf: "hatzedadin shelo, hanogea bahen" is "eino tamei mishum kira." The shelf's side walls, too, are not reckoned as parts of the stove. Normally, when an earthenware utensil has already contracted tumah, even its exterior transmits tumah to whoever touches it. In this case, though, the shelf never picked up tumah along with the kira in the first place, so one who handles the sides of the shelf is not rendered tamei through the stove.
There is one section of the shelf, however, about which the Tanna'im argue: "harachav shelo, Rabbi Meir metaher ve'Rabbi Yehuda metamei." The daf has roughly the outline of a triangle: broad where it meets the stove at the rear, tapering toward the front. That wide rear surface is exactly the seam between the two, which is why it can reasonably be attributed to either side. One and the same panel serves both as a portion of the kira's outer wall and as the shelf's hindmost side.
Rabbi Meir declares it tahor, holding that this surface is to be counted as part of the daf. Consequently, even when the stove is tamei, whoever touches that rear panel stays tahor. True, a tamei earthenware utensil ordinarily transmits tumah from its outside as well, but this segment of the stove's exterior is classified with the shelf rather than with the kira. Rabbi Yehuda takes the opposite position and declares it tamei: to his mind that broad rear surface belongs to the stove rather than to the shelf, and it therefore transmits the kira's tumah to whatever touches it.
The Mishnah then presents a second instance of an object put to work for a stove while retaining its own separate standing: "vechen hakofeh et hasal ve'oseh al gabav kira." Someone turns a basket upside down and constructs the stove above it, using the basket as a platform. The basket has a larger circumference than the stove and sticks out past it, in a manner similar to the shelf described earlier. And here as well the basket is not swallowed up into the stove. Tumah that reaches the kira does not carry over to it; it stays a utensil of its own, subject to tumah in the ordinary way of any wooden keli.
The thread joining both halves of the Mishnah is clear: physical attachment to a kira does not automatically make something a part of the kira. A shelf on its side and a basket beneath it each remain themselves, each judged by its own material and its own usefulness as a vessel. The only genuine question is where the boundary line falls, and that is the point over which Rabbi Meir and Rabbi Yehuda differ.