Keilim, Chapter 8, Mishnah 8. An earthenware stove, like every earthenware vessel, contracts tumah only through its inside: a sheretz that comes into the airspace of its interior renders it tamei, while a sheretz resting on its outside does not. The previous mishnah examined the vent hole of the stove from exactly this angle. Our mishnah continues along the same line and asks about several other parts of a stove, parts that are physically attached to it and even open into it, and asks whether the law calls them the interior of the stove at all.
The mishnah's first scenario is the mouth of the stove through which fuel is inserted: "nimtza mekom hanachat ha'etzim", a sheretz turned up at the spot into which the logs are laid. Does that spot count as the interior of the keli, or does it still belong to the outside?
Rabbi Yehuda rules most broadly. His words are "min hasafah hachitzonah velifnim", from the outermost rim of the wall going inward, and there a sheretz makes the stove tamei. The wall surrounding the fuel opening is a wall with thickness, and the tunnel running through that thickness is itself reckoned as interior space. As soon as the sheretz has crossed the outer lip, Rabbi Yehuda already regards it as within.
The Chachamim place the boundary at the far side of that same thickness: "min hasafah hapenimit velachutz", from the inner rim going out, and there the stove is tahor. To their mind, the tunnel under the fuel opening is not interior space; it is merely a corridor leading in. Only a sheretz truly within the body of the stove conveys tumah to an earthenware stove.
Rabbi Yose is more lenient than both, and he measures not by the walls but by the work the stove does. He rules "mikneged shefitat hakedera velifnim": only from the line directly under the resting place of the pot on top of the stove and further in does a sheretz make it tamei. And "mikneged shefitat hakedera velachutz", outward of that line, it is tahor, even where the sheretz sits within the area that the stove's walls surround.
The principle behind Rabbi Yose's ruling is worth noting. For him, the interior that matters is the space that actually serves the purpose of the keli, the hollow directly under where the pots rest and cook. A spot may be geographically inside the stove and still be a non-functional stretch of it, and a part of the vessel that does no work for the vessel does not draw in tumah. Such a place stays tahor.
Now the mishnah moves to a different category: stoves whose hollow is not deemed part of the stove whatsoever. "Nimtza mekom yeshivat habalan", a sheretz turned up at the bathhouse attendant's seat; "mekom yeshivat hatzaba", the dyer's seat; the seat of the olive boilers, "shel sholkei zeitim". Earthenware stoves of these types heated the bathhouse, boiled the dye and cooked olives, and each had a seat constructed onto it for the workman.
Each of these cases is ruled tahor. The cavity beneath such a seat does communicate with the stove's cavity, yet halachah treats it as lying beyond the interior, because it is not the space in which the stove performs its function.
A general principle seals the mishnah: nothing is tamei "ela min hasetima velifnim", meaning that only when a sheretz lies inward of the point at which the pot stops up the mouth at the top does the stove become tamei. This is precisely Rabbi Yose's line. Halachically, that region alone constitutes the stove's inside, and a sheretz anywhere else leaves it tahor.