Keilim, Chapter 8, Mishnah 5. This chapter continues to examine the earthenware oven and the special rule that a kli cheres contracts tumah through its airspace: a source of tumah suspended inside the hollow of the oven, even without touching its walls, renders the oven tamei. Along the way the chapter has taught several cases in which a sheretz is found within that airspace and yet the oven remains tahor, whether because of the rule of an inner space within an inner space or for some other reason. Our mishnah presents another such case, and it turns on an entirely different principle.
The mishnah opens: "Tarnegol shebala et hasheretz venafal la'avir hatanur, tahor." Picture a chicken that has swallowed a sheretz, and the bird then drops down into the hollow cavity of a clay oven. The ruling is that the oven remains tahor. On the surface, every condition for tumah through the empty space of a kli cheres appears to be met: a dead sheretz, which is an av hatumah, is now suspended inside the oven's cavity. Yet nothing is defiled, and the reason is that tumah which has been swallowed loses its force entirely. Sitting inside the body of a live creature, the sheretz has the status of tumah beluah, swallowed tumah, and in that condition it cannot impart tumah to anything.
It is worth being precise about why this oven stays tahor. This is not an application of the rule about an inner space contained within another inner space, which the chapter has discussed elsewhere. The reasoning here is simpler and broader: a source of tumah that is swallowed inside a living body is neutralized for as long as it remains there, and it makes nothing around it tamei.
The mishnah then teaches the flip side: "Ve'im meit, tamei." Should the bird die, the oven does become tamei. The line is drawn precisely where the previous ruling drew it. Swallowed tumah is neutralized only within a living body; a sheretz enclosed in a carcass is not treated as swallowed at all. From the moment the chicken has no life in it, the sheretz inside is once again an ordinary av hatumah, and it is now hovering in the hollow of a clay oven. Under those conditions the oven picks up tumah in the normal way.
The bread in the oven
A second case follows. The mishnah states: "Hasheretz shenimtza batanur", a sheretz discovered inside an oven, while loaves are baking there at the time. The ruling: "hapat shebetocho sheniyah, shehatanur techilah", the bread has second-degree tumah, since the oven itself holds first-degree tumah, called here a techilah.
Trace the sequence link by link. At the head of the chain stands the sheretz, an av hatumah. The oven draws its tumah from the sheretz by way of its hollow space, which makes the oven a rishon latumah. The loaves never touch the sheretz; their tumah comes from the oven alone, and anything defiled by a rishon takes on the status of a sheni. That is why the bread baking inside carries second-degree tumah.
Two clean principles emerge from this mishnah, then. First, swallowed tumah is inert: as long as the sheretz sits inside a living chicken it defiles nothing, and only when the chicken dies does the sheretz resume its power. Second, tumah descends in ordered degrees: the sheretz is an av, the oven it defiles is a rishon, and the bread the oven defiles is a sheni.