Keilim, Chapter 10, Mishnah 8. This is the final mishnah of the subject that has occupied us through most of the masechet: the tumah laws of earthenware utensils. Here the Mishnah turns to a case of larger earthenware vessels holding smaller ones inside them, where none of them is sealed shut with a tight lid. The question in every case is simple: whose airspace is the sheretz in? Because a keli cheres contracts tumah through its interior airspace, everything depends on which vessel's inside the sheretz occupies.
Nested pots with even rims
"Lefasin zo betoch zo": a series of earthenware pots arranged one within the next, each successive one narrower than the one holding it. The Mishnah adds, "vesiftoteihen shavot": every one of these pots reaches exactly the same height at its mouth. Even though the vessels grow smaller toward the center, they are not pushed down lower and lower inside the outer one; a person looking at the set from above sees all the mouths meeting along one flat line.
Now the case: "hasheretz ba'elyonah o batachtonah." The dead creature is found either in the pot called the upper one, which is the narrow vessel at the very center, or in the pot called the lower one, which is the wide vessel on the outside. The halachah is "hi temei'ah vechulan tehorot": tumah attaches to whichever pot actually contains the sheretz, while every other pot in the set stays tahor. Tumah reaches the one vessel because the creature rests within the space it encloses. Two considerations keep the rest tahor: the creature does not lie within the space that they enclose, and even a keli cheres that has become tamei has no power to transmit that tumah to another keli.
When the pots are punctured
"Hayu bechones mashkeh": now the Mishnah pictures the same nested pots, but each one is perforated to the degree that liquid can pass through, something like a strainer. Such a vessel occupies a middle status, and here is the key to the whole ruling: each perforated pot can be viewed as a keli or as a non-keli, and either way of viewing it can turn out to be the stringent one.
Begin with the pot itself. Classifying it as a keli means it is subject to tumah, which is the strict result; classifying it as a non-keli means tumah has no hold on it at all, which is the lenient result. When we turn to the pots surrounding it, the whole picture inverts, because only a real keli encloses an interior space that counts as its own. Suppose the creature rests inside one of these perforated pots. Treating that pot as a keli is the lenient position for the vessel around it, since the creature is then shut away in a space of its own. Refusing it the status of a keli is the strict position for that surrounding vessel, since the creature is then reckoned as lying in the space belonging to the vessel outside. Any single sheretz is located in exactly one interior at any given moment.
An illustration will make this concrete. Picture the creature resting in the narrowest pot at the center, and suppose that the holes in that pot lead us to deny it the status of a keli. The creature is then situated within the second vessel counting inward, inside the space it encloses, and that vessel picks up tumah. Because the classification of these perforated pots is genuinely in doubt, we rule strictly in whichever direction the doubt points.
This yields the Mishnah's ruling, "hasheretz ba'elyonah kulan temei'ot." A creature lying in the small central pot leaves the entire set tamei. For every single pot in the series, a strict reading is available in which the creature rests within the space that this pot encloses: simply treat all the pots inside it as non-keilim, and the creature is found in its interior. Take the third pot counting inward. Deny keli status to the two pots inside it and grant keli status to this one, and the creature is now lying in the space it encloses. That reading is the strict one, and it is the one we follow.
"Batachtonah hi temei'ah vechulan tehorot": where the creature lies in the widest pot on the outside, that pot alone contracts tumah. With respect to itself we grant it the status of a keli, and so tumah takes hold of it. Every pot nested inside it stays tahor, since no reading whatsoever puts the creature within them: grant them keli status and the creature lies outside the space they enclose, deny them keli status and it is no nearer to being inside them. Even after every available stringency is applied, tumah reaches only the outer pot.
Intact pots, when the outer rim is higher
The Mishnah now returns to unperforated, whole pots. Until this point the case has been one in which all the rims were level. Now it considers a set in which one rim rises above the others.
"Hasheretz ba'elyonah vehatachtonah odefet." The creature has landed in the small pot at the center, while the wide pot on the outside has walls and a mouth rising above all the other mouths in the set. The halachah is "hi vehatachtonah temei'ah": tumah takes hold of that central vessel and of the outer vessel too. The central one is affected because the creature entered it. As for the outer one, whose walls tower over the others, the creature had no way of reaching the center except by passing through the open space that those raised walls enclose. Pots standing in between, whose mouths reach only as high as the central pot, remain tahor.
The same case, with moisture in the middle pots
"Ba'elyonah vehatachtonah odefet": precisely the arrangement just described, with the creature in the small central pot and the wide outer pot rising above all the rest. One factor is now added: "kol sheyesh bah mashkeh tofe'ach temei'ah." Whichever of the intermediate pots holds damp liquid is tamei as well.
Tumah reaches the outer pot for the reason already given, since the creature crossed the space enclosed by its raised walls in order to get in, and it reaches the central pot because the creature came to rest there. The Mishnah's addition concerns the pots in between: they too are tamei, on condition that they contain liquid. The space each of them encloses is counted as a continuation of the space belonging to the outer pot, whose walls stand higher than theirs. A keli cheres, however, transmits nothing to another keli, so merely standing within the tamei outer vessel could never defile them. Liquid works differently: the moisture resting in those intermediate pots does contract tumah while it sits in the space belonging to the tamei outer vessel. And by force of a takanah derabbanan, liquids that have become tamei convey tumah to the vessel in which they sit. Each intermediate pot holding moisture therefore turns out tamei by way of the liquid it contains.
With this the Mishnah completes its treatment of the earthenware vessel and its airspace, having traced the principle to its finest cases: the sheretz defiles the vessel whose inside it occupies, and everything hinges on whose inside that is.