Keilim, Chapter 25, Mishnah 3. This chapter is built around the rule of achorayim vatokh, the distinction the Chachamim drew between the outside of a vessel and its inside. Our mishnah asks about a vessel whose base is hollow, so that when it is turned over the base itself can serve as a small container. Is that base treated as part of the vessel's outside, or does the fact that it can hold something give it the status of an inside?
First the background principle. The Chachamim decreed that a tamei liquid which touches the inside of a vessel renders the entire vessel tamei, inside and out. But if that liquid touched only the outside surface, the tumah stays where it landed: the outside is tamei and the inside remains tahor. Our question concerns a part of the vessel that is really its outside, yet on occasion is pressed into service as an inside.
The Vessels of the Mishnah
The mishnah enumerates a group of utensils: "Middot yayin vashemen", the graded measures with which wine and oil are sold; the zomalistera, a double implement carrying a spoon at one end and a fork at the other; "umesanenet shel chardal", the sieve through which mustard is filtered; and "meshameret shel yayin", the cloth strainer for wine. Of all of these the mishnah rules "yesh lahen achorayim vatokh": each one possesses an exterior and an interior.
Which portion counts as the tokh, the interior? Whichever cavity the utensil was made to work with in performing its task. Every other surface, the hollow base included, belongs to the achorayim, the exterior. True, a person may now and again invert the keli and let the cavity in its base serve as a receptacle, but such usage is unusual, and unusual usage does not grant a surface the standing of an interior. The base therefore retains its regular classification as part of the outside. So rules Rebbi Meir: divrei Rebbi Meir.
Rebbi Yehuda Disagrees
"Rebbi Yehuda omer: ein lahen." Rebbi Yehuda holds that these vessels are not subject to the rule of outside and inside at all. In his view, the base here practically never functions as a container; we simply do not turn these utensils over and use them that way. Since the base has no independent role, the vessel is not divided into two zones. It is one utensil with one function throughout. Consequently, if a tamei liquid touches the vessel anywhere at all, the whole vessel becomes tamei, because there is no separate "outside" whose tumah could be contained.
Rebbi Shimon's Middle Position
"Rebbi Shimon omer: yesh lahen." Rebbi Shimon rules that these vessels do stand within the law of achorayim vatokh, but only partially. With regard to food resting inside the vessel, the distinction holds: "nitme'u me'achoreihen, mah shebetokhan tahor." If the outside, meaning that base, contracted tumah from a tamei liquid, the food sitting in the utensil remains tahor. The inside is treated as untouched as far as its contents are concerned.
The vessel itself, however, is not divided. Once the tamei liquid has touched the exterior, the entire utensil becomes tamei, the inside included, "vetzarikh lehatbil," and it must be immersed in a mikveh to be purified.
The upshot is a careful balance. Where a vessel is a single undivided item with no genuine inside, the leniency of achorayim vatokh has nothing to work with, and tumah on any part of it spreads to the whole. Where a vessel does have a real inside, there are situations in which the inside is spared when the outside becomes tamei. Rebbi Shimon applies that leniency here in a limited way: the food held in the utensil stays tahor, while the keli itself becomes tamei and requires immersion.