Ohalos, Chapter 4, Mishnah 2. This chapter deals with a migdal, a large cabinet or closet standing in a house, and asks how tumah inside it relates to the house around it, and how tumah in the house relates to what is stored in the cabinet.
A little background from the opening of the chapter. The migdal being discussed is a very large piece of furniture, so large that it holds forty se'ah of liquid, and a vessel of that size is no longer mekabel tumah at all. The cabinet itself, then, never becomes tomei. The question is only about the airspace inside it and its relationship to the airspace of the house. There we learned the basic rule: if tumah is inside the migdal, the house becomes tomei, because the tumah will eventually have to be removed, and the only place for it to go is out into the house. But if the tumah is in the house, whatever is stored inside the migdal stays tahor.
Compartments Within the Cabinet
Now the Mishnah takes us one step further in, into the interior of the cabinet itself: "teyvas hamigdal", the boxes of the migdal. Picture a large closet fitted out with a series of small compartments or drawers, each one holding something different: keilim in one, food in another, drink in a third.
The Mishnah spells out the measurements of these compartments with care: "yesh bo poseiach tefach", the hollow within the compartment amounts to a tefach in each direction, a full cubic tefach of space. "Ve'ein biyetzi'aso poseiach tefach", yet the aperture through which one inserts and withdraws the contents does not reach that measure of a square tefach. Each compartment, in other words, is built on the model of a mailbox sunk into the wall of the cabinet: a narrow slot in front, and behind it a chamber enclosing a cubic tefach.
The Ruling of the Tanna Kamma
Here is the halacha. "Tumah besoycho, habayis tomei": if there is tumah inside one of these compartments, say a kezayis from a meis, the house becomes tomei. The tumah spreads out into the house even though the opening of the compartment is smaller than a tefach. Why? For the same reason we saw with the migdal as a whole: the tumah cannot remain there forever, it will have to be taken out, and there is nowhere for it to go except into the house. Therefore everything in the house is immediately tomei.
But the reverse is not true. "Tumah babayis, mah shebetoycho tahor": if the tumah is lying in the house, and I have keilim or food stored in one of these compartments inside the cabinet, they remain tahor, because the compartment has no opening of a tefach. That narrow slot through which things are inserted and removed is simply not large enough to serve as a passageway for tumah to enter.
How can a single opening serve tumah on its way out while barring it on its way in? The Mishnah supplies the rule in its own words: "shederech hatumah latzeis ve'ein darko likanes", tumah's normal course runs outward, not inward. Tumah lodged in a compartment is destined to be cleared away, and if a narrow slot is the only available route, that is the route it will travel. In the reverse direction there is no such expectation. Nobody takes tumah lying about the house and deliberately shoves it into a little cubbyhole. What would move a person to do such a thing? Hence, where the aperture measures a full tefach, tumah crosses it whether or not anyone wishes it to; where the aperture falls short of that measure, we rule that it does not cross at all. The commentators offer several explanations of exactly how this mechanism operates, but the halacha stands as the Mishnah states it.
Reb Yosi Disagrees
So when the compartment's aperture falls short of a tefach and the tumah is sitting inside, is the house tomei according to every opinion? The Mishnah does not present the matter as unanimous. "Reb Yosi metaher": Reb Yosi rules that the house is left tahor.
His reason: "mipnei sheyochol lehoytzi'o lachatzo'in o lesorfo bimkoymo". The whole argument of the Tanna Kamma rested on the assumption that the tumah must eventually emerge into the house. Reb Yosi challenges that assumption. Suppose the tumah is a kezayis. A person can divide it while it is still inside the compartment and remove it half at a time, and half a kezayis does not carry the shiur to be metamei. So nothing that is metamei ever actually passes out into the house. Alternatively, who says it must be removed at all? A person can burn it right where it lies, inside the compartment.
Because there exists a method of disposal in which either no shiur of tumah ever emerges or the tumah never emerges at all, Reb Yosi rules the house tahor. All of this concerns a compartment whose aperture does not reach the measure of a tefach.
Summing Up
The great migdal is fitted with compartments, each one enclosing a cubic tefach of space while being served by a narrow slot, under the required measure, for depositing and retrieving items. According to the Tanna Kamma, tumah placed inside such a compartment renders the contents of the house tomei, since that tumah is bound to emerge into the house; tumah in the house, however, cannot make its way into a compartment whose slot falls short of the measure, because tumah's course runs outward and not inward. Reb Yosi answers that the assumption of emergence is not compelling: the tumah can be divided and carried out in portions too small to be metamei, or destroyed by fire in place. In his view, then, not only are the compartment's contents tahor when the tumah lies in the house, but the house itself remains tahor when the tumah lies in the compartment, provided the slot does not reach the measure of a tefach.