Ohalos, Chapter 4, Mishnah 1. Our Mishnah introduces a new subject: a large cabinet, called a migdal, and the question of how tumah travels between the inside of that cabinet, the hollows in its walls, the house it stands in, and the narrow spaces above it, beneath it and alongside it. To follow the Mishnah we first need one piece of background from Maseches Keilim.
Why a Very Large Wooden Vessel Cannot Become Tamei
Maseches Keilim teaches that a wooden vessel large enough to hold forty se'ah of liquid, and anything larger, never becomes tamei. The source is the Torah's comparison of wooden vessels to sak, sackcloth. A sack has a defining feature: it is mitalteil malei v'reikan, it can be picked up and carried from place to place whether it is full or whether it is empty. Only a vessel that shares that quality is included in the category that receives tumah. A wooden container that holds forty se'ah, which is precisely the minimum measure of a mikvah, cannot be moved once it is filled. Practically speaking you could not budge it without breaking it. Such a vessel is called haba bamiddah, a container defined by its great capacity, and it is not mekabel tumah at all. Although it is technically a keli, for the halachos of tumah it behaves like a structure, an ohel in its own right. The migdal of our Mishnah is exactly such a piece: a large cabinet of that size.
A Migdal Standing in the Open Air
The opening case is a migdal shehu omeid ba'avir. Ordinarily the word migdal calls to mind a tower, but in our Mishnah it refers to a sizeable cabinet or cupboard. Nor should ba'avir be taken literally, as though the piece were suspended in mid air; it tells us that the cabinet has been set down outdoors, in a courtyard, on a street or in a field, with open sky overhead.
Examine the sides of this cabinet. The wood is not solid throughout. Cavities and openings pass through it from one face to the other, so that a lamp held on one side would be visible from the other. Still, not one of these gaps is a full tefach wide. Householders used such recesses as storage nooks, naturally for small articles: sewing needles, little implements and similar things.
Now assume tumah is sitting in the cabinet's interior, a kezayis of a meis for instance. The Tanna Kamma rules keilim sheba'aviyo tehorim: utensils resting in the cavity within the wood stay tahor. The converse holds too: tumah ba'aviyo, when the tumah occupies one of those recesses, keilim shebesocho tehorim, the utensils in the cabinet's interior remain tahor. Because the passage connecting the two areas falls short of a tefach, the two spaces never merge, and neither one absorbs what defiles the other.
Rebbi Yosi: Divide the Thickness in Half
Rebbi Yosi holds otherwise. To his mind the recess in the wood is not simply one undivided region lying outside the cabinet. Rather, mechtza lemechtza: take the wall's depth and split it evenly. Suppose the wood is one tefach deep. The half nearest the interior, measured outward from the inner surface, is reckoned together with the cabinet's inside; the half nearest the exterior, measured inward from the outer surface, is reckoned as being outside.
What follows from this? Should the tumah rest in the inner half of the recess, the utensils in the cabinet contract tumah. Should the tumah lie within the cabinet, any keli in that inner half becomes tamei too. Utensils in the outer half, however, are not reached by tumah within the cabinet, and tumah resting in the outer half does not defile what lies in the interior.
The Same Migdal Standing Inside a House
Everything until now concerned a cabinet in the open, where someone standing next to it and raising his eyes sees nothing but sky. What if he raises his eyes and sees the ceiling of his own home? That is the Mishnah's next case: hayah omeid besoch habayis, the cabinet has been placed indoors.
Now the rulings run differently. Tumah in the cabinet renders the house tamei, habayis tamei, and that is so even with the cabinet sealed shut. Conversely, tumah in the room leaves mah shebesocho, the cabinet's contents, tahor.
Behind this stands a basic rule: derech hatumah latzeis v'ein darkam lehikanes. Tumah does not stay penned in; its nature is to move outward. And once it leaves the cabinet, where can it go? Into the room. Hence the tumah shut inside the cabinet defiles the house even through closed doors. Entering, though, is not tumah's way, so tumah in the room does not force itself into a sealed cabinet. Closed up, the migdal counts as an ohel unto itself, and what defiles the room stops at its walls.
All of this rests on the cabinet being shut. Once its doors stand open, it is surely no more cut off than one room is from the next, and we already know that a window measuring four tefachim by four between two houses conducts tumah from one into the other. An open doorway on the cabinet is at least as large a breach, so the room's tumah reaches inside.
Vessels in the Spaces Around the Migdal
The Mishnah proceeds to three tight spaces. Imagine the cabinet resting on legs rather than sitting flush on the floor, leaving a gap underneath. How do we rule about utensils lying beino levein ha'aretz, in that clearance down to the floor? What about utensils beino levein hakoseil, in the narrow slot separating the cabinet from the house's wall? And utensils beino levein hakorah, in the gap between the cabinet's top and the ceiling beams overhead?
The Mishnah's answer hinges on how much open space the utensils are lying in. Where there is a posei'ach tefach, a clearance of a tefach, at any of the three places, underneath, along the side, or above the top, then tumah present in the room spreads into that gap and the utensils there become temei'im. V'im lav, where the clearance is short of a tefach, the spot is treated as sealed off: the room's tumah stays out, and even with tumah in the room the utensils there stay tehorim.
And if the tumah itself is what lies in one of those gaps, below the cabinet, at its flank or on its top? Then the size of the clearance is immaterial: habayis tamei. Given a tefach, the tumah obviously spreads through the rest of the room. And lacking a tefach, the cabinet still does not screen the house from tumah situated under it, next to it or over it, and so the house becomes tamei.