Ohalos, Chapter 13, Mishnah 4. Our chapter is occupied with openings in the wall of a house, and with the question of what size such an opening must reach before tumah can travel through it and reach whatever lies beyond the wall. The case before us is an opening that a person chiseled out to hold one particular keli, and Beis Shammai and Beis Hillel argue about how an opening of that sort is measured.
The Mishnah begins: "ha'oseh makom lekaneh." A person hollows out a place in his wall in order to hold his kaneh, his rod. Which rod is meant? The Bartenura explains that this is the rod used by weavers in their work. And why would a weaver store his rod inside a hole in the wall? The Tosfos Yom Tov offers an appealing explanation: the weavers would sometimes need to hand the rod from one room to the next, so a narrow opening was made to the exact measure of the rod. Someone standing on one side could call out, "pass me the rod," and instead of walking around to the doorway, his fellow would simply slide it through the hole.
The Mishnah continues: "o le'ispati." Or he made the hole in order to keep his ispati there. What is an ispati? The Bartenura writes that in Greek a sword is called ispati. It was not an actual sword, however, but a weaver's implement shaped like a sword. The mefarshim offer various identifications for these keilim, but the essential point for our halachah is the same in every version: there is a hole in the wall, it is there to hold a keli, and since the keli itself is far narrower than a tefach, the hole too is narrower than a tefach.
The Mishnah adds a third case: "o lener." Or he hollowed out the wall so that a lamp could rest there. The lamp itself did not sit inside the cavity; its grip was pushed into the opening while the lamp stayed outside it, so that the wall held the lamp steady and nobody had to stand there supporting it. Here too the mefarshim suggest more than one picture. Some hold that the opening served for handing the lamp across to the neighboring room, and the Mishnah Acharonah writes that a small lamp was set within the opening itself, so that its light would fall in both rooms together. Whichever way we understand it, this opening as well measured less than a tefach.
How large, then, must such an opening be for tumah to make its way across the wall? Beis Shammai rule: "shiuro kolshehu." The term kolshehu can carry the sense of the tiniest quantity, but that is not its sense here. In this Mishnah kolshehu means whatever amount of space that particular implement calls for. Was the cavity chiseled out for a rod? Then the space a rod occupies is the measure that lets the tumah cross. Was it chiseled out for a lamp? Then the space a lamp occupies is the measure. Every opening is assessed according to the keli it was designed to hold. So rules Beis Shammai.
Beis Hillel disagree: "Beis Hillel omrim: beposeach tefach." The measure is a full tefach.
The Three Categories of Holes
To grasp what divides them, it helps to remember that openings in a wall sort themselves into three groups. One made in order to admit light is measured by the drill that served in the Beis Hamikdash. One that opened up on its own, whether water wore it away or sheratzim gnawed through, is measured at melo egrof, a fistful. And one that was made because the owner wanted to put it to some use is measured at poseach tefach.
Beis Shammai concede that an opening intended for use ordinarily carries the measure of poseach tefach; their claim is that when the owner shaped the cavity around one designated keli, the requirements of that very keli become its measure. Beis Hillel reject this: an opening made for use belongs to a category, and it therefore receives that category's fixed measure, poseach tefach, a tefach square, and this is the measure through which tumah crosses the wall.
The Case Where All Agree
The Mishnah turns now to an opening about which no one argues. Suppose a man breached his wall "lazun es eino," literally to nourish his eye, meaning so that he could gaze through it at the chatzer or out into reshus harabbim; or "o ledaber chaveiro," so that he could hold a conversation with his fellow through the gap; or "o letashmish," for ordinary use, resting a keli in it or handing a keli across. In every one of these the measure that allows tumah to travel through the wall is "beposeach tefach," a tefach by a tefach.
Review
Let us review. Where a man carved out a cavity for one designated keli, whether a weaver's rod, the sword-shaped tool of the weavers, or a lamp, Beis Shammai measure it by whatever that keli requires in order for tumah to pass, while Beis Hillel maintain that an opening of this sort falls under the heading of openings made for holding and handing things, and therefore takes the fixed measure of poseach tefach. And where the opening was not made for any single keli, but so that a person could look out at the chatzer or into reshus harabbim, or converse with his fellow, or for everyday needs, all agree that the measure is a tefach square.