Ohalos, Chapter 13, Mishnah 2. This chapter is occupied with openings in a wall: which openings are large enough to allow tumah to travel from one side to the other, and which are too small to serve as a passageway for tumah. The answer does not depend on size alone. It depends on what the opening was made for. An opening whose purpose is to let in light is treated as significant even when it is tiny, while an opening made merely as a shelf or a niche to store things in requires a full square tefach before tumah passes through it. Our mishnah takes that principle and applies it to a case where the very purpose of the window changes in the middle of its existence.
A window made for air
The mishnah's first words present a "chalon shehi le'avir." Translated plainly, that is an aperture whose stated function is air, which sounds like a fresh category, since the discussion until this point revolved around windows built for light. The Bartenura clears this up at once: to make a window le'avir is to make it so that the outdoor atmosphere of the world enters the house, and there is no difference between that and making a window in order to receive light. Air and light are being counted here as a single classification rather than two.
The meforshim discuss why the mishnah shifts its wording at all, and one explanation emerges from the continuation of the mishnah itself. The mishnah wants to stress that this window's whole standing rests on its being open to the outside air. Its status as a window of light is not a permanent label stamped on it; it lasts only as long as there is open sky on the other side of it. The moment something outside blocks it, the window loses that standing. That is why the mishnah chose the language of avir, of open air.
The shiur, and how it changes
The measure assigned to such a window is "melo makdei'ach," the amount of space taken up by the drill, meaning the sizable drill bit that was stored in the lishkah of the Beis Hamikdash. That is a modest opening, well under a tefach, and nonetheless tumah crosses through it, because an aperture that draws in light matters halachically even when its dimensions are slight.
Now the mishnah introduces "banah bayis chutzah lah," he built a house on the outside of that window. Where there was once open air, there is now a structure standing in the way. The window no longer brings in air and no longer brings in light. Its function has been taken from it, and with that its measurement changes: "shiurah biposeach tefach," it now requires an opening of a square tefach. Having lost its purpose as a window of light, it is judged like any ordinary opening, the kind made to place objects in, which needs a full tefach for tumah to travel through it.
One window with two measurements
The mishnah then presses the case further: "nosan es hatikrah be'emtza hachalon," the ceiling of that newly built house was laid across the window at its midpoint. What emerges is a window that is partly stopped up and partly clear. Looking through its bottom section, one faces the new structure; looking through its top section, one still faces the open sky.
Here the mishnah teaches something striking: a single window splits into two different halachos, each part with its own measurement.
- Hatachton, the lower half, which is covered by the roof and no longer receives light or air: "biposeach tefach." This portion has become an ordinary opening, the kind used for storing things, and tumah passes through it only if there is a square tefach.
- Veha'elyon, the upper half, which rises above the roof and still faces the open air: "melo makdei'ach." Since light still comes through it, it retains its importance as a window of light, and the small measurement of the drill's opening suffices for tumah to pass.
A question from the Rashash
There is a case earlier in the chapter of a man who set out to seal up his window of light and did not manage to finish the job, leaving part of it open. There the halachah does not leave the remnant with the small measurement of a window of light; the leftover opening receives an entirely different shiur. Why, then, should our case be any different? Here too part of the window has been closed off and part remains open, so why does the open upper half keep its original standing as a window of light?
The Rashash draws the distinction. In that earlier case the man's intention was directed at the window itself: he wanted the whole window gone, and the only reason anything remains open is that he did not complete his work. His unfinished project cannot leave the remnant with its old status. In our case nobody set out to close the window at all, not even a part of it. The owner of the neighboring plot simply built his house and laid his roof where he laid it. The window was never the object of anyone's plan to seal it.
So the two halves are judged by what they actually are. The lower half, cut off from the light, has effectively become the sort of opening one places objects in, and its measurement rises to a full tefach. The upper half still does exactly what it was built to do, bringing the light of the world into the house, and a window that still gives light carries real chashivus. For that reason its measurement remains the small one, melo makdei'ach, the space of the drill.