Negaim, Chapter 13, Mishnah 4. This chapter deals with a house afflicted with tzara'as and with what must be torn down together with it. Our mishnah asks about the parts of the structure that stand between two levels or on the fringes of the building: to which unit do they belong, what is spared from demolition, and in what quantity do the ruins of a bayis menuga convey tumah.
A house with an upper chamber
"Bayis shenir'eh bo nega, haysa aliyah al gavav." A nega has broken out in a house, and above that house stands an aliyah, an upper chamber. The aliyah counts as an independent unit; it is not swallowed up into the dwelling underneath it, and no nega has appeared in the aliyah itself. Now, that upper chamber rests on koros, beams, and those beams touch both floors. So to which of the two do they belong?
The mishnah decides: "nosnin es hakoros la'aliyah." The beams are reckoned as belonging to the chamber above, and they consequently remain tahor. Because they are joined to the aliyah, we take the lenient route, l'kula, and treat them as its property rather than as part of the stricken house. The Rav fills in what this means in practice: "v'somchan b'amudim, v'eino notetzan im habayis": pillars are placed underneath the beams so that they will remain in position while the house beneath is being knocked down. The very need to prop them up teaches that the mishnah is speaking of timbers lying horizontally, since a vertical post would hardly require a pillar beneath it, though the Rav's wording about pillars could be read otherwise, and quite possibly uprights as well as crossbeams are covered by the halachah.
"Nir'eh ba'aliyah, nosnin es hakoros labayis." Should the affliction instead show itself in the upper chamber, the same leniency operates in reverse: the beams are now credited to the house below, which has no nega, and we do not pronounce them tamei.
When there is nothing built above
The mishnah's next case: "Lo haysah aliyah al gavav" (nothing was built over the house), and therefore "avanav v'eitzav va'afaro nitotzin imo." With no second unit in existence, there is no other owner to whom the beams might be credited, and so everything goes: the stones of the house, its wood and its dust all come down with it.
"U'matzil al hamalbeinim v'al srigei hachalonos." Yet even a house that is dismantled in its entirety leaves certain articles untouched. What are malbeinim? The Rav records two views. According to the first, they resemble squared bricks laid out over the roofs. On this the Tosfos Yom Tov raises a difficulty: the Rav never tells us for what purpose they are laid out there. Were they simply resting on the roof with no role at all, it would be self-evident that they are no part of the building, and the mishnah would be teaching nothing; clearly they perform some job up on the roof, and nevertheless they survive the demolition. The second view the Rav brings, "v'yeish meforshim k'ein mezuzah," understands a malbein as a sort of frame, along the lines of a doorpost that juts out from the wall. In our mishnah it is a window frame, and the Rav specifies "osin bachalonos mibachutz l'noy": such frames are fitted to the exterior side of a window purely as ornamentation, contributing nothing to the strength of the house. Moreover, "v'ein mechubarin batit bachalon": no plaster bonds them to the window at all; they are simply set snugly into place from without. Precisely because they never became a genuine component of the building, the nega does not reach them, and they remain tahor.
The window screens
"V'al srigei hachalonos." The screens fitted over the windows are also rescued from the demolition. The Rav pictures them as a mesh or grating, something woven, installed in a window so that a person can gaze out from inside toward the street.
The Tosfos Yom Tov points to a remarkable detail in the way the Rosh puts it in his commentary to the mishnayos. He gives that same description, and then appends a short clause: "v'osin hasevachah shelo yiplu misham hatinokos." A mesh, after all, partially blocks one's view rather than improving it, so its real function must have been protection: to prevent little children from tumbling out through the opening. That is exactly the situation in our own time, when the law in New York City obligates a homeowner to install guards on his windows so that a child will not fall through, rachmana litzlan. Apparently they, too, fitted their windows with safety devices for the identical reason.
Rabbi Yehudah's malbein
"Rabbi Yehudah omer: malbein habanuy al gavav nitatz imo." Here the malbein is something else entirely. The Rav explains it as "eitz aroch she'al hakir," a long piece of wood laid along the wall, "ha'asuy l'hoshiv alav roshei hakoros," into which the ends of the beams are set, "shelo yirkevu milachluchis hachomah," so that they should not rot from the dampness of the wall. The walls had a certain porosity, moisture seeped through, and one did not want the beams that carry the structure and hold up the upper story to rot away. Such a malbein, built into the house, is demolished together with it.
The measure of tumah
The mishnah closes with a shiur: "avanav v'eitzav va'afaro" of the stricken house are "mitam'in b'kazayis." Should a piece of the stone, the wood or the dust of the bayis menuga be carried into some other house, it transmits tumah once it amounts to a kazayis. The Rav traces the source: tzara'as is likened to a meis, "dichsiv: al na tehi kameis," the words of Aharon begging Moshe Rabbeinu that Miriam should not become like one who has died, which indicates that tzara'as bears the character of death. That analogy is not restricted to the afflicted person himself; it reaches the substance of the afflicted building too. Hence, exactly as a meis defiles through a kazayis of flesh, the rubble of this house defiles through a kazayis.
"V'Rabbi Elazar Chisma omer: b'mishehen." That is the girsa in our texts, while the Rav had before him b'mashehu, meaning the tiniest quantity whatsoever. His reasoning runs as follows: flesh of a meis indeed demands a kazayis, but a complete limb is subject to no minimum at all. Take a very small person whose finger does not contain a kazayis of flesh: that finger still conveys tumah, since a limb is metamei b'mashehu, with no shiur, even below a kazayis. Rabbi Elazar Chisma transfers that yardstick to the debris of the afflicted house.
The Rav rules, "v'ein halachah k'Rabbi Elazar Chisma." The accepted halachah follows the Tanna Kamma: a fragment of dust, timber or stone from the bayis menuga conveys tumah to another house or to a person only when it reaches the measure of a kazayis.