Negaim, Chapter 13, Mishnah 3. This chapter continues the laws of a house afflicted with tzara'as, and our mishnah takes up a case that touches two families at once: a single stone whose width is shared by two adjoining houses, when the nega appears on one side of it.
Which stone are we speaking about?
The mishnah's wording is "even sheb'zavis," a stone set at a corner. A number of the commentators observe that its placement at the corner is not what generates the halachah. A corner stone is simply the familiar illustration of a block so wide that, where two houses meet, part of its thickness sits inside one dwelling and the remainder inside the one beside it. Any broad stone built into a shared wall in that fashion, corner or not, is governed by the identical rule.
Chalitzah: the whole stone goes
The mishnah teaches "bizman shehu choletz": when the moment arrives to extract the afflicted stone, "choletz es kulo", he draws out the stone in full, notwithstanding that a portion of it sits in the property of a neighbor whose own house shows no nega whatsoever. The Rav supplies the derivation: "ki darshinan vechiltzu", and the plural form of the verb, "melamed sheshneihem choltzim", teaches that where the nega surfaces in a wall held in common, the extraction involves both owners.
This is exactly the thought captured earlier in the perek by the phrase "oy larasha v'oy lischeino": alas for the wicked man, stricken on account of his lashon hara, and alas as well for the man living next door, who gets pulled into the whole affair and forfeits a stone of his own in the bargain.
Netzitzah: only his own half
Now the mishnah continues, "u'vizman shehu notetz": at the stage of tearing down, "notetz es shelo", he breaks apart only what belongs to him. Picture the sequence that led here: the stone was drawn out, ketzi'ah was performed, substitute stones were installed, new plaster was spread in the ticha, and the nega returned. The house is now condemned to demolition, and here the ruling shifts. The stone may be broken apart with tools, and the owner takes down nothing beyond the section standing within his own walls, "u'mani'ach es shel chaveiro", allowing his neighbor's part of the stone to stay in place.
"Nimtza chomer b'chalitzah min hanetitzah." A stringency therefore emerges in chalitzah that demolition does not carry. Extraction claims the stone in its entirety, the neighbor's share included; demolition claims only the stretch of wall belonging to the man whose house was afflicted, and whatever belongs to the neighbor is left standing.
The view of Rabbi Elazar
"Rabbi Elazar omer: bayis habanuy rosh u'psoyin." Rabbi Elazar takes up a wall constructed of two different courses. At the bottom sits a rosh, a thick block (the corner stone we have been discussing, or any wide stone whose thickness reaches into both houses), and resting above it are psoyin, a word also vocalized pisin or psosim and linked to the term for fragments or crumbs: smaller stones, each of them only half as thick as the block beneath. Being narrow, every one of these lies wholly inside one dwelling or the other, so those on this side are the property of this owner and those opposite belong to the man next door.
"Nir'eh barosh, notel es kulo." Should the nega show itself in the thick block that reaches into both homes, the chalitzah sweeps up everything, the small stones overhead included, his neighbor's among them. "Nir'in b'psoyin, notel es shelo u'mani'ach es shel chaveiro." Should it appear in the small stones, which are already sorted and identified by owner, he takes his own and lets his neighbor's remain in the wall.
How do we rule?
The Rav offers no verdict here between the Tanna Kamma and Rabbi Elazar, and I did not find the Meforshim on this mishnah settling the question either. The Hilchesa Gevirta, the digest of halachos appended to the close of each perek, does decide, and it follows the Tanna Kamma: the neighbor's small stones are not taken away even when the nega turns up in the thick block, and all the more so when the affliction is located in the small stones themselves.
The Rambam
In Hilchos Tumas Tzara'as the Rambam never introduces this distinction. Halachah 4 sets down the din of the previous perek: "hayah kosel beino l'vein chaveiro, shneihem choltzim", a wall standing between the two of them is extracted by both, "u'shneihem kotzim es he'afar", both scrape off the surrounding dust, "u'shneihem mevi'im avanim acheros", and both supply the replacement stones. The earth, however, falls to one man alone: "aval ba'al habayis l'vado" is the one who "mevi es he'afar", since the pasuk writes "v'afar acher yikach v'tach" of a single individual, and therefore "ein chaveiro mitapel imo b'ticha", the neighbor bears no part in plastering over the stones newly set into the wall.
Halachah 5 opens with "even sheb'zavis," a stone at the corner, and the Rambam does not add what other commentators add, that a thick stone spanning both houses is treated identically even away from a corner. "Bizman shehu choletz", at the point of extraction, "choletz es kulo", the entire stone comes out. "U'vizman shehu notetz", at the point of demolition, "notetz es shelo u'mani'ach shel chaveiro", the afflicted owner destroys his own share and leaves his neighbor's standing. About a wall composed of a thick block with small stones above it the Rambam says nothing at all. It may be that in his understanding it is the wide stone alone, the one whose thickness reaches into both homes, that is taken away, and nothing beyond it, which is why no distinction was called for. On that reading he holds with the Tanna Kamma.
The Rambam then appends a remark that echoes a Gemara in Chullin: "v'yeish badavar safeik" whether "tihyeh zo shel chaveiro k'mo yad", whether the neighbor's share of the stone functions as a handle serving "l'even shelo", the stone of the afflicted house. He leaves the matter unresolved, standing as a safeik.
So this mishnah leaves us with the sober lesson that stands behind it: the metzora's affliction does not remain private. His neighbor's stone comes out of the wall together with his own, and only when the house is finally condemned does each man's property go back to being his alone.