Negaim, Chapter 12, Mishnah 3. This perek is the first of two that deal with nigei batim, the tzara'as of houses. Having established which houses can come under this halachah at all, the Mishnah now turns to the materials a house must be built of, and to the timing of the nega's appearance.
The clearest entry point is the mishnah's final clause, which sets out the principle governing everything that precedes it: "Ein habayis mitamei benegaim," a house does not contract the tumah of negaim, "ad sheyehei vo," until there are present in it "avonim ve'eitzim ve'ofor": stone, wood and earth. The Rav cites the possuk from which this is drawn, which speaks of "es avonav ve'es eitzav ve'es kol afar habayis," its stones, its wood, and all the earth of the house. For the purposes of this halachah, a house is precisely that combination of three materials.
One unfit wall disqualifies the house
With that principle before us, turn to the mishnah's first case: "Bayis she'echad mitzdadav mechupeh beshayish," a house one of whose four sides is overlaid with marble; "echad besela," a second side consisting of natural rock; "ve'echad bilveinim," a third of kiln-baked bricks; "ve'echad be'ofor," and a fourth of earth. The ruling: "tahor."
At first glance nothing more than the concluding principle is needed to account for this: a building of that description is missing the three required materials. Two of its sides are stone, brick is not among the substances that take on the tumah of negaim, and wood is absent altogether. The Tiferes Yisroel, though, extracts an additional teaching here. Marble is a surface upon which a nega does not settle, and once a single wall of the house is unsuited, even those walls of that same house which are suited remain free of tum'as negaim. Two distinct halachos therefore emerge from this mishnah: the house must be composed of stone, wood and earth; and should even one of its walls be incapable of bearing a nega, the rest are exempt along with it. In his Boaz he points out that this is the position of the Rosh and of the Rav.
He then raises the difficulty that suggests itself: neither of them spelled out why marble should be a material upon which no nega arises. Marble, after all, is stone hewn out of the ground, and stone as such is a fit host for a nega. His resolution rests on one additional word in the language of the Rosh, who does not write that marble cannot become tamei, but that a nega is not fit "lavo," to come, upon it. That single word carries the whole answer: marble is extraordinarily smooth, and by its nature a nega does not take hold on such a surface. This is no halachic exclusion of marble from tum'as negaim; it is a statement about where a nega does and does not appear. In several places the Gemara describes a person who needed a particular procedure being brought into a room of marble, precisely because such a chamber is smooth, clean and hygienic. That very quality is what makes it unlikely ground for tum'as negaim. This appears to be the intention of the Rosh, and of the Rav with him.
The four kinds of walls
The Rav works through the mishnah's four items. The first of the four walls is coated with shayish, that is, marble. Sela he defines as "tzur shehi shom misheishes yemei Bereishis," a rock that has stood in that spot since the six days of creation, natural bedrock rather than stone shaped later on. Leveinim are bricks, "asuyin bekivshon," produced in a kiln. And ofor describes a wall built "migushei ofor," out of lumps of soil, on the model of a mud or adobe dwelling whose side is pressed together from clods of earth. Every one of these leaves the house tahor, because the requirement is that all four of the house's walls be able to contract the tumah of negaim; where even a single side consists of a substance that cannot become tamei, in any of the varieties listed here, the house is tahor.
Why does the Rav stress that the sela is a rock standing there since Ma'aseh Bereishis? Apparently his point is that the owner did not build this wall out of stones at all. Rather he joined the other three walls of his house to an ancient mass of rock that was already in place, the flank of a mountain, or a boulder protruding from the ground since the six days of creation. Since this side of the house was never built, but was simply an existing rock pressed into service as a wall, it does not receive tum'as negaim. The mishnah's word mechupeh, faced or covered over, fits this reading if we understand that the house was set up against that rock. And it emerges from the Rav's explanation of ofor as well that a plain earthen wall likewise does not receive negaim. Each item on the list is a wall that cannot bear a nega, and any one of them renders the whole house exempt.
A nega that appeared too early
The mishnah proceeds to a case that turns on timing: "Bayis shelo hoyu vo" the three components, a house lacking "avonim ve'eitzim ve'ofor," "venir'eh vo nega," and a nega appeared in it, "ve'achar kach heiviu vo" stone, wood and earth, which were only later brought and built up around the original walls, the nega remaining where it first showed itself: "tahor." Because the nega surfaced at a moment when the structure stood outside the laws of tum'as negaim, supplying the missing materials afterward cannot lend that nega any force.
"Vechein beged shelo arag bo sholosh al sholosh, venir'eh vo nega, ve'achar kach arag bo sholosh al sholosh" is the parallel case, and for a moment the perek returns to nigei begadim. A garment on the loom had not yet been woven to three etzba'os by three etzba'os, and a nega appeared on it, a gris of adamdam or yerakrak, and only afterward the weaving was completed to the required three by three. It is tahor. The sign of the nega presented itself while the garment was still too small to be susceptible, and reaching the minimum size later does not retroactively give that sign any standing.
Once we have the closing rule that a house needs stones, wood and earth, one might wonder what the opening clause of the mishnah adds. The answer lies in the second halachah we drew from it: the opening clause teaches that even where the other three walls are properly composed of stone, wood and earth, if one single wall is built of a material that cannot receive a nega, or that is by its nature not prone to one, as the Rosh explains regarding marble, the entire house is not subject to tum'as negaim.
Hence the mishnah's concluding words: "Ein habayis mitamei benegaim," no house contracts the tumah of negaim, "ad sheyehei vo" the three components named in the possuk, stone, wood and earth. A nega in a house is judged against the house of which the Torah spoke, and only a building answering to that description, in every one of its four walls and already before the nega makes its appearance, falls under these halachos at all.