Negaim, Chapter 12, Mishnah 2. This mishnah opens the discussion of nigei batim, the tzara'as that appears on houses, a subject that occupies this perek and the one that follows it. Before the Mishnah can teach us how such a nega is inspected and what is done about it, it must first tell us which houses are candidates at all: who must own them, where they must stand, how many walls they must have, and what they must rest upon.
A note carried over from the previous perek
The large editions of Mishnayos, the Yachin uBoaz volumes and similar sets, print at the close of each perek a section called Hilchos Ketzuvos, which lists the settled halachos that emerge from the mishnayos one by one. It is worth returning for a moment to what it records on the final mishnah of the previous perek, which taught "beged hamusgar shenis'arev ba'acheirim, kulan tehorim," that a quarantined garment which became mixed among other garments leaves all of them tahor.
The Hilchos Ketzuvos makes a point there that changes how we understand that ruling: this holds even chad bechad, even one quarantined garment against a single tahor one. It is therefore not a matter of the tamei garment being nullified in a majority at all. The reason is different. A garment placed under hesger is not muchzak b'tumah. True, we treat a garment in hesger as tamei to the degree that a pisyon, a spreading, can emerge from it, and a pisyon must always emerge from tumah. Still, the status of hesger is never fixed and settled; it can yet change in either direction. Consequently, where there is a doubt there is no chazakah of tumah upon which to lean, and even one against one the garments remain tahor, midi lo huchzak letum'ah adayin, since it was never established in tumah. Recall the case: the nega on the garment had been painted over, so that the garment looked exactly like its neighbor and could not be identified.
That same section goes on to the mishnah's second halachah. When a garment under hesger is cut apart into small squares, none of them retaining the minimum measure of three etzba'os in each direction, the tumah is gone. Suppose those scraps were later worked back together, their threads becoming matted and interlocked as felt is compressed, until the resulting fabric is once again fit to accept tumah going forward: the tumah of before still does not revive. Nisbatel meihem tum'ah demeikara: the moment the pieces fell short of the required shiur, their original tumah dropped away, and reuniting them has no power to bring it back.
All houses, except those of a non-Jew
The mishnah begins: "Kol habatim mitam'in benegaim chutz mishel ovdei kochavim," all houses can become tamei with negaim except a house belonging to a non-Jew. "Haloke'ach batim me'ovdei kochavim yeira'u katchilah," one who purchases houses from a non-Jew, they are inspected as though the matter were beginning now: the kohen comes and views the nega as a first viewing.
The Rav notes at the outset that our subject is specifically houses in Eretz Yisroel, for the possuk speaks of "beveis eretz achuzaschem," a house in the land of your possession. Only a house in Eretz Yisroel is subject to nigei batim. And from the same word the exclusion of the non-Jew is learned: achuzaschem, your possession, becomes tamei with negaim, but the possession of an akum does not become tamei with negaim, even when it stands in Eretz Yisroel. This is a gezeiras hakasuv.
The Rav adds that just as their holdings do not become tamei with negaim, so too their garments and their bodies do not become tamei with negaim. In other words, the halachos we have already learned, that the body of a non-Jew is not subject to negaim and that his garment is not subject to negaim, all flow from this very possuk about houses, from the word achuzaschem.
Why a purchased house differs from a convert's body
Body and property, however, are not on the same footing. Imagine a non-Jew with a baheres on his skin who then converted. That mark will never carry tum'as basar, so long as its color stays as it was. The reason is that during the time it sat on a non-Jew's flesh it bore no shem nega whatsoever; halachically it was a baheres tehorah, a mark whose very status was one of taharah, unable to make anything tamei. And even granting the rule that ger shenisgayer ketinok shenolad dami, that one who converts is regarded as a newly born child, the mark had already been assigned a status of taharah of its own, and that assignment stays with it once he has entered Klal Yisroel.
A house or a garment is not comparable. There the nega never acquired a status of taharah. Nothing about its appearance was ever declared tahor; it was only the domain, the fact that the object stood in the ownership of a non-Jew, that held the halachos of negaim away from it. Once the ownership is transferred to a Jew, that single impediment is removed, and the mark can now function as a nega.
Here the Rav comments, "vehayamim shehayah bahem hanega kol zman shehayah beyad oved kochavim einam olim min hacheshbon": whatever days the nega spent on the house while the house belonged to the non-Jew are not credited to the tally. On the corresponding line earlier concerning nigei begadim, the Tosfos Yom Tov already asked what tally could possibly be meant. Neither a house nor a garment begins counting days by itself; the entire process rests with the kohen. He must first come and look at the house, or at the garment, and days begin to run only once he has pronounced it under hesger, so that before his viewing there is simply nothing being counted. For the full treatment the Tosfos Yom Tov points back to his own extended discussion of the question in the preceding perek, by the negaim of garments. Whatever the resolution, the halachah in practice is yeira'u katchilah: the kohen inspects at this point and rules hesger or hechlet on the basis of what is before his eyes.
Four walls, no more and no less
The mishnah proceeds to houses ruled out by their very shape: "bayis agol," a dwelling built in a circle, which possesses no corner whatsoever, and "bayis trigon," one framed with three corners.
The Rav explains the principle: any house that does not have precisely four corners, whether fewer or more, is not subject to nigei batim. The source is the wording of the parshah, where the word kiros, walls, appears twice. The plural kiros means no fewer than two, and the word is stated two times, yielding four walls. A house must therefore have four walls, not three and not five. So the Rav writes on trigon that the same applies to a house with two corners or with five.
A house must be joined to the ground
Now the mishnah adds a separate disqualification: "bayis habanui bisfinah," a house built on a ship, "o be'askarya." The possuk says "beveis eretz achuzaschem," a house of the land, teaching that we require a house joined to the ground; until it is attached to the earth it is not subject to negaim.
Regarding the term askarya, the Rav offers that it means a toren, a ship's mast. One might imagine a little compartment set high up on a tall mast for a lookout to stand in. The Tosfos Yom Tov, though, quotes Rashi, who takes the word as a description of the vessel itself: an enormous ship fitted with a mast, whose interior contains rooms of four corners, the ship being labeled after the mast it bears, "kelomar bisfinah gedolah sheyesh bah askarya, vekara lo hachi al shem askarya shebah." The term itself still denotes a mast; in this mishnah it serves as a name for the great ship that carries one.
The Tosfos Yom Tov also brings the Rambam, who reads the word together with what follows, askarya al arba koros, a structure raised off the ground upon beams or poles. Most of the meforshim, however, read askarya as a unit of its own, as the Rav does, and then "al arba koros," built upon four beams, as a further case that is not subject to negaim.
The Rav also notes an alternate reading, aksadya in place of askarya, and explains that these are logs which are tied together and floated down rivers and out to sea, called in the language of Scripture rafsudos. The Rav says the like in a mishnah in Maseches Berachos. Such a raft, carrying a dwelling upon it, is a house that does not become tamei with negaim, since it is not joined to the ground.
Beams and pillars
The mishnah concludes: "ve'im hayah meruba, afilu al arba'ah amudim, mitamei," if the house was square, even standing on four pillars, it is subject to negaim. The Mishnah's choice of the word amudim, pillars, in contrast to koros, beams, demands explanation.
The Tosfos Yom Tov cites the Maharam miRothenburg on this. Earlier Rishonim explained "al arba koros" as a house built up on top of beams: for instance, beams projecting outward from an upper story, upon which one builds a dwelling out over open air. In the language of the Gemara a korah is generally a horizontal beam, and this is how these Rishonim understood the case, a new structure resting on beams and thus off the ground.
The Maharam explains otherwise: "dekoros haino sheheim rechavim, uru'in la'amod belo neti'ah bakarka." He reads the koros as uprights, and draws this distinction: to count as a house standing on the ground, part of the structure must actually be planted in the ground, no'utz bakarka. Koros are thick and broad enough to bear a house while merely resting on the surface, without being driven into the earth, and such a house is not joined to the land. The amudim of the mishnah's final clause, by contrast, are pillars sunk into the ground, and therefore that house is subject to negaim.
What emerges looks like a dispute among the Rishonim, with the Maharam holding an approach all his own. The prevailing view takes koros as horizontal beams on which a structure is erected in mid air, a wing extending outward from an upper floor, while arba amudim describes a four-cornered house standing on columns that bear upon the ground. On that reading, even columns that were never driven down into the earth leave the house counted as joined to the land, and it is therefore subject to negaim.
The mishnah has thus set the boundaries of the entire subject: a house in Eretz Yisroel, in Jewish ownership, with four corners, resting upon the ground. Only such a house comes before the kohen.