Negaim, Chapter 13, Mishnah 9. This chapter deals with the tumah of a bayis menuga, the house in which the nega of tzora'as has appeared. Our mishnah measures traffic in two directions: what happens when a tohor person or a tohor garment enters an afflicted house, and what happens when a tomei person or an afflicted garment enters a clean house. In both directions the Mishnah is telling us how much has to go in for the entry to count.
The Mishnah opens with human beings. Its first clause, "Tohor shehichnis rosho verubo," pictures a man who is clean pushing his head together with the majority of his body "lebayis tomei," into an afflicted house; the ruling is "nitma," he has contracted tumah. The clause that follows turns the picture around: "Vetomei shehichnis rosho verubo" speaks of a metzora, who carries tumah in his own person; when that much of him crosses "lebayis tohor," into a house that has no nega, the Mishnah rules "tim'uhu," the house has been rendered tomei by him.
From people the Mishnah moves on to fabric. "Talis tehorah shehichnis mimenah shalosh al shalosh lebayis tomei, nitmeis": when a clean shawl has a section of three by three etzba'os (fingerbreadths) taken into a stricken house, the shawl contracts tumah. Such a square is the minimum quantity of cloth capable of receiving tumah in the first place. The second half reads "utmei'ah shehichnis mimenah afilu kezayis lebayis tohor, tim'ahu": where the shawl itself bears the nega, an olive's bulk of it inside an unafflicted house is already enough to make that house tomei. The two halves are deliberately unequal, since a kezayis is a lesser quantity than a square of three fingerbreadths.
The Rambam's Framework
The Rambam, in his Peirush HaMishnayos, had a gift for reducing a mishnah to the rules that lie beneath it, and that is what he does here. For a human being he formulates the yesod: kol haba rosho verubo, whoever has brought in his head along with the larger share of himself, is judged keba kulo, as though every part of him had come inside. That one rule accounts for both halves of the Mishnah's first ruling, the clean man who steps into a stricken house and the metzora who steps into a sound one.
With fabric the Rambam reasons by analogy. A begged is capable of catching the nega of tzora'as only when it amounts to a square measuring three fingerbreadths on each side; correspondingly, a begged carried into a bayis menuga picks up nothing unless the section taken inside comes to that same measure. Once it does, the tumah takes hold of the garment in its entirety, even while the greater part of the cloth is still outside the doorway.
But if the begged itself is the menuga, the moment a kezayis of it is inside a clean house it renders the house tomei. The Rambam groups this with the parallel cases: afflicted garments, and the stones, the wood and the plaster dust of a bayis menuga, are all metamei bebiah, by entering, with the measure of a kezayis.
We saw earlier in this masechta a machlokes tannaim as to whether the tumah of these items requires a kezayis or whether even the slightest amount suffices. We pasken that a kezayis is required. The model is a meis, where flesh is metamei with a kezayis, while a complete limb is metamei bechol shehu, in any amount at all.
The Source in the Sifra
Where do we learn that an afflicted garment is metamei bebiah in the first place? The Rambam cites the Sifra on the pasuk "uletoras habeged ulabayis", which mentions the law of the garment and the law of the house together. Makish begged labayis: the Torah sets the begged alongside the bayis. Just as the bayis is metamei bebiah, so too the begged is metamei bebiah.
The Sifra presses on: ein li ela begged, from that comparison alone I would know nothing beyond a woolen garment. Minayin lerabos es kulam, what brings in the remaining forms of tzora'as, nig'ei adam among them? The Torah answers, talmud lomar, with the phrase that heads the whole section, "zos toras nega tzora'as," said with reference to "begged hatzemer." Because Scripture wraps all of it into one toras nega tzora'as, every one of them is set beside the begged: mah begged mitamei bebiah, af kulam metamin bebiah.
The Rav's Explanation
The Rav explains the Mishnah's measures. Regarding a person, anything less than his head and the bulk of his body is not considered a biah, an entry, at all. Regarding the shawl, once three etzba'os by three etzba'os have entered, it is treated as though the entire talis had come in, and the whole garment is tomei. Less than that, however, and nothing happens: a piece smaller than three by three is not susceptible to tumah and is halachically insignificant.
As for the stricken talis of which no more than a kezayis came inside a clean house, the Rav explains that the house becomes tomei because tzora'as is likened to a meis, and a meis conveys tumah through an olive's bulk of flesh. And the very principle that such a talis is metamei bebiah comes from the pasuk that speaks in one breath of the law of the garment and the law of the house, "uletoras habeged ulabayis": the begged is thereby set beside the bayis, makish begged labayis. Since the house openly conveys tumah to whoever comes inside, as the pasuk says "vehaba el habayis," the garment must do the same. And the heading "zos toras nega tzora'as," said of "begged hatzemer," carries the rule over to the other materials as well, to shesi va'eirev, warp and weft threads, and to oros, animal hides: mah begged mitamei bebiah, af kulan metamin bebiah.
The Mishnah has thus given us two clean measures to carry away: for a person, rosho verubo, since whoever enters with his head and most of his body is regarded as having entered entirely; and for cloth, three by three etzba'os to receive tumah from the house, but only a kezayis when the cloth is itself the carrier of the nega.