Negaim, Chapter 6, Mishnah 7. Our mishnah picks up the very case the previous one was engaged in, so we begin by laying that case out. Picture a large baheres. Inside it sits a michyah of healthy flesh, an adashah (a lentil) in area, laid out as a square ring. In the hollow center of that ring sits a second baheres, a gris in measure. The outer baheres is declared muchlat on the spot, since a michyah stands within it, while the inner one is merely placed in hesger. The preceding mishnah worked through what happens when that michyah gets covered over, whether by the inner baheres growing outward or by the outer baheres growing inward. Here Rebbe Shimon comes to set limits on that ruling.
Rebbe Shimon: only when the michyah was exactly an adashah
"Omer Reb Shimon, eimasai, bizman shehi k'adashah muveis." The word "muveis" is being used here in a most unusual sense; it means m'tzumtzemes, precisely measured, exactly that size and no more.
The Rav frames Rebbe Shimon's "eimasai" this way: under what circumstances does the outer baheres turn tahor when the michyah is blotted out by the spreading of one baheres or the other? Only where the michyah measured a bare adashah and not a hairsbreadth more, so that once it is covered over, nothing of it survives.
The mishnah continues: "Hoysah yeseirah me'adashah, hamosar siman pisoyon lapnimis" - should the michyah have exceeded an adashah in area, that extra portion serves as a pisoyon for the inner baheres; "v'hachitzonah tmeiah," and the outer baheres stays tamei. The Rav spells out the picture: "im pos'sah hapnimis al oso mosar, shteihem l'hachlit." The inner baheres spread over nothing beyond that surplus, and a full adashah of michyah is left untouched. The outer baheres therefore retains its michyah, a siman tumah in full force, while the inner baheres has just undergone a pisoyon. Each of the two is tamei.
This position stands against Rebbe Akiva, who holds that an expansion taking place inside the territory of a larger baheres is no pisoyon at all. The halachah does not follow Rebbe Shimon here; it follows Rebbe Akiva.
The case of the bohak
"Hoyah bohak pachas me'adashah, siman pisoyon lapnimis v'ein siman pisoyon lachitzonah." A bohak is a dull white blotch, dimmer than the krum beitzah, which is the faintest of the four shades of white listed in the opening mishnah of the first perek. A bohak, standing on its own, has no connection to tumas tzoraas whatsoever.
The Rav describes the case: there is a bohak smaller than an adashah lying adjacent to the inner baheres, with the outer baheres surrounding the bohak. If the inner baheres grew and covered the bohak, that is a pisoyon and the inner baheres is muchlat, since this is an expansion outward, mibachutz. If, however, it was the outer baheres that grew and covered the bohak, that is not a pisoyon for it, since a nega does not spread into its own interior, "v'ein hanega poseh l'socho."
The Rav then explains why the mishnah specified a bohak smaller than an adashah: "d'im hoyah k'adashah hoyah mitamei mishum michyah." Had the bohak been the size of an adashah it would have been tamei as a michyah, since we hold that a michyah is mitamei in all colors. But less than an adashah there is no michyah here, for there is no michyah smaller than an adashah. There is, however, still room for the law of pisoyon according to Rebbe Shimon.
Take note of what lies beneath Rebbe Shimon's ruling: an inner baheres, though hemmed in on all sides by a larger baheres, can still register a legitimate pisoyon onto ground beyond itself. Rebbe Akiva, whose opinion is the accepted halachah, maintains that growth taking place within the domain of a bigger baheres never qualifies as a pisoyon at all.
The Tosfos Yom Tov challenges the Rav
The Rav's claim that a bohak's washed out white can double as the color of a michyah disturbs the Tosfos Yom Tov considerably. The Rav did not invent the idea; the Rambam rules the same way. Other Rishonim, the Rosh foremost among them, reject it outright. For the Rosh, a michyah means this particular individual's own healthy flesh in its natural shade. Ruddy skin yields a ruddy michyah, dark brown skin a dark brown one, olive skin an olive one, whatever tone that person's sound skin happens to carry. A white michyah could qualify only for someone whose entire body is that faded bohak white, namely an albino.
Why, then, does the mishnah trouble itself with a bohak at all? The Tosfos Yom Tov responds: "v'hashta tuva kamashma lan." Set aside the notion of a bohak functioning as a michyah, which it cannot do, and the mention of the bohak still delivers plenty. Back in the first perek of Negaim we were taught that a bohak makes an interruption, "d'bohak mafsik." Where a bohak lies between the om (the original nega) and the ground onto which the nega has spread, it forms a chatzitzah, a divider, and the person is spared the tumah of pisoyon. Our mishnah comes to teach that a bohak of less than an adashah carries no such power to divide and rescue, and the spreading counts as a genuine pisoyon (in the view of this Tanna, Rebbe Shimon).
The Tosfos Yom Tov continues: "u'matzasi l'Maharam," in the Maharam meRothenburg he located backing for the Rosh's approach, that same Rishon who never entertained a bohak serving as a michyah. And this is exactly what the mishnah's careful wording conveys: pisoyon comes into discussion here only because the bohak falls short of a full adashah. Were the bohak a whole adashah in size, its being covered over would not amount to a pisoyon, not even for the inner baheres, since a bohak of that measure creates a hefsek separating the om from the new growth, precisely as the first perek taught, and it blocks the fresh area from attaching itself to the original nega.