Negaim, Chapter 6, Mishnah 4. Our chapter is occupied with the measurements of the nega and of the michyah (the patch of healthy flesh) found inside it. This mishnah takes the familiar pairing of a baheres together with a michyah and asks a simple question: what happens when one of them grows and what happens when one of them shrinks?
A thought on Parshas Beha'aloscha
Before we turn to the mishnah, here is a delightful observation in the parshah that anyone who has learned Negaim this far will appreciate. Describing what befell Miriam, the Torah relates that once the anan withdrew from above the tent, Miriam stood there metzora'as, white as snow. The pasuk then continues that Aharon faced his sister, "ve'hinei metzora'as", and behold, metzora'as. The same fact appears to be stated twice over: first that she was white as snow, and then again, after Aharon looked her way, that she was metzora'as.
Targum Yonasan ben Uziel does not render the two clauses in the same way. For the first he uses the description chivra ketalga, white like snow, precisely the wording he employs when Moshe Rabbeinu spoke against Am Yisrael and his own hand became leprous. For the second he writes that Aharon faced his sister "ve'ha segira", and behold, she was segira: she had now become a musgeres, formally placed into hesger.
The explanation is beautiful in its simplicity. Aharon was a kohen. He looked at his sister, and the words "ve'hinei metzora'as" are not a repetition of the description but a ruling: he examined her and pronounced the psak, and with that psak she became a musgeres.
Now recall the machlokes we learned in the second perek. The Tanna Kamma holds that a person may examine every nega except his own, and his wife's nega is included in what he may examine. Rabbi Meir holds that one may not examine the negaim of any of his relatives. The Targum here clearly follows the Tanna Kamma, that a kohen may examine and rule on the nega of a relative.
Curiously, Rashi further on in the parshah takes the opposite position. Aharon begs Moshe to daven on her behalf, asking that she not become kemeis, like one already dead, whose flesh was half consumed as he emerged from the womb. Rashi offers several readings, and among them a davar acheir: if you will not cure her through your tefillah, then who is left to place her in hesger and who is left to pronounce her tahor? I cannot inspect her myself, being her relative, and a relative may not examine negaim, and there is no other kohen available. And this, Rashi explains, is the force of the phrase about emerging from the womb: I came from that same womb, I am her brother, and it is precisely my being her brother that prevents me from declaring her tahor.
When Aharon says there was no other kohen, the point is presumably not that literally no other kohen existed, and certainly one does not need a Kohen Gadol to declare a metzora tahor. But the other kohanim of that moment were his own sons, so they too were Miriam's relatives. In any case, Rashi here is following Rabbi Meir. Rashi on Chumash often brings the view that best fits the pshat even when it is not the accepted halacha, and here that is exactly what he does. The halacha follows the Chachamim, as the Targum renders it: a kohen may examine and rule on the negaim of his relatives.
The mishnah
"Baheres kegris uvah michyah pechusah mikadashah", a baheres the size of a gris with a michyah inside it that is smaller than an adashah (a lentil). Such a person will be placed in hesger: the baheres itself is a full shiur of a gris, but the michyah is under the size of an adashah, so it is not a siman of tumah. He is therefore musgar, not muchlat.
Should the baheres expand, "ravsah habaheres, temei'ah": whether the growth appears when the first week of hesger concludes or when the second one does, such growth is pisyon, spreading, which serves as a siman of tumah, and he is tamei.
"Nis'matah, tehorah", if it shrank below the size of a gris, or the spreading simply receded and returned to what it had been, he is tahor.
"Ravsah hamichyah, temei'ah", if the baheres stayed at a gris and the michyah grew until it reached the size of an adashah, that is a siman of tumah and he is muchlat, fully tamei.
"Nis'matah", it shrank, and here a fresh concept enters the mishnah. The michyah contracted, and the reason it contracted is that the baheres itself advanced into the ground the michyah had held. Rabbi Meir rules that he is tamei, a muchlat. His logic: granted, what remains of the michyah no longer has the shiur that constitutes a siman tumah; still, the baheres has expanded, having taken over territory that previously belonged to the michyah, and in Rabbi Meir's view expansion inward, into the area enclosed by the nega, qualifies as pisyon.
"Vachachamim metaharin, she'ein hanega poseh lesocho", the Chachamim declare him tahor, because a nega does not spread into itself. Growth into the michyah in its own midst is not called pisyon at all. And the halacha follows the Chachamim.
So this mishnah gives us both directions of change, in the baheres and in the michyah, and closes with the principle that will guide us further: ein hanega poseh lesocho.