Negaim, Chapter 4, Mishnah 7. The subject of this chapter is the simanei tumah that attach to the arba'ah mar'os, and one point deserves to be made clearly before anything else: the four shades of white are not themselves signs of tumah at all. They merely define which sort of blemish is capable of becoming tamei. There are three actual signs: sei'ar lavan, hairs that have turned white; michyah, an island of live, healthy flesh within the nega; and pisyon, spreading, which is reckoned only when a week of confinement has run its course, whether the first or the second. Our mishnah takes up a nega that carries two of these signs simultaneously, and asks what the halachah is when one of the two vanishes.
Nine lentils of white and one of live flesh
The mishnah begins: "Baheres kagris, uvo michyah k'adashah." There is a baheres that measures a gris, and set within it is a michyah the size of a single adashah, a lentil.
The Rav supplies a helpful figure here: one gris is equivalent to ten adashos. Our nega, then, consists of nine adashos worth of baheres plus one adashah of michyah, and the two together add up to the required gris. The Rav also observes, at the opening of his comment, that if not for that dot of living flesh, we would simply have an ordinary baheres of a full gris.
The mishnah goes on: "vesei'ar lavan besoch hamichyah." The white hair sprouts out of the michyah and not out of the white area of the nega. Under those circumstances the hair does not render the man tamei. Back in perek alef we were taught that living skin interposes, separating the hair from the baheres, so that a hair rooted in the michyah cannot function as a siman tumah at all.
When one of the two signs disappears
Now the mishnah says "halchah hamichyah": should the live flesh disappear, the nega becomes tamei mipnei sei'ar lavan, on account of the white hair. What remains is a complete gris of baheres with a white hair rooted in it, and when the kohen comes back he pronounces the man muchlat by virtue of that hair.
The reverse case: "halach sei'ar lavan", the white hair disappears, whether by falling out or by reverting to black. Here the man is tamei because of the michyah.
Follow the logic. Even beforehand, while the hair was still there, he was already tamei on account of the michyah. The relationship runs in one direction only: the live flesh disables the hair, since it lies between the hair and the white patch, but the hair has no effect whatsoever upon the michyah. Whether or not a white hair happens to be present, the michyah is a functioning siman tumah in its own right.
The Tiferes Yisroel uses this to answer a difficulty posed by the Tosfos Yom Tov. Why does the mishnah change its verb? Where the live flesh vanishes it says temei'ah on account of the white hair, yet where the hair vanishes it says tamei on account of the michyah. The Tiferes Yisroel proposes vocalizing the first word as tim'ah: the kohen now issues a fresh ruling of tumah based on the hair, because only at this moment has the hair acquired any force. When the hair drops away, by contrast, nothing whatever has changed; the man's status continues as it already stood, tamei because of the michyah. From the michyah's perspective, a white hair beside it neither adds nor subtracts.
Rabbi Shimon's objection
"Rabbi Shimon metaher." Where the live flesh vanished, Rabbi Shimon rules the man tahor, "mipnei shelo hafachato habaheres," since it was not the baheres that had turned the hair white. Recall where the hair had been rooted: in the living flesh. Rabbi Shimon maintains that a white hair qualifies as a siman only if the baheres is the very thing that changed a black hair into a white one, and a hair anchored in healthy skin was never changed by the baheres.
The Chachamim take the opposite view. They too hold that the white hairs must arise only once the nega is on the skin, but they do not insist on Rabbi Shimon's additional condition. Given an area of a gris composed of baheres plus michyah together, if black hairs within that area turn white, the whitening counts, and the nega is treated as the cause of it.
The second case: the hair rooted in the white
Thus far the hair stood in the live flesh. The mishnah now shifts to a hair anchored in the white itself: "Baheres hi umichyaso kagris, vesei'ar lavan besoch habaheres." The baheres and its michyah come to a gris between them, nine adashos of white and one adashah of live flesh, and on this occasion the white hair grows out of the baheres. Notice what is missing here: there is no full gris of baheres by itself.
Once again "halchah hamichyah": if the live flesh disappears, he is tamei by virtue of the white hair. And "halach sei'ar lavan": if the hair disappears, he is tamei by virtue of the michyah. The shift in wording teaches the same lesson as before. When the live flesh goes, the kohen pronounces a new ruling of tumah grounded in the hair; when the hair goes, he is tamei regardless, precisely as he had been, on the strength of the michyah. All of this assumes that the michyah is positioned as it must be: sitting at the center of the nega, square in its shape, and intact rather than broken up into scattered specks.
In this case as well, "Rabbi Shimon metaher." His position is far more striking now. In the earlier case one could follow his reasoning, for the hair had come up out of the live flesh and there was room to argue that white skin had never acted upon it. Here, though, the hair rises from the baheres itself, and still Rabbi Shimon rules tahor: "mipnei shelo hafachato baheres kagris," because no baheres of a gris turned it white. In his view it does not suffice for the hair to sit in white skin; the white skin it sits in must itself measure a full gris. In our nega the gris is reached only with the michyah's help, and while the michyah does round out the shiur, it is not baheres.
The mishnah closes with Rabbi Shimon's concession: umodeh, he agrees, that if there is a full gris of white bimkom sei'ar lavan, in the very spot where the white hair stands, the man is tamei. In other words, even when a michyah is found elsewhere inside the nega, so long as the hair is surrounded by an expanse of a gris of the white shade itself, Rabbi Shimon concurs that once the michyah departs the man is tamei.
A closing note on the halachah: in neither of these disagreements is Rabbi Shimon followed. Ein halachah keRabbi Shimon, and in both cases of our mishnah the nega is tamei.