Negaim, Chapter 10, Mishnah 4. The whole of this perek is given over to the third of the nigei adam, the nesek, which strikes those places of the body that grow hair: the zakan, that is, the part of the face on which a beard grows, and the rosh, the hair of the head. Our mishnah takes up the black hairs that have the power to keep a person out of the tumah of a nesek, and it draws a sharp line between two very different cases.
The first is called hatzomeach, "that which sprouts": two black hairs that came up inside the nesek after the nesek was already there. The second is called hameshuyar, "that which was left over": two black hairs that were standing there from before, and which remained in place once the nesek appeared. The mishnah rules on each case separately, and, as we will see, the two are not treated alike.
What the black hairs are saving us from
To follow the mishnah we have to have the simanei tumah of a nesek in front of us. The nesek itself consists of a change in the color of the skin under the hair, and the palette here is wider than in tzaraas habasar: we are not confined to the eight shades (the four levels of white together with their four reddish mixtures), for other discolorations qualify as well, a baheres among them. Once such a discoloration is present, tumah can arrive by one of two routes. The first is se'ar tzahov, a pair of golden hairs, the siman treated earlier in the perek; that is an instant hachlata, and the man becomes muchlat then and there. The second is pisyon, spreading, examined when the first week of hesger runs out or when the second does, and it too brings about a hachlata.
Against both of these stands the black hair. When two black hairs are present in the nesek, they are matzil, they rescue the man from tumah: not only from the immediate hechlet of the golden hairs, but even from the hechlet that would have come through pisyon at the end of a week of hesger.
Hatzomeach: black hairs that sprouted afterwards
Regarding hairs that sprouted after the nesek appeared, the mishnah teaches that they save "miyad se'ar tzahov umiyad pisyon," from the hand of the golden hair and from the hand of the spreading, and it then lists every possible arrangement of those hairs. "Mechunos umefuzar": whether the two hairs are bunched together side by side or scattered at a distance from one another. "Mevutzar veshelo mevutzar": whether they are centered in the middle of the nesek or not centered at all. In every one of these positions, hair that sprouted anew rescues the man.
Hameshuyar: black hairs that were left over
Hair that stood there beforehand and outlived the arrival of the nesek is likewise matzil, both against the golden hairs with their instant hechlet and against a pisyon that would render him tamei after hesger. And here too the arrangement is irrelevant: clustered or far apart, in the center or off center. Should a whole crop of golden hairs come up, or should the nesek spread, these two surviving black hairs still keep the tumah away. There is, however, one limitation on left-over hair from which sprouting hair is free: "ve'eino matzil min hatzad," it is of no help when it stands at the side, unless there is a gap of two hairs' width separating it from the komah.
The word komah requires explanation. A nesek, by its very nature, produces baldness: the patch where it sits loses its hair, and around that bald patch there remains a perimeter of hair belonging to the part of the head that the nesek has not yet reached. That surrounding fringe is the komah. Left-over black hairs standing at the very edge of the nesek must therefore be a distance of two hairs' space inward from the komah, so that they can be identified as black hairs that genuinely remained within the nesek and are not simply part of the untouched perimeter of hair around it.
Why the difference between sprouted and left over
The Tiferes Yisroel explains the logic. When hairs sprout up anew, their very growth is a proof that the nega has begun to heal; new growth is a sign of recovery. That being so, they testify to the healing wherever they stand, and they save even when they are not in the center but near the perimeter.
Hairs that were merely left over are another matter. So long as they stand in the middle we reason this way: the nega is pressing in on them from every direction, and even so this pair of black hairs is still standing right in the heart of it. Evidently the affliction is too feeble to consume even two hairs at its center, and a nega of such weakness leaves the man tahor. But once the surviving hairs are at the rim, what proof do we have that they held out because the affliction lacks force just there? It may well be that they are nothing more than a leftover piece of the encircling komah, hair that the nega has not yet touched in the first place. Hair like that can hardly serve to save him.
One golden hair and one black hair
The mishnah continues: "achas tzehuvah ve'achas shechorah, achas tzehuvah ve'achas levanah, einan matzilos," one golden hair and one black hair, or one golden hair and one white hair, do not save. The Rav (the Bartenura) explains that the golden hair spoken of here is one that preceded the nesek, so it is not itself a siman tumah; and precisely because it is no siman tumah, the mishnah must tell us that it nevertheless does not join together with the black hair to save the man from the golden hairs and from the pisyon. On this reading, our anonymous mishnah follows Rebbi Eliezer ben Yaakov, who says further on that a se'ar tzahov which came before the nesek is neither metamei nor matzil.
That means our mishnah is not going according to Rebbi Yehudah. Earlier in this chapter we learned the dispute between Rebbi Yehudah and Rebbi Shimon on exactly this point. Rebbi Shimon holds that just as in tzaraas habasar the baheres must precede the white hairs and turn them, so here the nesek must precede the golden hairs. Rebbi Yehudah holds that the Torah does not use the word hafach, "turned," with regard to a nesek, so whether the nesek turned the hairs golden or not makes no difference, and the two golden hairs are metamei. According to the Rav, our mishnah takes the third position, that of Rebbi Eliezer ben Yaakov: a golden hair that preceded the nesek is not metamei, but neither is it matzil.
The Baal Tiferes Yisroel prefers to explain the mishnah in a way that fits even Rebbi Yehudah, since it is his view that we pasken. True, Rebbi Yehudah holds that se'ar tzahov is metamei even when the nesek did not turn it, but that applies to two golden hairs that preceded the nesek. Here we are dealing with a single golden hair, and a single hair on its own is not metamei at all. Precisely for that reason one might have thought that since it carries no tumah of its own it takes on a different value and can join with the black hair to save from pisyon. The mishnah comes to tell us that it cannot.
Why the Torah speaks of black hair
The mishnah's last words, that one golden together with one white does not save, lead the Tiferes Yisroel to a further conclusion: white hairs do save. Were a white hair incapable of saving even when paired with a proper partner, the clause would be teaching nothing. So the "black" of the pasuk is not meant with precision; it is merely how people speak, since Yisroel are for the most part dark-haired, and hair in good health is called black hair. So it is taught in Toras Kohanim, and the source is cited both by the Rash and by the Rosh.