Negaim, Chapter 10, Mishnah 9. The subject of this chapter is the nesek, the nega that appears on the head or in the beard. There are two indications that make a nesek tamei: se'ar tsahov, hair that has turned a golden yellow, and pisayon, the spread of the nesek beyond its original borders. Sound black hair growing inside the nesek works in the opposite direction and is a siman taharah. Our mishnah puts a pointed question on the table: when a siman taharah has already taken effect and then vanishes, while a siman tumah is still sitting in the nesek, where does that leave the man?
The Tanna Kamma
The case opens with "mi shehayah vo nesek" - a person who had a nesek on him, "u'vo se'ar tsahov" - and inside that nesek there was golden hair. Such a man is "tamei": the golden hair has made him muchlat. Then comes a change: "nolad bo se'ar shachor" - black hair sprouted within the nesek, and the ruling is "tahor," because a pair of black hairs purifies (the word se'ar in this context refers to two hairs). The chiddush of the mishnah, however, lies in what follows: "af al pi shehalach lo se'ar shachor" - even should that black hair later drop out, leaving the yellow hair alone in the nesek as before, the verdict remains "tahor."
The Rav supplies the framework behind these rulings. Golden hair conveys tumah at any stage at which it shows itself: right at the first examination, at the close of the first week of hesger, at the close of the second week, or even later on, after the kohen has already sent the man away as a tahor person. Black hair, for its part, purifies when it is born inside the nesek, and it does so even if its growth came only after the nesek had already formed, and even when it is not mevutzar, not fenced off. That condition of being mevutzar (that the hair stand at a distance of two hairs' breadth from the kamas harosh, the sound hair ringing the nesek) is demanded only of hair belonging to the mishur, hair that predated the nesek and merely survived inside it. Hair that sprouts anew is held to no such standard.
On the closing words of the case the Rav is careful with his wording: the yellow hair has already had a verdict of taharah pronounced upon it through the black hair, and therefore it is permanently spent as a source of tumah, even once the black hair is gone.
When the hachlatah came from pisayon
At this point, on the Rav's reading, the Tanna Kamma separates the two simanei tumah from one another. Suppose the man's hachlatah rested on pisayon, and black hair then appeared and purified him despite the spread, and after that the black hair disappeared. Now the pisayon reasserts itself and makes him tamei all over again. The spread never went anywhere, and its force returns with it.
The identical rule governs a nesek that spread, then shrank back, and afterward spread a second time. We do not argue that a nesek which once earned a ruling of taharah is beyond the reach of tumah. Wherever pisayon is the siman, whether the taharah arrived through black hair or through the retreat of the spread, the departure of the siman taharah brings the tumah back.
Rabbi Shimon ben Yehudah in the name of Rabbi Shimon
Against this stands the position of Rabbi Shimon ben Yehudah, quoting Rabbi Shimon: "kol nesek shetaher sha'ah achas" - any nesek that held a status of taharah even for one hour - "ein lo tumah l'olam," it is never again subject to tumah. He lays out the situations one by one: the hachlatah was by pisayon and the spread then drew back in; or the hachlatah was by pisayon and black hair arrived and purified it; or the hachlatah was by golden hair and black hair arrived and purified it, and the black hair subsequently left; or the area that had contracted opened up again and turned into a nesek a second time. In every one of these, this nesek is closed to tumah from then on.
The Rav stresses just how broad this is. Although the simanei tumah are physically present, golden hair or a spread among them, they have lost their power to render tamei. Nor is the rule confined to a nesek that was purified by black hair and left as it was. Even if the nesek spread after the black hair departed, and even if a fresh yellow hair was born within it, the nesek once tasted taharah and is therefore immune. Simanim that arise upon it afterward simply find no tumah to attach themselves to.
Rabbi Shimon
A third opinion cuts the principle down considerably. Rabbi Shimon restricts it to a single siman and to a single hair: "kol se'ar tsahov" - any golden hair - that once stood in a nesek at the hour of taharah, "ein lo tumah l'olam." The hair present in the nesek when the black hair came is the hair that will never convey tumah again, and nothing beyond it.
Consequently, should another golden hair be born in that nesek once the black hair has fallen away, or should the nesek spread at that later point, the man is tamei. Rabbi Shimon will not accept the broad claim that a nesek which held taharah for a moment is permanently sealed off from tumah.
The halachah
We rule with the Tanna Kamma. A man carried a nesek containing golden hair and was tamei; black hair was born inside it and he became tahor; and when that black hair later drops out he stays tahor, since, in the Rav's formulation, a yellow hair upon which taharah was already pronounced has no tumah left to give.
Yet the Rav has the Tanna Kamma treating the two simanim unequally. Where the hachlatah rested on pisayon, and black hair came and purified it, and then the black hair left, the pisayon that remains renders him tamei anew. The purified golden hair does not revert to tumah; the pisayon does. And a golden hair newly born after the departure of the black hair likewise makes him tamei.
It is worth noting that this is not the only way to learn the mishnah: the Tosfos Yom Tov brings the Rambam, whose explanation runs along different lines. We have kept to the Rav from beginning to end, and the halachah, as he decides it, follows the Tanna Kamma.