Negaim, Chapter 4, Mishnah 5. Our chapter is devoted to the simanei tumas tzaraas, the three signs that transform a white blotch on the skin into a nega: seir lavan, a pair of hairs inside the blotch that have turned white; pisyon, an expansion of the blotch beyond its original borders; and michyah, a pocket of ordinary flesh surrounded by the white. After weighing the three against each other, the Mishnah now turns inward and asks what the first of them actually consists of: when do we say that a hair is a white hair?
Where on the hair does the siman lie?
"Shtei sa'aros ikkaran mashchir veroshan malbin, tahor." Take two hairs whose lower ends, at the skin, are still black, while their upper ends have gone white: this man is tahor. "Ikkaran malbin veroshan mashchir, tamei." Reverse it, so that the whiteness sits at the bottom and the black at the top, and he is tamei. Everything is measured at the base of the hair, the spot where it pushes out of the afflicted skin, because that is the place where the nega's power is felt. Pale ends attached to a black base testify to nothing at all.
How much white does the hair need?
"Kamah yehei belavnonis?" What quantity of white is called for? For Rebbi Meir it is "kol shehu": the barest trace suffices, as long as the eye can detect it. Rebbi Shimon holds "kedei likrotz bezog": there must be enough white to be gripped and clipped off by a scissors, and that is the standard measure given for the length of a hair.
We rule like Rebbi Meir. The core of the argument seems to lie here. In Rebbi Shimon's view, the demand belongs to the hair itself: since the siman is called a white hair, the white section has to possess the proportions of a hair. In Rebbi Meir's view, the demand belongs to the color: the hair already has a hair's proportions, and all that remains is for whiteness to be discernible within it, and with that it has become a seir lavan.
The Rav states this simply. There is indeed a shiur for the hair that has whitened, namely its length, which must be at least what a scissors can catch and cut at its blades. That is a measure of the hair. The lavnonis, though, the white coloring that confers the standing of a white hair, has no quantity attached to it other than being visible. Once the strand is long enough, even a faint speck of white showing in that black hair makes it a seir lavan. And the siman is never a single hair: two of them are always required.
A single hair that presents itself as a pair
"Haysah achas milmatah venechlekah milmalah veniros keshtayim, tahor." One hair emerged from the skin and then parted into two at its upper portion, in the manner of the spine of a lulav that separates, so that the eye takes it for two hairs. He is tahor. The siman is not defined by two strands appearing before us; the Torah asks for two hairs, and here we have one hair that happened to split.
Black hairs growing inside the white
"Baheres uvo seir lavan" continues the Mishnah, and at this point the girsaos part ways, each yielding its own lesson.
The Rav's text reads "veseir shachor": a white patch holding the two white hairs, with black hairs sprouting in it as well, and the verdict is tamei. What we learn is that those black hairs do not spoil the patch by taking up room within it; the gris that the halachah demands is still fully there.
The Rosh, whose position is brought by the Tosefos Yom Tov, keeps the reading familiar to us, "o seir shachor," and finds in the Mishnah two distinct rulings. Should the patch hold white hairs, the kohen pronounces him a confirmed metzora, hechlet. Should there be no white hairs and only a black one, the kohen assigns him hesger, leaving his status pending until the examination at the close of the second week resolves it. On either ruling the subject is tumah, once as a settled fact and once as a suspended one.
The Mishnah concludes: "Ein choshshin," we harbor no worry, "shema mi'et," lest it have reduced, "mekom seir shachor es habaheres, mipnei she'ein bo mamash." The area taken up by the black hair is not suspected of cutting the baheres down below its shiur. The point sits particularly well with the Rav's girsa, yet it holds on both readings: black hairs standing in the midst of the white are never counted as swallowing space and shrinking the gris. A hair carries no real bulk; it is simply too slender to be reckoned as filling any area.