Negaim, Chapter 4, Mishnah 4. The subject of this perek is the simanei tumah of tzaraas: the marks that turn a questionable white spot on the skin into a genuine nega. Three such marks exist. Seir lavan means a pair of hairs inside the spot that have gone white. Pisyon means that the spot has grown larger, whether upon the completion of one period of hesger or upon the completion of the second. Michyah means a bit of ordinary living flesh surrounded by the whiteness. The perek pairs these three off, two at a time, to show in what respect each one is heavier than the sign it is being compared with. Seir lavan was weighed against pisyon; michyah was weighed against pisyon; and the mishnah before us closes the circle by weighing seir lavan against michyah.
A word about vocabulary before we start. Our mishnah says baheres, but the term is serving here as a convenient label. The Torah names four grades of white capable of forming a nega (se'eis, baheres, and the fainter secondary shade of each, called sapachas), and everything stated here holds equally for all of them. So wherever the mishnah says baheres, understand: the whitened area, in whichever of the four shades it happens to present.
Two lists, one in each direction
"Yeish b'seir lavan mah she'ein b'michyah, v'yeish b'michyah mah she'ein b'seir lavan." The white hair carries chumros that the living flesh lacks, and the living flesh carries chumros that the white hair lacks. Each of the two signs owns a domain in which it outweighs its counterpart.
The advantages of seir lavan
"Sh'seir lavan metamei bish'chin uv'michvah." A shchin is flesh laid raw by a boil or a bruise; a michvah is flesh laid raw by fire. Each of these is its own class of nega with its own halachos, and in such a place a white hair is effective: the person is pronounced tamei immediately, a full hechlet, with no week of hesger required.
"Umetamei mechunas umfuzar." Whether the two hairs are bunched together in one spot or lie far apart at opposite ends of the whiteness, they render tamei. How they are positioned relative to each other makes no difference whatsoever.
"Bimvutzar veshelo bimvutzar." So too it makes no difference whether the hairs stand deep inside the patch or close to its border. And here is exactly where michyah is the more exacting of the two, since a michyah counts only when it lies in the interior of the white.
The advantages of michyah
"Shehamichyah metamei bekerachas uvegabachas." Kerachas and gabachas are negaim of a bald head: the discoloration at the rear of the scalp and the discoloration at the front. In such a place, living flesh showing in the center of the discoloration constitutes a complete siman tumah. A white hair has no role there at all; on a bald head it does not function as a sign.
"Hafuchah veshelo hafuchah." The basis of this clause is the Torah's own wording, "hafach lavan," that the hair turned white. The premise underlying the siman is that the nega is what bleached the hair: the whiteness was present first, and under its influence a dark hair lost its color. Sequence therefore counts. Should the two white hairs have been there already, with the white patch forming around them only afterward, nothing was turned white by any nega; those hairs merely happen to be located there, and the man remains tahor. Michyah imposes no such condition. Living flesh that was present before the whiteness spread around it and beneath it, and living flesh that emerged within a patch already in place, are equally a siman tumah, provided the usual requirements of michyah are met: that it be square in shape, positioned in the interior of the white, and one unbroken piece of flesh rather than an assortment of tiny specks.
"Um'akeves es hahofeich kulo lavan." There is a principle that a man over whose whole body, head to toe, the whiteness has spread is tahor. White hairs scattered throughout that whiteness do nothing to obstruct such taharah. Michyah does obstruct it. A single piece of healthy flesh the size of an adashah, a lentil, situated anywhere on him, prevents the purity that a wholly whitened body would otherwise bring.
"Umetamei bechol mareh, mah she'ein kein beseir lavan." Earlier in the perek we saw that seir lavan surpasses pisyon in being metamei bechol mareh lavan, in any shade of white, including a white fainter than k'rum beitzah, the dimmest of the four shades listed in the opening perek. Michyah reaches further still. Michyah is metamei inside a discoloration of absolutely any color. Whatever hue the affected skin has taken on, living flesh appearing in its middle produces tumah. Seir lavan, by contrast, operates only in skin that is white, understood broadly enough to include whites fainter than the four, but white all the same.
The mishnah has thus laid out the account on both sides: the white hair extends to shchin and michvah, and it disregards both the arrangement of the hairs and their place within the patch; the living flesh extends to kerachas and gabachas, disregards which of the two appeared first, holds back the taharah of a body covered entirely in white, and functions within a discoloration of whatever color it may be.