Negaim, Chapter 7, Mishnah 3. This perek is occupied with beharos that lie beyond the grasp of tumah, and the Mishnah before us raises a simple yet weighty issue: suppose the nega's color does not stay fixed, but shifts while the process is still under way. The Mishnah begins, "Nishtanu mareihen bein lehokel bein lehachmir": their appearances changed, in the direction of leniency or in the direction of stringency.
The Four Shades
Back in the opening Mishnah of the first perek we were taught that the maros nego'im form a scale of brightness. At the top stands kasheleg, the white of snow. Beneath it is kesid haheichal, matching the plaster of the Heichal. Next comes ketzemer lovon, the shade of white wool, meaning the fleece of a newborn lamb after it has been rinsed. Faintest of the four is uvechrum beitzah, the shade of the thin skin inside an eggshell. The question here is what to do when a nega that presented as one of these shades now presents as another, whether it has dropped to a fainter shade, which is a move lehokel, or climbed to a stronger one, which is a move lehachmir.
Keitzad Lehokel
"Hoysah kasheleg vena'asis kesid haheichal, ketzemer lovon, uvechrum beitzah": the spot began at the top of the scale, snow white, and then appeared instead as the plaster of the Heichal, or as white wool, or as the membrane of an egg. The Mishnah's own words "na'aseis mispachas se'eis o mispachas azah" describe the same thing: a se'eis that now shows as the toldah belonging to it, one rung lower, or the strong baheres that now shows as its toldah. In every one of these instances the whiteness has weakened, and that is what the Mishnah calls lehokel.
Keitzad Lehachmir
The opposite case is described next: "Hoysah kechrum beitzah vena'aseis ketzemer lovon, kesid haheichal o kasheleg" - the spot started out at the faintest end of the scale, like an egg's membrane, and then intensified, showing as white wool, or as the plaster of the Heichal, or as snow itself.
The Three Opinions
Reb Elozor ben Azaryah metaher. As he sees it, once the shade has moved at all, the beharis under examination is not the one we began with, and that very shift renders the person tohor.
Reb Elozor Chismah divides the cases. Where the change went lehokel and the nega faded, the ruling is tohor. Where the change went lehachmir and the nega strengthened, however, teira'eh bat'chilah: the spot is looked at as though it had only now appeared.
Reb Akiva omer: bein lehokel bein lehachmir, teira'eh bat'chilah. Whether the beharis slipped from a stronger white to a weaker one within the four maros, or rose from a weaker white to a stronger one, both directions are handled the same way, as a nega that is starting its course now.
The Halachah
We rule like Reb Akiva. Given that a beharis is kept under observation through as much as two weeks of hesger, a change of shade in the interim is hardly unusual. It carries no consequence whether the shift was lehokel, the white growing weaker, or lehachmir, the white growing stronger: the change does not render the nega tomei, yet neither may we simply resume the count already in progress. The nega is treated as an entirely fresh one, teira'eh bat'chilah, beginning again from the start. The kohen imposes hesger anew, and when that first week is up we check whether a pisyon has appeared or any other siman tumah has emerged.