Negaim, Chapter 5, Mishnah 2. This mishnah lays down a broad principle about doubtful cases in nega'im, and then immediately marks off the exceptions to it. It opens with a sweeping statement: "Kol safek nega'im tahor," every safek in nega'im is tahor. Wherever we cannot resolve the facts, the man is not declared a metzora.
"Chutz mizeh ve'od acher": except for this case, and one more. The first exception is the case taught just before ours, where a bahares of half a gris was joined by a second half-gris in which two hairs turned white, and the mishnah ruled that even a safek about the order, whether the bahares preceded the se'ar lavan or the white hair preceded the bahares, leaves the man tamei. Our mishnah now adds the second exception and spells it out.
The case
"Ve'eizeh zeh?" And which is this one? "Mi shehayah bo bahares kegris vehisgirah": there appeared on a man a bahares measuring a gris, the smallest shiur that qualifies as a nega, and it contained no siman tumah at all, so the kohen shut him up for a week of hesger, quarantine. "Uvesof shavua veharei hee kesela": when the week is out and the kohen comes back to look, the white patch now measures a sela.
To appreciate the problem, keep the measurements in mind. A gris is a split bean, roughly the area of ten adashim, still a fairly small spot, something like a dime or a nickel. A sela is substantially larger, closer to a half dollar. It is unusual for a nega to spread that much in the course of a single week.
Two possibilities
Only two things can account for what the kohen is now looking at, and neither is quite ordinary. Perhaps the bahares of last week is still here and has grown, which is pisyon, spreading, a siman tumah, and the kohen would then pronounce the man muchlat. Or perhaps the first bahares vanished and a fresh nega arose where it had been, since a patch as wide as a sela normally surfaces already at its full measure rather than growing into it. On that reading nothing from the previous week attaches to it: no siman tumah has ever been seen on it, so the man would be shut up once more as a musgar and not declared muchlat.
In the words of the mishnah: "Safek shehee hee, safek she'acheres ba'ah tachteha, tamei." It is a doubt whether this is the very same bahares that has now spread, in which case he is certainly tamei and muchlat, or whether another nega came in its place, in which case he would only be musgar. And the mishnah rules that in this safek too he is tamei.
This, then, is the second of the two exceptions to the rule with which the mishnah opened. As a general matter every doubtful case in nega'im is resolved leniently and the man is tahor, but in these two cases, the safek about which came first, the bahares or the white hair, and the safek here about whether the patch before the kohen is the old nega spread or a new nega altogether, the halachah rules tamei.