Negaim, Chapter 5, Mishnah 5. This perek is occupied with sofek nega'im, cases in which the facts of a nega cannot be established, and with the rule that in such a situation the person is left tohor. Before our mishnah lays down its own principle, it is worth recalling the two exceptions to that rule which this perek has already presented.
The first is the well known case with which the preceding perek concludes: a beheres containing a sei'ar lavan (a white hair), where the order in which the two appeared can no longer be established. Did the patch arise first, "beheres kadmah lesei'ar lavan", or did the hair turn white before the patch appeared, "sei'ar lavan kadmah labeheres"? Only the first sequence produces tumah. The Mishnah nevertheless issues a verdict of tomei. The Gemara in Bava Metzia recounts that Hakadosh Baruch Hu Himself declared such a doubt tohor, and yet the Tanna Kama of our Mishnah, along with the Rambam in his Yad Hachazakah, hold that the man is tomei.
The second exception is stated in the mishnah that opens our own perek, which likewise set out the rule together with its exclusions: "Kol sofek nega'im tohor", every questionable nega leaves its owner tohor, "chutz mizeh ve'od acher", apart from the case just described and one additional case. That additional case is this: a man whose beheres measured a gris was put into hesger, so that the patch could be observed for seven days, and when the kohen came back, what stood before him was a patch as large as a sela. Two possibilities now compete. "Sofek shehi hi": perhaps this is the original gris, which spread until it reached the dimensions of a sela. "Sofek she'acheres ba'asah tachtav": or perhaps a second beheres took the place of the first. Growth from a gris to a sela within a single week is unusual; on the other hand, a fresh outbreak of one of the arba'ah mar'os could easily have surfaced during those seven days already measuring a gris. We therefore cannot determine whether a piso'yon lies before us, which would require hachlatah, or a new nega, which would require a new week of hesger. The Mishnah resolves this doubt in the direction of tumah, and the man is pronounced muchlat.
The principle of our mishnah
"Kol sofek nega'im b'tchilah tohor, ad shelo nizkak letum'ah." Every doubt in nega'im at the outset, before the person has become bound up with tumah, that is, before there has been any hachlatah upon him, is resolved as tohor. "Mishenizkak letum'ah, sfeiko tomei." Once he has been declared a metzora muchlat, any doubt that arises afterwards regarding his status is resolved as tomei.
"Keitzad?" How so? "Shnayim sheba'u eitzel kohen": two people presented themselves to a kohen, "bizeh beheres kegris uvizeh kesela", one of them carrying a beheres of a gris, which is the smallest measure that qualifies as a nega, and the other a patch as large as a sela, a considerably bigger measure. Both were placed in hesger. "Besof shavu'a", when the seven days were up, "bizeh kesela uvizeh kesela ve'ein yodei'a be'eizeh meihen posah": each of them now displayed a sela-sized patch, and there is no telling in which of the two the nega had spread. The kohen has lost track, and in all likelihood the two men themselves can no longer say which of them was which. Consider what ought to have followed. The one who arrived with a gris and now shows a sela has an unmistakable piso'yon, and his week of observation should close with hachlatah. His companion, whose patch measured a sela from the outset and remains exactly a sela, has shown no spreading at all and needs nothing beyond a second week in hesger. But since the two can no longer be told apart, no declaration can be issued for either of them.
"Bein be'ish echad, bein bishnei anashim, tohor." The same verdict holds not only where two separate people are involved, but even where a single person carried both patches, a gris in one place and a sela in another, and after the seven days both measure a sela. Here too one patch has definitely spread while the other definitely has not, and the man's status is tohor.
Why even one man is tohor
Why even a single person is tohor
From piso'yon to the other simanim: the proof of the Rosh
The Tosefos Yom Tov cites the Rosh, whose perush is printed alongside the Mishnayos. The Rosh observes: "de'af al gav dehai kro", although this posuk was formulated "bepiso'yon ksiv", with reference to spreading and not to the other simanei nega, "hu hadin besei'ar lavan", a white hair is governed by precisely the same law. His proof is drawn from the Tosefta. A beheres measuring half a gris holds two hairs, and on its own it is not yet a nega; then "nolad beheres kechatzi gris", a further half gris of beheres came into being beside it, "uvo shtei se'aros", containing two hairs of its own, so that a complete gris with hairs now stands there (the correction of the Tosefos Yom Tov directs this to the Tosefta in the second perek). "Harei zeh lehachlit": such a case calls for hachlatah. Yet if the kohen was "to'ah ve'eino yodei'a", mistaken and unable to establish "eilu shehafchu ve'eilu shekadmu", which hairs whitened once the patch had appeared and which of them were already there beforehand, the man remains tohor, "detzarich sheyeida", for he is obliged to know "be'eizeh se'aros hu machlito", on account of which hairs he is issuing the declaration. So far the Rosh. The Tosefos Yom Tov adds, "vechein he'etik haRambam", that the Rambam likewise recorded this halachah, at the close of the seventh perek of Hilchos Tumas Tzora'as. The Rav, then, derives the halachah from the posuk about spreading; the Rosh applies it equally to sei'ar lavan, where the uncertainty concerns which came first, the hair or the patch; and the Tosefos Yom Tov concludes with the observation "venir'eh im kein", that presumably "dehu hadin nami bemihyah", the very same rule governs mihyah. Should a comparable doubt arise between two areas of healthy flesh, with no way of knowing which is which, no hachlatah can be pronounced.
Rebbi Akiva
The Tanna Kama declared tohor in both situations: the single man bearing two patches, one of which certainly spread, and the pair of men, one of whom certainly requires nothing more than a further week of hesger. "Rebbi Akiva omer": Rebbi Akiva draws a distinction between them. "Be'ish echad tomei, uvishnei anashim tohor." Where a single body is involved, the certainty that a nega with a piso'yon is present on that body is sufficient to render the man tomei. Where two bodies are involved, and one of them is definitely tohor, neither of them may be pronounced tomei. The Rav points out that the halachah does not follow Rebbi Akiva; we rule with the Tanna Kama, and in both situations the men are left tohor.