One indication that a nega is tameh (siman) is if the nega spreads. The fourth perek discusses cases where the nega's size fluctuated. One case is where the nega shrunk and then grew back to its original size. R' Akiva maintains that the nega is tameh while the Chachamim disagree.
The Bartenura explains that the argument is whether the return back to the original size can be considered "spreading". R' Akiva maintains that since it spread, the nega should be treated as tameh. The Chachamim however reason that had the nega been that size the entire time it would be tahor. Consequently returning to that size should also be.
The Rambam (Hilchot Tumat Tzaraat 4:9) rules like the Chachamim, understanding that the discussion is about a nega that was after the petur. In other words, a nega had been through the two weeks of quarantine, had not developed any siman and the kohen deemed it as being tahor.
The Raavad comments, that the position of the Chachamim only makes sense if we are dealing with a nega that was originally a gris (half bean) -- the minimum size of a nega -- and shrank to less than a gris. In other words, once it shrank, what remains is not considered a nega. If however we are dealing with a nega that never shrank to less than the size of a gris then the Chachamim would rule that the nega is tameh.
The Ravaad's proof comes from a later Mishnah (9:3) that discusses a nega that filled a shchin (boil) on the inside of the hand. Since there is nowhere for it to spread and it is in a place that does not grow hairs, it appears that no siman can develop. Does the nega require quarantine? One suggestion is that it would, since it might shrink and the spread again, which would be an indication of tumah. The Mishnah continues by asking what the ruling would be if it was the minimum size. We will learn in time the Mishnah's conclusion. Nevertheless the Raavad understands that it is clear that if it was always larger than a gris and spread back, the Chachamim would agree that the nega would be tameh.
The Ohr Sameach however differentiates between the two cases. In the later Mishnah, the concern is that by the end of quarantine, the nega would be smaller than the original size and the kohen would rule that the nega is tahor. If it spread after that, then the Chachamim would agree that it is a siman tumah. The case in our Mishnah, is if the changes in size all occurred after the kohen ruled the nega was tahor. Since the fluctuations result in the nega returning to the size that the kohen deemed as tahor, the Chachamim maintain that the nega is tahor. Why would this be the case? The Chazon Ish (Negaim 3:27) explains that a reduction that has not been assessed by a kohen is not considered anything. (This would be similar to fluctuations in size during quarantine, in between inspections by the kohen.)
The Chazon Ish presents a question on the Raavad. If the case is where the nega reduced to less than a gris and then spread back, it should require quarantine. When the nega becomes less a gris it is tahor. Once it grows back it should be considered a new nega. He argues that we cannot suggest that since it shrunk to less than a gris, what remains is permanently tahor, such that that when it grows back, the new part is too small to be considered a nega on its own. The Chazon Ish cites the later Mishnah (5:3) as proof that addresses the case of a se'ar pekudah. The Mishnah rules that if a nega had two white hairs (a siman tumah) then shrunk to less than a gris with the two white hairs inside it, then grew back to a gris, the nega is still tahor. This is because the two white hairs, did not appear in this newly formed nega, but rather in an old nega that became tahor. R' Akiva however argues that the two hairs are an indication of tumah. The Mishnah implies however that if two new white hairs formed in the nega everyone would agree that it would be tameh. This implies that when it spread back, we do not discount the part that was present when it shrunk below a gris. Note that according to this understanding, the nega would not be tahor completely, as the Mishnah suggests, but would be considered a new nega requiring quarantine.
Perhaps we can defend the Raavad based on the Mishnah Achrona. He understands that once the nega is less than a gris it is considered nothing. Consequently, when the Chachamim state that the nega is tahor it is completely tahor -- we disregard the small amount that previously remained. Why then does R' Akiva argue? The Mishnah Achrona cites the Raavad (!) that R' Akiva understands that the presence of the hairs is an indication that despite being smaller than a gris, the nega has not healed.
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