Don't Cut It

השמר בנגע הצרעת לשמר מאד ולעשות ככל אשר יורו אתכם הכהנים הלוים כאשר צויתם תשמרו לעשות (Ki Teitzey 24:8.)  The Gemirah in Shabbos (132b) is clear that this not only applies to tamah negaim but to tahor negaim as well.  Rashi identifies the tahor nega as the בהק and one who the nega covered their entire body.  The Tosfot Rid disagrees with Rashi's mentioning of בהק because he argues that is not called a nega at all and the case of when the nega spread over the whole person also doesn't make sense for the person is not going to skin himself.  Instead he learns the Gemorah as referring to a case of a nega that stood for two weeks that was susceptible to tumah if it developed signs of tumah on such a nega there is an issur to cut it off.  This is the opinion of Tosfot Shavuot (4a) as well.  We see Rashi holds that the בהק is considered a nega and that the issur of cutting off the nega has nothing to do with attempting to rid one's self of tumah.  The Rashba in Shabbos (133a) asks why is cutting off the nega to do a milah considered a דבר שאינו מתכוין just because the person's intent is merely to do the mitvah, the person still has full intent to do the act of cutting?  He answers that the issur of cutting the naga is not to do the act of cutting it off, the issur is to try to cause taharah without the proclamation of the kohan as the possuk says ככל אשר יורו אתכם הכהנים הלוים.  Since the person is not trying to cause tahara, rather to do the mitzvah, it is a דבר שאינו מתכוין.  The Sfas Emes asks that the aforementioned Gemorah contradicts this for the Gemorah says the issur applies to a tahor nega as well?  The Rashba must have learnt like the Rid and Tosfos and since there was a possibility of tumah it is included in the issur of trying to be metahar the nega.  This approach is supported by the Gemorah Shabbos (94b) that says if there are three white hairs in the nega and one cuts off one of them there is no lashes because the nega is still tamah because of the two hairs.  We see the issur is to be metahar the nega.  (See Mishne L'melech Tumas Tzaaras 10:1.)  A נ"מ between Rashi and the other Rishonim may be on a nega that is less thabn the size required for it to become tamah, less than a גריס if there will be an issur in cutting it off.  According to Rashi yes, for it is still a nega but according to the other Rishonim no for there is on possibility of tumah (see Igrot Moshe back of volume 1 Kodshim siman 25.) 

The Gemorah Arachin (3a) has a רבוי to include a katan that will have tumas negaim.  The Kli Chemda in Lech Lecha brings a question from the Kutna Rav according to the הו"א that a katan does not have tumas negaim why would we need a possuk to tell us that you can cut off the nega of a katan to do milah, there is on tumas negaim on a katan?  His question assumes like the approach of the Rishonim not like Rashi