Negaim, Chapter 3, Mishnah 1. Until now the Mishnah has been occupied with the conditions under which a Kohen examines a nega: the light, the place, the posture of the person being examined. Now it turns to the examiner himself and asks whose nega he may rule upon, and from there it goes on to two parallel halachos in entirely different areas of Torah.
Chutz Minig'ei Atzmo
The Mishnah's opening ruling: "Kol hanega'im adam ro'eh," every nega falls within a person's competence to inspect, "chutz minig'ei atzmo," apart from the ones appearing on his own skin. Someone else's affliction he is fit to examine and pasken on, and that includes members of his own family; a nega on his own flesh he may not rule on at all, and having other people present with him does not change that.
This line has had a long life outside the beis midrash as well. People often quote it as a wry comment on human nature: a man sees every blemish in everyone around him, chutz minig'ei atzmo, except the blemishes that are his own. It is a beautiful play on the words, and worth remembering, but the Mishnah itself is speaking pure halacha about the examination of tzara'as.
"Rebbi Meir omer": Rebbi Meir broadens the exclusion to cover his family as well, "af lo nig'ei kerovav." The basis is a hekesh drawn from the pasuk "lechol riv ulechol nega," which sets quarrels and nega'im beside one another. Since a man is disqualified from judging his relatives when the matter is a monetary riv, that same disqualification carries over to deciding a relative's nega.
Nedarim: A Man Does Not Release His Own Vow
The Mishnah now attaches two further halachos cut from the same cloth: a role a person fills on behalf of other people while being barred from filling it on his own behalf. The first is nedarim: "Kol hanedarim adam matir," every vow lies within a person's power to dissolve, "chutz minidrei atzmo," other than the vows he took upon himself.
A word on hataras nedarim. By Torah law a single mumcheh, a true expert holding semichah, has the power to dissolve a neder by himself. Where no such expert is to be found, and nobody today carries that rank, three men substitute for him. Those three are not a beis din, and a good deal follows from that: their session may be convened at night; a neder that bears on Shabbos may be dissolved on Shabbos itself; and relatives are free to sit together, a father alongside his son, or two brothers, as long as the vow under discussion is not theirs. Our Mishnah accordingly teaches that even the expert who is empowered to dissolve vows has no power over his own, and that a man seated as one of the three cannot use that seat to undo a neder of his.
An aside that fits here. I heard Rabbi Soloveitchik zt"l mention in shiur that he had obligated himself to finish a certain quantity of material, and he treated an undertaking of that sort as belonging to the category of a vow to perform a mitzvah or to give tzedakah, which requires a proper hatarah. When the quota went unfinished, he took pains to locate three men he considered genuinely competent, men who grasped what they were engaged in. The lesson is obvious. Rounding up whichever three fellows happen to be standing nearby on Erev Rosh Hashanah is not how hataras nedarim ought to be done. One should seek out people who understand what charatah, regret, is, and what a pesach, an opening, is.
His Wife's Nedarim
"Rebbi Yehuda omer": his addition concerns "af lo nidrei ishto," a wife's vows, specifically those "shebeino levein acheirim." A husband does have a mechanism of his own where his wife's vows are concerned, hafaras nedarim, but its reach is limited: it extends to vows bearing on the relationship between the two of them, beino leveinah, and to vows of inuy nefesh, which inflict affliction on the wife herself (and that too touches the marriage, seeing that she is in distress). Rebbi Yehuda is dealing with the remaining class, vows that hafarah cannot reach and that can only be undone through ordinary hataras nedarim.
The Rav explains: kegon nedarim she'einam shel inuy nefesh udevarim she'einam beino leveinah, shehein tzrichim chakiras chacham. Such vows need a chacham to examine them, and there we are concerned: shema yakel gabei ishto velo yevakeir yafeh, that with his own wife he will lean toward leniency and will not investigate as carefully as he should, since his wife is like himself. Therefore even a yachid mumcheh, an expert qualified to act alone, may not release his wife's vow by himself. But the Rav holds that he may join a panel of three: aval mitztareif hu bichlal shelosha, since once there are three sitting together, we do not suspect that all of them will fail to examine the matter properly, and they will arrive at an honest ruling as to whether the neder can be released.
Bechoros
"Kol habechoros adam ro'eh," every firstborn animal is his to examine, "chutz mibechoros atzmo," excluding the firstborn animals belonging to him. While the Beis Hamikdash stood, the firstborn of a kosher animal was handed over to a Kohen and brought as a korban. With no Mikdash the kedushah of a bechor has not lapsed, and the arrangement farmers commonly depend on nowadays is to sell a nochri a share in the animal, since a nochri's partnership takes it out of the law of bechorah. Suppose, however, that this was never arranged, and a Kohen finds himself holding a bechor. As long as it is unblemished he may not eat it, and there is no Mikdash in which to offer it, so he waits for the animal to develop a lasting mum. Once that blemish is there the animal becomes permitted as food, and it is exactly this inspection that our Mishnah addresses: he may examine the mum on other people's firstborn animals and declare them fit to eat, but not the mum on his own.
The Tosfos Yom Tov points out that the Gemara in Bechoros, at the close of the fourth perek, establishes that the case here is a yachid mumcheh, an expert holding semichah who is authorized to permit bechoros on his own. His own animal is beyond his authority. Should two others sit with him, however, "de'i bitlasa lo chashidi," precisely as the Rav explained by hataras nedarim, a group of three is above suspicion. The Tosfos Yom Tov adduces a proof from the end of the second perek of Yevamos: a woman who carried out mi'un or chalitzah before a dayan is permitted to marry him afterward, "mipnei shehu beis din," for when the ruling issued from a court no one suspects that he arranged matters for his own benefit. The same applies here: a man may examine his own bechor as one of three, just as a husband may participate in dissolving his wife's neder as one of three, even though those three are not a court at all but merely stand in for a qualified expert.
Three separate areas of halacha, one thread running through them all: a person's own interest clouds his eye, and the Torah removes him from the ruling, unless others are sitting with him to keep the judgment honest.