Mikvaos, Chapter 9, Mishnah 7. This chapter is devoted to the subject of chatzitzah, the substance that clings to a person or to a garment and interposes between it and the water of the mikveh. Our mishnah continues that theme with regard to begadim, articles of clothing that themselves require tevilah. Something has become stuck to the garment, and the question is whether that substance blocks the water from reaching the cloth and invalidates the tevilah.
The mishnah lists three kinds of working garments. "Matpachos shel zafasin": the aprons worn by those who coat barrels with tar. In order to waterproof a barrel, a man would smear it over with pitch, and while he worked he wore an apron. "Veshel yotzrin": the aprons of potters, who fashion vessels out of clay. "Veshel mafsolei ilanos": the aprons of those who prune and trim trees. In all three cases the apron is worn for one purpose, to shield the man's real clothing from the mess of his trade.
Concerning such aprons the mishnah rules "ein choitzin", they do not interpose. The dirt that has collected on them is not a chatzitzah. Here we are speaking of a mi'ut, a substance covering only a small part of the garment, and the reason it does not interpose is that the owner of such an apron is simply not makpid. He has no objection whatsoever to the stains on it, because that is precisely what the garment is there for. And even where the wearer is a talmid chacham, about whom we have learned that he is particular about a spot on his clothing, in this case even he is not particular. The apron is meant to absorb the dirt.
"Rebbi Yehuda omer: af shel kayotzin keyotzei bahen." Rebbi Yehuda adds the aprons of those who work among thorns, garments worn so that the thorns should catch on them rather than tear the clothing underneath. These, he says, have the same halachah.
What is Rebbi Yehuda coming to add, and where does he part company with the Tanna Kamma? The mefarshim explain the reasoning. This apron exists in order to be torn in place of the garment beneath it. Now, if a man is prepared to let his apron be ripped and shredded and raises no objection, then all the more so he raises no objection to a bit of dirt settling on it, and there is no hakpadah at all. The Tanna Kamma, however, does not accept this reasoning. In his view one can be reconciled to the garment being torn, since that is the very function it serves, and still be particular that it not become soiled. Therefore the Tanna Kamma did not include the aprons of the thorn workers in his list.
The mishnah closes by drawing everything together into a principle: "Zeh klal: kol hamakpid alav choitzetz, veshe'eino makpid alav eino choitzetz." Any substance about which a person is makpid, that is, he wants it off his garment and would remove it, interposes and invalidates the tevilah. And a substance about which he is not particular, in a case such as ours where it is a mi'ut about which the owner has no hakpadah and the chatzitzah would only be miderabbanan, does not interpose, and the tevilah is valid.
The lesson of the mishnah is that chatzitzah is not measured by the eye alone but by the attitude of the owner. The very same tar stain that would render one man's cloak unfit leaves the tarrer's apron perfectly fit for tevilah, because to him it is not a blemish at all.