Mikvaos, Chapter 9, Mishnah 6. This chapter deals with chatzitzah, a substance that interposes between the water of the mikveh and the item being immersed and thereby invalidates the tevilah. Having dealt with vessels and other articles, the Mishnah now asks about a garment: what happens when dirt is found on a beged that needs to be immersed?
The Mishnah rules: "Al habegadim mitzad echad, eino chotzetz." If the dirt sits on the garment from one side only, meaning it has not soaked through the fabric and there is not enough of it to penetrate to the other side, it is not a chatzitzah. "Mishnei tzedadim, chotzetz." But if it has worked its way through and is absorbed in the material so that it shows on both sides of the cloth, it does form a barrier and the tevilah is invalid.
A dissenting view follows. Rabi Yehuda reports "mishum Rabi Yishmael," in the name of Rabi Yishmael, that "af mitzad echad," soiling visible on a single surface, already interposes. His reasoning: nobody is content to leave any grime whatsoever on his clothes, and since a person objects to it, that objection is what turns even surface dirt into a barrier to the water.
"Rabi Yose omer" introduces a third view, one that draws a line between one owner of a garment and another. On the clothing of the "banayim," the dirt interposes "mitzad echad," from a single side alone. Banayim refers to talmidei chachamim, and the title traces back to the description of Torah scholars as "bonayich," those who are the real builders of the world. Our Sages taught that a scholar's garment must be free of any stain, and consequently even soiling that never soaked through the cloth is something he takes to heart, so it stands between him and the mikveh water.
"Veshel bor," but the garment of an unlearned man, who is not exacting about his appearance, is only rendered invalid for tevilah when the dirt is "mishnei tzedadim," showing on both sides of the cloth.
Once again the halachah of chatzitzah is measured by hakpadah, by whether the owner of the garment is bothered by what clings to it, and Rabi Yose teaches that the standard of a talmid chacham in his dress is itself a halachic reality with consequences for his tevilah.