Mikvaos, Chapter 9, Mishnah 3. This chapter is devoted to the laws of chatzitza, the substances that come between the water of the mikvah and the body of the person immersing. For a tevilah to be valid, the water must reach every part of the body; anything clinging to the skin or hair that blocks the water can invalidate the immersion. Our mishnah continues the chapter's list, turning now to items that do not constitute a barrier.
The mishnah opens: "Eilu she'ein chotzitzin" - these are the things that do not interpose. It then lists three cases: "kilkulei harosh", hair on the head that has become knotted and tangled; "u'veis hashechi", the hair of the underarm; "u'veis hasesarim", the hair of the private areas. And the mishnah adds one crucial word: "b'ish" - in a man.
Why should tangled hair be different from any other clinging matter? The governing principle throughout the laws of chatzitza is hakpada, a person's insistence on removing something. Matter that a person minds and would want gone is treated as foreign to the body and blocks the water. Matter that a person simply lives with, and never troubles himself to remove, is not regarded as a separate substance standing between him and the mikvah at all. Tangled hair, and hair growth in the underarm and in the concealed areas, are things a man ordinarily pays no attention to. Since there is no hakpada, there is no chatzitza.
What, then, of an individual man who does happen to be particular about these things? The Tanna Kamma's ruling still stands, and his immersion is valid. Because the general run of people are not particular here, this one man's fastidiousness is set aside: batla da'ato etzel kol adam, his personal view is nullified before the practice of everyone else. Hakpada is measured by the standard of people in general, not by the sensitivities of a single person.
Rabbi Eliezer disagrees: "echad ish v'echad isha, kol hamakpid alav chotzeitz, v'she'eino makpid alav eino chotzeitz." He draws no distinction between a man and a woman. For him the entire question is decided by the individual standing in the water: whatever this person is particular about interposes, and whatever he is not particular about does not interpose. Rabbi Eliezer's position is the stricter one, since he refuses to let the general practice override the particular person's own hakpada, and he judges each case by the individual.
The mishnah thus leaves us with the two ways of measuring hakpada: the collective standard of people at large, by which a man's unruly or hidden hair is never a barrier, and Rabbi Eliezer's personal standard, by which every person is bound by his own particularity, man and woman alike.