Niddah, Chapter 2, Mishnah 2. The subject of this chapter is bedikos, the internal examinations through which a woman clarifies whether she stands in a state of taharah or of tumah. The chapter's first ruling states a sweeping rule about such checking: "Kol hayad hamarbeh livdok," a hand that busies itself with repeated examination, is "banashim meshubachas" but "uva'anashim tikatzeitz" (in women it earns praise, in men it deserves to be severed). Why the difference? Every extra examination a woman makes is pure benefit to her: she establishes anew that the taharos she works with have not been compromised, and that diligence is called meshubachas. A man who would repeatedly inspect himself for shichvas zera gains nothing of the sort. The handling itself arouses him, and the Mishnah therefore speaks of such behavior in the harshest terms. From this principle our mishnah moves to two concrete matters connected with bedikah: those women who are incapable of examining themselves, and the cloths that accompany marital relations.
"Hachareshes, vehashotah, vehasuma, veshenitrefah da'atah." Four types of women appear here, all of whom lack the capacity to conduct a trustworthy examination unaided. The first can neither hear nor speak. The second lacks sound judgment. The third has no vision, so that even if the cloth is properly inserted she has no way of inspecting it afterward. And the fourth is a woman whose faculties have become deranged. Since the second and the fourth both describe impairment of the mind, the Gemara asks what the precise difference between them is.
"Im yesh lahen pikchos, maskinos osan vehen ochlos bitrumah." If these women have with them other women who are lucid and competent, those women attend to them and perform the bedikah on their behalf. Once such an examination has been carried out for them, they are considered checked, and they may eat terumah. In other words, the inability to examine oneself does not automatically leave a woman in a state of doubt; the bedikah may be done for her by someone capable of doing it properly.
The Mishnah then continues with a halachah that follows directly upon the teaching at the close of the previous chapter, that a woman who handles taharos performs a bedikah before relations with her husband. "Derech bnos Yisrael meshamsos bishnei edim, echad lo ve'echad lah." The practice of Jewish women is to use two cloths, called edim, witnesses, in connection with marital relations. One is for the husband, to wipe himself afterward, and one is for the wife, to wipe herself afterward. Each cloth is then available as evidence should blood be found on it.
"Hatznuos maskinos lahen shlishi lesaken es habayis." Women of unusual modesty and care ready a third cloth in addition to the two, and with it they check themselves internally before relations. What does this third cloth accomplish? Its purpose emerges where husband and wife are together more than once over the course of a single night: such a woman renews her bedikah before each occasion. The standard conduct called derech bnos Yisrael already provides for a check afterward, alongside the examination beforehand that we have been taught. The added care of the tznuos lies in inserting a fresh examination in between, so that she does not rest on one check when the night begins and another when it ends, but rather confirms her status anew ahead of every act.
So this mishnah teaches us two sides of the same value. A woman who cannot examine herself is not simply left in doubt, for a competent woman checks on her behalf and she may then eat terumah. And a woman who can examine herself is encouraged to do so generously, the modest ones adding a cloth beyond what practice requires, because in the realm of taharah every additional bedikah is a step toward clarity.