Literary critic Adam Kirsch is reading a page of Talmud a day, along with Jews around the world.This week, Daf Yomi readers entered the home stretch of our 7 1/2-year journey through the Talmud, as we began the last tractate in the cycle—Tractate Nidda, which is devoted to the laws governing menstruation. Nidda belongs to the sixth of the Mishnah’s six orders, Seder Taharot, which deals with the complicated subject of ritual purity—a comprehensive system of taboos that, in ancient Judaism, governed many aspects of life, including food preparation and burial practices. The concepts of pure (tahor) and impure (tamei) have come up regularly in other sections of the Talmud as well.But of the 12 tractates in this mishnaic order, only one, Nidda, has Gemara in the Babylonian Talmud. That is because, after the destruction of the Temple in 70 C.E., most of the system of ritual purity fell into disuse. Impurity was mainly of concern because priests and others who entered the sacred grounds of the Temple had to be pure—that is why, for instance, priests were not supposed to enter cemeteries, to avoid contamination by the dead. Once the Temple was gone—and with it the possibility of offering certain sacrifices that restored ritual purity—Jews generally stopped observing these taboos.The exception is the taboo against sexual intercourse between a man and a menstruating woman, or nidda, which is still observed by Orthodox Jews today. This prohibition is rooted in Leviticus 15:19: “And if a woman has an issue, and her issue in her flesh is blood, she shall be in her menstruation seven days, and whoever touches her shall be impure until the evening.” Deliberately violating this rule is a serious sin, punishable by karet, the “cutting off” of the offender’s soul.But because menstrual cycles can be unpredictable, it’s not always easy to determine exactly when a woman’s impurity begins. That is the problem addressed in the first chapter of Tractate Nidda, which Daf Yomi readers studied last week. If a woman examines herself and finds menstrual blood, can she assume that her period has just started and that she has only now become ritually impure? How does she know that it didn’t start earlier and she just failed to notice it? The answer matters because if she might have been menstruating earlier, then any vessels she handled in the interim might be tamei, ritually impure. She also wouldn’t be certain when to begin the count of her days of nidda, in order to determine when it is permitted for her to have sex again.In Nidda 2a, we see that this problem was a matter of debate between the great rabbis Hillel and Shammai. Usually when these authorities are at odds, Shammai is more strict, but in this case the opposite is true. Shammai says that a woman’s impurity begins from the moment she finds menstrual blood, and she doesn’t have to worry about having missed the signs earlier. Hillel, on the other hand, says that she was potentially impure ever since her last “clean” self-examination, since menstruation might have begun at any time between then and now.But while the law usually follows Hillel in these disputed matters, that is not the case here. Instead, the rabbis propose a sensible compromise: The woman only has to be concerned about the 24 hours before she discovered blood, not all the way back to the time of her last self-examination, on the assumption that she would not be likely to have missed the signs of menstruation for more than 24 hours. And if she examined herself more recently than that, then she only has to be concerned about the time since her last examination. These rules, however, only apply to women who do not have a fixed menstrual cycle, so that they can’t predict by the calendar when they will begin menstruating. Women whose cycle is predictable don’t have to worry about retroactive nidda at all, since they presumably know just when to start checking for signs of blood.These formulations in the mishna give rise to a long discussion in the Gemara, in which the rabbis try to draw analogies between detecting menstruation and detecting other types of impurity. For instance, to be effective at removing impurity, a ritual bath must contain 40 se’a of water—a Talmudic measurement that works out to somewhere between 300 and 600 liters. What if people are using a ritual bath on the assumption that it has the necessary amount of water, and then one day someone measures it and discovers that it has less than 40 se’a?The law in this case is that the bath is considered retroactively invalid back to the time it was last measured, since it could have lost water at any time between then and now. As the Gemara points out, this seems to contradict the reasoning of Shammai in the case of menstruation, since he says that a woman can assume she has only become impure once the impurity is discovered. It is closer to the reasoning of Hillel, who backdates a woman’s potential impurity to her last examination, but even here there is a difference: For Hillel the woman is only potentially impure, while the ritual bath is considered definitely impure.The rabbis take up other examples as well. What about a barrel of wine that is discovered to have turned into vinegar, which renders it unusable as teruma, the tithe given to the priests—does the owner have to worry about exactly when it went sour? Or what about an alleyway where a dead “creeping animal”—one of the categories of impure, unkosher animals—is discovered: Do we assume that it has been there for a long time, or that it turned up just before it was found? All of these are ways of asking about the same problem—how to deal with uncertainty when it comes to ritual purity.But are the cases really parallel? The assumption is that a woman doesn’t know whether she’s menstruating unless she examines herself and finds blood, just as we don’t know the exact volume of water in a ritual bath until we measure it. Eventually, however, the Gemara gets around to asking an obvious question: Is it possible that a woman could begin menstruating without knowing it? Shammai’s logic suggests that “a woman senses within herself” when the flow of blood has begun, which is why she can be certain she hasn’t been menstruating before she notices it.Hillel’s logic, on the other hand, suggests that a woman could be mistaken about whether she is bleeding—for instance, because “she thought it was the sensation of her flow of urine.” And what about a woman whose flow begins when she is asleep—would that necessarily wake her up, or might she sleep through it? The rabbis assume that she would wake up: “due to her discomfort she would awaken, just as it is with the sensation of the need to urinate.”What’s faintly comic about the whole discussion, of course, is that none of the rabbis knew what it feels like to menstruate, because they were all men. It’s possible that they derived their beliefs about this subject from talking to their wives, but then again maybe not—there’s certainly no reference to any woman’s input in the Talmud. Here more than anywhere else, the modern reader has to wonder how different the Talmud, and the history of Judaism, might have been if women had been involved making Halacha.***Adam Kirsch embarked on the Daf Yomi cycle of daily Talmud study in August 2012. To catch up on the complete archive, click here.