The Gemara explains their reasoning: And both tanna’im derived their respective opinions only from the case of a woman suspected by her husband of having been unfaithful [sota], who is forbidden to engage in intercourse with her husband just like a definite adulteress.

The Rabbis hold: This case of a ritual bath is like the case of a sota. Just as in the case of a sota it is uncertain whether she was actually unfaithful, and nevertheless the Torah rendered her like one who definitely committed adultery in that she is forbidden to her husband until she drinks the water of a sota; here too, in the case of a ritual bath, it is uncertain whether it was lacking the requisite measure of water and yet the Torah rendered it as though it was definitely lacking water, to the extent that teruma that touched an item that had been immersed in it must be burned.

The Gemara raises a difficulty: If the halakhot of the ritual bath are derived from those of a sota, then one can say that it should be like the halakha of a sota in another respect: Just as a sota who is suspected of having been unfaithful in a public domain is considered pure, i.e., she does not undergo to rite of a sota, here too, any impure item that was immersed in a currently deficient ritual bath that is situated in a public domain should be considered pure.

The Gemara answers: How can these cases be compared? One cannot apply the halakha of a sota in a public domain to any other case. There, the reason why a sota is treated as having definitely been unfaithful is due to her seclusion with another man. And as a proper seclusion in the public domain is not possible, she does not assume the status of a sota. By contrast, here, in the case of the ritual bath, the uncertainty is due to the lack of the requisite measure of water in the ritual bath. If so, what difference is it to me if the ritual bath is lacking in a public domain or if it is lacking in a private domain?

The Gemara comments: And if you would say that the guiding principle in any case of uncertainty involving impurity in a public domain is that it is ritually pure, and therefore all items immersed in a ritual bath situated in the public domain should be pure even if there is uncertainty about its status of purity, that suggestion can be rejected, as follows: Since there are two factors that weaken the possibility that the items are ritually pure: First, the ritual bath is currently lacking, and second, the item has a presumptive status of impurity, it is therefore considered as an item of definite impurity.

After analyzing the reasoning of the Rabbis, the Gemara turns to the opinion of Rabbi Shimon. And Rabbi Shimon holds: This case of a ritual bath is just like the case of a sota. Just as a sota who is suspected of having been unfaithful in a public domain is considered pure, here too, any impure item that was immersed in a currently deficient ritual bath that is situated in a public domain is considered pure.

The Gemara asks: If the halakhot of the ritual bath are derived from those of a sota, then one can say that it should be like the halakha of a sota in another respect: Just as a sota who secluded herself with the man in a private domain is deemed definitely impure, i.e., she is forbidden to her husband until she undergoes the rite of the sota, here too, any ritually impure item that was immersed in a currently deficient ritual bath situated in a private domain should be considered definitely impure. If so, any teruma that comes in contact with the immersed vessel should be burned. Why, then, does Rabbi Shimon rule that its status is suspended and it is neither consumed nor burned?

The Gemara answers: How can these cases be compared? There, in the case of a sota, there is a basis for the matter. She is considered definitely impure, as her husband issued a warning to her about this particular man and she then secluded herself with him. By contrast, here, in the case of a ritual bath, what basis for the matter is there? Why should one assume impurity with certainty?

And if you wish, say instead that this is the reason for the opinion of Rabbi Shimon: He does not base his ruling on the case of a sota; rather, he derives the end of impurity from the beginning of impurity, i.e., he derives the halakha of the immersion of an impure item into a ritual bath from the halakha of the initial contraction of ritual impurity.

The Gemara explains: Just as with regard to the beginning of impurity, if there is uncertainty whether or not a pure item came into contact with a source of impurity, if this occurred in the public domain it is considered ritually pure; so too in the case of the end of impurity, if there is uncertainty whether or not the impure item was immersed in a ritual bath with the requisite amount of water, the halakha is that if the ritual bath is located in the public domain, the item is considered pure.

The Gemara asks: And how would the Rabbis respond to this claim? They would respond: How can these cases be compared? There, with regard to the beginning of impurity, the man who might have touched an impure item remains with the presumptive status of ritual purity. Consequently, we do not lower his status to one who has contracted ritual impurity merely due to uncertainty. Here, in the case of the ritual bath, the man who immerses in that ritual bath has the presumptive status of ritual impurity. Therefore, we do not remove him from his status of ritual impurity due to uncertainty.

§ The Gemara returns to its analysis of the mishna. Shammai said: For all women, their time is sufficient, i.e., women who discern that menstrual blood emerged do not need to be concerned that perhaps the flow of blood began before they noticed it. The Gemara asks: In what way is this case different from that of an alleyway? As we learned in a mishna (66a): With regard to the carcass of a creeping animal that is found in an alleyway, it renders any item in the alleyway ritually impure retroactively to the time that a person can say: I examined this alleyway and there was no carcass of a creeping animal in it, or until the time of the last sweeping of the alleyway.

The Gemara answers: There too, one can explain: Since there are two types of carcasses of creeping animals that are likely to be found in the alleyway, creeping animals from the alleyway itself and creeping animals that came from the world at large, it is compared to a case where there are two factors that weaken the possibility that the items are ritually pure. Therefore, even Shammai agrees that in such a case the impurity extends retroactively back in time.

And if you wish, say instead that this is the reason for the opinion of Shammai: Since a women senses within herself if she is experiencing a flow of blood, if she felt the flow only at present, it is certain that she did not experience a flow previously. And Hillel holds that she might have experienced a flow earlier while she was urinating and she thought it was all the sensation of her flow of urine.

The Gemara asks: And according to Shammai, isn’t there the case of a sleeping woman, who would not sense her flow of menstrual blood? The Gemara answers: A sleeping woman would also sense her flow, and due to her discomfort she would awaken, just as it is with the sensation of the need to urinate.

The Gemara raises another difficulty with regard to the opinion of Shammai: But there is the case of a mentally incompetent woman, who does not properly understand what she is sensing. She might have previously experienced a flow of menstrual blood that she did not notice. The Gemara answers: Shammai concedes in the case of a mentally incompetent woman that she is impure retroactively. The Gemara asks: But the mishna explicitly teaches that Shammai mentioned all women, which apparently includes even the mentally incompetent. The Gemara answers: When the mishna teaches: All women, it is referring to all mentally competent women.

The Gemara raises a difficulty: But if so, let the mishna teach that Shammai’s opinion applies to: Women, rather than referring to all women. The Gemara answers: The inclusive statement: All women, serves to exclude the opinion of Rabbi Eliezer. As Rabbi Eliezer said in a mishna (7a): There are four women with regard to whom the halakha is that their time is sufficient. This indicates that there are only four, and no more. Therefore, the mishna teaches us: All women, to include all mentally competent women, not only the four mentioned by Rabbi Eliezer.

The Gemara raises yet another difficulty against the opinion of Shammai: But there is the case of blood stains. The mishna teaches (66a) that a woman who finds a blood stain is impure retroactive to the last time she examined her clothing and found them clean. Shall we say that we learned the mishna with regard to stains not in accordance with the opinion of Shammai? Abaye said: Shammai concedes in the case of stains. The Gemara asks: What is the reason? The reason is that she neither engaged in handling a slaughtered bird nor did she pass through a marketplace of butchers. If so, from where could this blood stain on her clothing have come? Since it must be from her last menstrual flow, Shammai agrees that she is impure retroactively.

The Gemara suggests another analysis: Or if you wish, say that this is the reason for the opinion of Shammai: She is impure only from that point onward, as, if it is so that there was any menstrual blood previously, it would have come out at the outset, i.e., at the earlier time. The Gemara explains why Hillel maintains that there is ritual impurity retroactively, in light of this analysis: And as for Hillel, he holds that the walls of the womb, i.e., the walls of the vaginal canal, held back the menstrual blood from leaving the body entirely, and therefore there might have been a previous emission from the uterus into the vaginal canal that was not visible on the outside. The Gemara asks: And how does Shammai respond to this claim? The Gemara answers: Shammai maintains that the walls of the womb do not hold back blood.

The Gemara asks: With regard to a woman who engages in intercourse while using a contraceptive absorbent cloth in the form of a wad that she inserts in her vagina at the opening of her womb so as not to become pregnant, what is there to say? In other words, how does Shammai explain why there is no retroactive impurity status in such a case, as it cannot be claimed that any previous menstrual blood would have flowed out earlier. Abaye says: Shammai concedes with regard to a woman who engages in intercourse while using a contraceptive cloth that she is impure retroactively.

Rava says: Here as well, in a case where a woman engages in intercourse while using a contraceptive absorbent cloth, any previous menstrual blood would have flowed out, as the absorbent cloth does not hermetically seal the womb. The reason is that the cloth wrinkles due to perspiration, leaving space for blood to pass through. The Gemara adds: And nevertheless Rava concedes to Abaye that Shammai agrees that she is retroactively impure in a case where a woman engages in intercourse while using a tightly packed absorbent cloth.

§ The Gemara suggested that the reasoning for Shammai’s opinion is that the woman would have felt any previous menstrual flow, or that any prior menstrual blood would have flowed out previously. Yet, earlier the Gemara suggested that his reason was that the woman retains her presumptive status of ritual purity. With regard to the explanations given for the opinion of Shammai, the Gemara asks: What difference is there between these versions and that version suggested earlier?