The Gemara answers: The difference between them is with regard to the possibility of raising a contradiction between Shammai’s ruling here and the cases of a barrel, a ritual bath, and an alleyway. According to that version, that Shammai’s reasoning is based upon the principle that an item retains its presumptive status, there is room to raise these contradictions, as the Gemara explained. By contrast, according to these other versions, that the woman would have felt any previous menstrual flow or that any prior menstrual blood would have flowed out previously, there is no room to raise such contradictions, as those rulings do not contradict the halakha here, where a unique factor is relevant.

The Gemara further asks: And what difference is there between this version, that any prior menstrual blood would have flowed out, and that version, that she would have felt any previous menstrual flow? The Gemara answers: According to Abaye, who maintains that Shammai agrees that a woman who engages in intercourse while using a contraceptive cloth is impure retroactively, there is a difference in a case where the woman engages in intercourse while using a contraceptive cloth. If Shammai’s reasoning is that a woman senses the emission of blood, in this case too it is assumed that she did not emit blood earlier. By contrast, if his reasoning is that any prior menstrual blood would have come out at the outset, it is possible that the cloth blocked the emission of blood, and she already emitted blood. Consequently, she is impure retroactively.

According to Rava, who maintains that blood can emerge even if the woman engages in intercourse while using a contraceptive absorbent cloth, there is a difference between the two interpretations in a case where she engages in intercourse while using a tightly packed absorbent cloth, as such a cloth will prevent the blood from flowing out, but it does not affect her ability to sense an emission.

It is taught in a mishna (Teharot 4:4) in accordance with this version, i.e., that Shammai’s reasoning is that if it is so, that there was menstrual blood previously, it would have come out at the outset: Hillel said to Shammai: Don’t you concede to the following halakha: In the case of a basket that was used as a container for ritually pure items, and those items were placed in one corner, and subsequently the carcass of a creeping animal was found in a different corner of the basket, the halakha is that the items initially considered ritually pure are now retroactively considered impure? Shammai said to him: Indeed, that is correct.

Hillel continues: And if so, what difference is there between this case of the pure items stored in the basket, which are now considered retroactively impure, and that case of the ritually pure woman who experiences a menstrual flow, who is impure only from that point onward? Shammai answers: This one, the basket, has a base on which it rests, and therefore the creeping animal might have been there earlier unseen. That one, the menstruating woman, has no base, i.e., there is nothing to impede her menstrual blood from flowing out. Shammai’s response indicates that she is not retroactively impure because if she had experienced a previous menstruation it would have flowed at that stage.

§ Rava says: The reason for the opinion of Shammai that a woman who experiences menstruation is not deemed ritually impure retroactively is due to the dereliction of the mitzva to be fruitful and multiply. If woman is always deemed retroactively impure, she and her husband might abstain from engaging in sexual intercourse out of fear that she is already impure even if she has not yet experienced bleeding. The Gemara adds that this explanation is also taught in a baraita: Shammai said to Hillel: If it is so, that a woman is rendered retroactively impure as you claim, you will have caused the Jewish women to be derelict in the mitzva to be fruitful and multiply.

The Gemara asks: But according to the one who taught this version, i.e., Rava, there is a difficulty: Isn’t it taught in the aforementioned mishna in accordance with that other version, that Shammai’s reasoning is that if it is so, that there was any menstrual blood previously, it would have come out at the outset? The Gemara answers: There, in that mishna, it is Hillel who erred. He thought the reason of Shammai is that if it is so, that there was any menstrual blood previously, it would have come out at the outset. And therefore, he raises a difficulty against Shammai’s opinion from the case of the basket.

And Shammai said to Hillel: You are mistaken. My reasoning is actually due to the dereliction of the mitzva to be fruitful and multiply. But even according to your erroneous reasoning that led you to raise a difficulty from the case of the basket, I have an answer for that as well: This basket has a base, and therefore the creeping animal could have been there earlier, unseen; but that menstruating woman has no base.

The Gemara asks the reverse question: And according to the one who teaches this other version, that Shammai’s reasoning is that if there was previous menstrual blood it would have come out at the outset, isn’t it taught in a baraita in accordance with that other version, that Shammai’s reasoning is due to the dereliction of the mitzva to be fruitful and multiply?

The Gemara answers: That baraita was recorded at a later point in their discussion, as this is what Hillel is saying to Shammai: Yes, the reason that you say for your opinion has merit, that if it is so, that there was any menstrual blood previously, it would have come out at the outset. Nevertheless, you should enact a safeguard for your statement and render her retroactively impure, as in what way is this case different from the rest of the entire Torah, where the Sages enact safeguards?

Shammai said to Hillel: If so, i.e., if she is considered impure retroactively as a safeguard, you will have caused the Jewish women to be derelict in the mitzva to be fruitful and multiply. The Gemara asks: And how does Hillel respond to Shammai’s reasoning with regard to the mitzva to be fruitful and multiply? The Gemara explains that Hillel would answer: Although I rule that she is retroactively impure, did I say that she is forbidden to her husband and must therefore abstain from the mitzva to be fruitful and multiply? I said only that she renders impure any ritually pure items that she touched.

The Gemara asks: And how would Shammai reply to this? The Gemara answers: Shammai maintains that even a safeguard that renders ritually pure items impure should not be imposed. The reason is that if it were so, the heart of a scrupulous husband might strike him with pangs of conscience over sins that he could transgress and he would separate from his wife, out of fear that she is impure.

The Gemara provides a mnemonic for the ensuing discussions: A base; examined; covered; in a corner. It was stated: If one has a basket that was used as a container for holding ritually pure produce, and those items were placed in one corner of the basket, and subsequently the carcass of a creeping animal was found in a different corner of the basket, Ḥizkiyya says: That produce initially considered ritually pure is still pure. Rabbi Yoḥanan says: The produce initially considered ritually pure is now retroactively deemed impure. The Gemara asks: But how can Ḥizkiyya rule that it is pure? After all, don’t Shammai and Hillel agree in the case of the basket that the produce initially considered pure is now retroactively impure?

The Gemara answers: When do Shammai and Hillel agree that all the produce is retroactively impure? With regard to a basket that has a base. In such a case, the carcass of the creeping animal might have previously been in the basket and it went unnoticed when one removed the pure produce items. When Ḥizkiyya and Rabbi Yoḥanan disagree, it is with regard to a basket that has no base, i.e., both ends were open and the basket was used while it was lying on its side. In that situation, the contents of the basket are emptied by raising one end, which causes all items inside to fall out. Consequently, any creeping animal that was found subsequently must have entered the basket only after that produce was removed, which is why Ḥizkiyya rules that the produce remains pure.

The Gemara asks: If this is referring to a case where the basket has no base, what is the reasoning of Rabbi Yoḥanan for ruling that the produce is ritually impure? The Gemara answers: The basket has no base but it does have an inward-curved rim. Therefore, it is possible that the carcass of a small creeping animal might have been held back by the rim and remained inside the basket even after it was turned over. Rabbi Yoḥanan rules that the produce is retroactively impure, due to the possibility that the creeping animal might have been in the basket previously together with the initially pure produce.

The Gemara raises a difficulty: But isn’t it taught in a baraita: With regard to one who draws ten pails of water one after another, i.e., he draws out water in one pail and from it he fills another ten pails with water, and the carcass of a creeping animal is found in one of them, that pail of water is ritually impure but all the rest of them are pure. And Reish Lakish says in the name of Rabbi Yannai: This halakha was taught only in a case where the pail has no inward-curved rim. Consequently, its contents are entirely emptied each time it is overturned, which means that the creeping animal must have entered only this pail. But if the pail has a rim, all of the water that had been drawn is impure, as the creeping animal might have entered the pail earlier and the rim held it back each time the pail was overturned.

The Gemara explains the difficulty: Shall we say that Ḥizkiyya, who is not concerned that the rim of the basket might have held back the creeping animal, does not agree with the opinion of Rabbi Yannai? The Gemara answers that there is a difference between the two cases: Water is slippery, whereas produce is not slippery. In order to pour water out of the pail it is not necessary to turn it over completely. Therefore, the rim might hold back a creeping animal. By contrast, a basket must have one end fully raised in order to empty out all the produce it contains. In such a case Rabbi Yannai would agree that nothing could have been held back, and therefore the emptied produce remains pure.

Alternatively, there is a different reason why one does not overturn a pail of water entirely: In the case of water, one is not particular about it, i.e., he does not mind if some is left over. Therefore he does not overturn the pail completely. By contrast, in the case of produce one is particular about it, so he raises one end of the basket entirely to ensure that every last piece of produce is emptied. Consequently, in the case of produce Rabbi Yannai would agree that even if the basket has a rim, all the produce remains pure.

As an alternative to its previous resolution that Ḥizkiyya is discussing the case of a rimmed receptacle, the Gemara suggests another resolution of the apparent contradiction between Ḥizkiyya’s ruling and the opinions of Hillel and Shammai. And if you wish, say: When do Shammai and Hillel agree? In the case of a basket that was not examined. Since he did not examine the basket when he placed the pure produce there, a creeping animal might have already been inside. Consequently, they agree that the produce does not retain its presumptive state of purity but is considered retroactively impure.