THE SOURCE FOR THEIR OPINIONS
Chachamim and R. Shimon both learn from Sotah;
Chachamim say, just like Sotah is Safek Teme'ah and the Torah considers her Vadai Teme'ah, the same applies to Taharos immersed in a Safek Mikveh.
Question: They should say that just like a Sotah is considered Tehorah if she was in Reshus ha'Rabim (i.e. never secluded), also the Taharos!
Answer: Regarding Sotah we distinguish Reshuyos, because seclusion is only in a Reshus ha'Yachid;
The Taharos depend on the deficiency of the Mikveh. There is no difference between Reshus ha'Yachid and Reshus ha'Rabim.
Question: The general rule is, Safek Tum'ah in Reshus ha'Rabim is Tahor!
Answer: That is when there is only one Re'usa. When there are two, it is considered Vadai Tamei.
Also R. Shimon learns from Sotah. Just like a Sotah is considered Tehorah in Reshus ha'Rabim, also Taharos.
Question: He should say that just like a Sotah is considered Vadai Tamei in Reshus ha'Yachid (if she was secluded), also Taharos!
Answer: Regarding Sotah, there is Raglayim l'Davar (circumstantial evidence) that she is Teme'ah -- she was warned, and she was secluded;
Regarding Taharos, there are no Raglayim l'Davar that they are Teme'im.
Alternatively, we can say that R. Shimon learns the end of Tum'ah (immersion) from becoming Tamei:
If Tum'ah doubtfully touched a person in Reshus ha'Rabim, he is Tahor. Likewise, in a case of Safek immersion, he is Tahor.
Chachamim disagree. If one has a Chezkas Taharah (and Tum'ah doubtfully touched him), we do not change the Chazakah to consider him Tamei;
Likewise, if one has a Chezkas Tum'ah (and doubtfully immersed), we do not change the Chazakah to consider him Tahor!
Question (against Shamai): Why is Nidah different from the case of a Mavuy (alleyway)?
(Mishnah): If a Sheretz was found in a Mavuy, all Taharos that were made there are retroactively Teme'im, up to the last time it was checked for Sheratzim or swept. (One looks for Sheratzim when sweeping.)
Answer: Because Sheratzim can come from the Mavuy itself or from the outside, this is not considered one Re'usa, rather, two.
OTHER EXPLANATIONS OF SHAMAI
Answer #2 (to Question 1:h on Daf 2a, i.e. what is Shamai's reason?): Shamai holds that a woman is (usually) Margish (senses when Dam leaves the Makor. Since she was not Margish before, we assume that it came now. Even if she saw blood now during a Bedikah without Hargashah, perhaps she mistakenly attributed the Hargashah to the Bedikah. Poskim argue about whether Hargashah means feeling something wet flowing, or the Makor opening, or an agitation of her whole body.)
Hillel says, perhaps it flowed before when she was urinating, and she attributed the Hargashah to urinating.
Question (against Shamai): Perhaps the blood came when she was sleeping, therefore she was not Margish!
Answer: The Hargashah would have woken her, just like an urge to urinate wakes up a person.
Question (against Shamai): A Shotah (insane woman) does not know that the Hargashah is due to Dam Nidah!
Answer: Indeed, Shamai admits that a Shotah is retroactively Teme'ah.
Question: The Mishnah says "all women"!
Answer: It refers to all sane women.
Question: It should have omitted the word "all"!
Answer: "All" teaches unlike R. Eliezer:
(R. Eliezer): Only four categories of women are Dayan Sha'atan.
Question: The Halachah is, (in certain cases) we consider a woman to be a Nidah due to a Kesem (blood stain, e.g. on her garment). Does Shamai argue with this (because she had no Hargashah)?!
Answer (Abaye): Shamai admits that a woman who found a Kesem is Teme'ah.
Question: What is the reason?
Answer: If she did not handle a bird or pass through a meat market, we cannot attribute the blood to anything except for her. (Most blood comes with Hargashah. Perhaps she forgot or attributed the Hargashah to something else.)
Answer #3 (regarding Shamai's reason): Shamai holds that had the blood entered the Beis ha'Chitzon earlier, it would have left her body right away.
Hillel argues. He holds that the walls of the Rechem (womb) could hold it in.
Shamai holds that the walls do not hold in blood.
Question: If a woman uses a Moch (soft material inside the Rechem to impede semen and prevent pregnancy), perhaps that held in the blood!
Answer #1 (Abaye): Shamai admits that a woman who uses a Moch is retroactively Teme'ah when she sees blood.
Answer #2 (Rava): A Moch shrivels up. It does not block the entire width of the Rechem, so it does not hold in blood.
THE DIFFERENT EXPLANATIONS OF SHAMAI
Question: What is the difference between these latter explanations of Shamai to the first answer (that she retains her Chazakah)?
Answer: The questions from the cases of a barrel, Mikveh, and Mavuy (and the answer we were forced to give, to distinguish between one Re'usa and two) apply only if Shamai's reason is Chazakah, but not if it is Hargashah or that the blood would have come out.
Question: What is the difference between the answer of Hargashah, or that the blood would have come out?
Answer: If he holds that the blood would have come out, he admits (to retroactive Tum'ah) for one who uses a Moch (Abaye; Rava - a Moch Dachuk (tight-fitting). Even after it shrivels, it blocks the entire width of the Rechem)), but not if his reason is Hargashah. (Tosfos - also, he admits regarding a Shotah only if his reason is Hargashah.)
Support (for answer #3 - Beraisa #1): Hillel asked Shamai: Don't you agree that if one corner of a box was used for Taharos, and a Sheretz was found later in a different corner, that the Taharos are Teme'im? (Perhaps the Sheretz was there from the beginning, and it was Metamei the box)!
Shamai: I agree. A box has a bottom to hold a Sheretz, but the Rechem has no bottom wall to hold blood.
Answer #4 (regarding Shamai's reason - Rava): Shamai says Dayah Sha'atah, lest we inhibit procreation. (If she is retroactively Teme'ah, her husband will worry that he had relations with a Nidah, and he will refrain from relations in the future.)
Support (for Rava - Beraisa #2): Shamai told Hillel: If you make her retroactively Teme'ah, you will inhibit procreation!
Question: Beraisa #1 shows that Shamai's reason is that the blood would have come out!
Answer: Hillel thought that this was Shamai's reason, therefore he asked about a Sheretz in a box.
(Shamai answered "my reason is due to procreation." The Beraisa records only that) he (also) told him "even if that were my reason, a box is different, for it has a bottom."
Question: How does Answer #3 explain Beraisa #2?
Answer: At first, Hillel agreed with Shamai's reason that the blood would have come out, but said that even so, it is proper to be stringent, just like we always make "fences" to distance people from transgressing;
Shamai answered that such a fence would inhibit procreation.
Question: Hillel is retroactively Metamei regarding Taharos. He does not say that she was forbidden to her husband!
Answer: Shamai holds that the husband will reason that if she was (retroactively) Teme'ah regarding Taharos, she was also forbidden to him, and he will refrain from relations.
A SHERETZ FOUND IN A BOX
(Chizkiyah): If one corner of a box was used for Taharos, and a Sheretz was found later in a different corner, the Taharos are Tehorim;
(R. Yochanan): They are Teme'im.
Question (against Chizkiyah): Even Shamai agreed that they are Teme'im!
Answer #1: Hillel and Shamai discussed a box with a bottom. Chizkiyah and R. Yochanan argue about a box without a bottom. (It rests on its side on the floor. One may put things in or take out from the openings at either end.)
Question: If so, what is R. Yochanan's reason? (Surely, if the Sheretz was there before, when he poured out the Taharos it would have spilled out with them!)
Answer: The case is, it has no bottom, but it has Ognim. (The boards near the bottom bend in. The Sheretz could have gotten caught there when he poured out the Taharos.)
(Beraisa): If Reuven drew water from a pit ten times in a bucket, each time he emptied it into a barrel, and the last time he saw a Sheretz in the bucket:
All the water in the barrel is Tahor. (Surely, the Sheretz was not in the bucket before this time, for it would have fallen into the barrel when he emptied the bucket into it. Water in the pit is considered attached to the ground, so it cannot become Tamei.)
(R. Yanai): This applies only to a bucket without Ognim. If it has Ognim, the water in the barrel is Tamei. (Perhaps the Sheretz was in the bucket before, and the Ognim kept it from falling into the barrel.)
Suggestion: Chizkiyah (is not concerned lest Ognim held in the Sheretz. He) argues with R. Yanai.
Rejection: He could agree with R. Yanai. One merely tilts a bucket to spill out the water, but one turns a box until it is vertical to spill out the produce (even Ognim would not hold in a Sheretz);
Reason #1: This is because water flows out easier than produce.
Reason #2: This is because a person is particular to take all the produce, but he is not particular to get every drop of water (especially if he plans to draw more and empty it again into the barrel).