Answer: The Yerushalmi answers that it is as thin as a hair, and it is called a 'Shefifon'. The ground is permitted to split open before it, but not vessels.

Implied Question: It would not be possible to cite the Gemara's reasoning here in Perek Kol ha'Yad (Nidah 15b), in Perek Kol ha'Tzelamim (Avodah-Zarah 41b) and in the first Perek of Pesachim (9a), where the Gemara also asks that Safek should take away from Vaday ...

Answer: ...because it is different here, since le'Chumra, we say that Safek Isur takes away from Vaday Heter.

Implied Question: ... And by the same token the S'varos that the Gemara cites there are not applicable here ...

Answer: ... because there it is taking about a Safek that is frequent.

Implied Question: Here the Gemara does not differentiate between whether he Toveled close to the 'Chafifah' or not, as it does in the last Perek of Nidah (66b), where Rava states that if, after coming up from the Tevilah the woman found something that is Chotzetz on her, then provided she Toveled close to the Chafifah, she does not need to Tovel again. Otherwise ... '?

Answer: Explanation #1: Rabeinu explains that the Gemara here is speaking where the Chatzitzah is found on the body, whereas the Gemara in Nidah is speaking where she found it on her head - because Ezra only instituted Chafifah on the hair.

Proof #1: ... like we say in Perek Merubeh (Bava Kama 82a) 'min ha'Torah one needs to examine in case there is a knot', and knots apply exclusively to the hair.

Proof #2 (Part 1): Furthermore we learned in the Mishnah in Nazir (42a) that a Nazir is permitted to make Chafifah and to separate, but not to comb. And in the last Perek in Nidah (66b) Rabah learned that a woman should make Chafifah specifically with hot water, but not with cold water, since it causes the hair to become matted.

Proof #2 (Part 2): But as far as the rest of the body is concerned, the Lashon 'Hadachah' (washing) would have been more appropriate, as Rabah says there 'A person should always teach in his house that his wife should wash the folds in her skin with water - implying Davka the folds, but not the rest of the body.

Implied Question: One could say that he mentions 'the folds' to teach us that even though the water does not actually need to reach those spots, it does needs to be able to do so (in which case we would have a proof that Chafifah incorporates the entire body)?

Answer: One cannot however, say that, since the Gemara does not say 'even the folds', implying that it is specifically the folds that need washing.

Implied Question: And when Rav Nachman asked his mother-in-law whether she was short of a caldron (in which to bathe) - implying that the entire body requires Chafifah?

Answer: ... it is not because it is obligatory that he said it, but because it was her custom to bather in hot water, just as one tends to do nowadays.

Explanation #2: Furthermore, Rabeinu Tam said that here the Gemara is talking about Taharos, whereas the Gemara in Nidah is talking about Toveling for her husband, and the Chachamim were generally more stringent regarding Taharos than they were regarding a woman's husband, as is implied in Nidah in a number of places.

Explanation #3: Finally, it is possible to establish our Sugya by Kohanim, who do not require Chafifah, because they constantly need to Tovel, as is implied in the Yerushalmi.

Clarification: This means it is almost certain that he Toveled properly, seeing as he subsequently dealt with that species.

Question (Part 1): Why can we not give the same answer in the case of 'a Mikvah that is measured and found to be lacking' at the beginning of Nidah (2b), where the Gemara rules that 'all the Taharos that were performed relying on it are retroactively Tamei, because a. it is Chaser in front of us, and b. one places the Tamei person on his Chazakah. Why do we not say there too 'that the Mikvah has a defect, but the man doesn't?

Question (Part 2): Alternatively, why do we not place the animal on its Chazakah, and say that it has not been Shechted, since the knife has a defect before us?

Answer (Explanation) #1: It is different there, because it is possible to apply the S'vara that the water in the Mikvah gradually diminished, as the Gemara explains there.

Answer (Explanation) #2: Alternatively, it is different here, since it is definitely the bone that causes the defect.

Implied Question: And if one will ask that there too, he definitely Toveled, like the Gemara says here with regard to the case of 'Taval ve'Alah'?

Answer: ... one cannot compare the cases, since by virtue of the fact that he continued to handle the same species after the Tevilah renders it 'Vaday Taval', because it probably comes from the species with which he was dealing. Whereas there (in the case of 'Mikvah she'Chasar', how can one possibly suggest that 'Vaday Taval', seeing as he Toveled in a Pasul Mikvah, which is not considered Tevilah.

Observation: ... And when the Gemara will shortly cite Rav Yosef who declared T'reifah up to thirteen animals, this cannot go according to Rav Chisda ...

Reason: ... according to whom even the last one will be Kasher, due to the probability of the fact that the knife became defected only on the neck-bone, seeing as he still intends to Shecht other animals; but he is not concerned about the last one of this batch.

Explanation #3 (Part 1): What difference does it really make whether the defect is found on the knife or on the animal?

Explanation #3 (Part 2): And what the Gemara must therefore mean when it says that the knife is defected is that there are a number of S'feikos, perhaps it became defected on the bone, and even if it became defected on the skin, perhaps he did not Shecht the Simanim with the defected part of the knife. Consequently, it is s S'fek S'feika.

Explanation (Part 1): Now, when the Gemara asks afterwards whether 'Kol Safek' does not come to include such a case, it means to ask whether it comes to include even a case where the Heter is more likely than the Isur.

Explanation (Part 2): ... And the Gemara answers 'No, it comes to include Safek Shahah, Safek Daras'; even though, one does not tend to make Shehiyah and D'rasah, but rather to Shecht the way that Shochtim do.

Explanation (Part 3): ... and the Gemara then asks that, in that case, what is the difference, seeing as they are both more like likely to be kasher than T'reifah?

Implied Question: We cannot ask how one can possibly prove from here that one places on a Chazakah, perhaps here he is Tamei mi'Safek, because 'Safek Ttum'ah bi'Reshus ha'Yachid, S'feiko Tamei'?

Answer: ... because the Pasuk declares him Tamei under all circumstances, even if it was in as R'shus ha'Rabim where there were many people, and even regarding something that does not have the understanding to ask.

Question: That is fine as long as there is no Re'usa (fault), because where there is - such as a knife which has a defect, or a case where one of two people who certainly became Tamei in the R'shus ha'Rabim, where we declare them both Tahor, on account of Chazakah. In such a case, from where do we know that we go after Chazakah?

Answer: There where it is found to be lacking after seven days, where we nevertheless do not declare Tahor retroactively all those who entered the house during the time it was quarantined.

Question (Part 1): This matter is comparable to a Mikvah that was measured and found to be lacking - where we ignore the Chazakah of the Mikvah, and declare the man who Toveled in it Tamei, because we place the Tamei person on his Chazakah, in conjunction with the fact that the Mikvah is lacking in front of us.

Question (Part 2): By the same token we ought to say here that we ignore the Chazakah of the Nega, and go after the man, who has a Chezkas Taharah, in conjunction with the fact that the Nega is lacking in front of us.

Implied Question: We cannot answer that this case is different, since we go le'Chumra ...

Answer #1: ... because min ha'Torah, one cannot make such a distinction.

Answer #2: And besides, even le'Kulah we declare him Tamei as well, with regard to burning Kodshim and bringing a Korban, should he enter the Mikdash, even though this will entail bringing a Safek Chulin into the Azarah.

Answer #3 (Part 1): Furthermore, even the Gemara specifically establishes him on his Chazakah, even Le'kula, because if it was a large Nega which was found to have diminished after one week, we declare him Tahor. Why are we not afraid that even as he was leaving, the Nega grew smaller, to the Shi'ur that it was in the middle of the week (see Tosfos ha'Rosh), in which case the Nega actually remained in its appearance exactly as it was, and needs to be quarantined again?

Answer #3 (Part): The reason must therefore be because, at the time of the quarantine, we place it on its Chazakah.

Question: The Gemara in Perek Yedi'os ha'Tum'ah (Shevu'os 17b) states that someone who enters a Bayis ha'Menuga backwards, is Tahor, even if his entire body except for his nose is in the house, because the Torah writes "ve'ha'Ba el ha'Bayis", implying that he enters the house in the regular manner?

Answer: One can differentiate between entering a house and leaving it, since it is normal to leave a house backwards, like a Talmid taking leave of his Rebbe.