1) A "REVI'I L'TUM'AH" OF LIQUIDS
OPINIONS: According to the opinion that liquids cannot be Metamei foods, is there a d'Oraisa level of Revi'i l'Tum'ah for Kodshim, or is the Tum'ah of Revi'i only mid'Rabanan?
(a) RASHI (DH Amar Lei) explains that it must be that Revi'i b'Kodesh is mid'Rabanan according to the opinion that liquids cannot be Metamei foods mid'Oraisa. According to this opinion, a liquid must be the end of the chain of Tum'ah, because it cannot be Metamei anything after it. A utensil, on the other hand, cannot be Tamei with less than a Rishon l'Tum'ah. Consequently, if a Tamei utensil (a Rishon) touches a liquid, the liquid becomes a Sheni and is the end of the chain of Tum'ah. If the utensil touches a food, then the food becomes a Sheni and can be Metamei only a liquid (to make it into a Shelishi), since food cannot be Metamei a like item (another food). How, then, is it possible for there to be a level of Revi'i? It must be that Revi'i is only mid'Rabanan.
(b) TOSFOS (15a, end of DH Vlad) explains that there can be a case of a Revi'i mid'Oraisa, because there are other items besides foods and liquids that can become Tamei as a Rishon or Sheni. The Gemara later teaches that "Chibas ha'Kodesh" makes it possible for even non-edible objects to become Tamei if they are Kodesh. Consequently, one more level of Tum'ah d'Oraisa can be added to the chain: A utensil (a Rishon) touches Etzim or Levonah (the wood or incense used in the Beis ha'Mikdash, which become Tamei because of "Chibas ha'Kodesh" (24b)) and makes them a Sheni, the Etzim or Levonah then touch food (and makes it a Shelishi), and the food then touches a liquid (and makes it a Revi'i).
It should be noted that Rashi, who disagrees with Tosfos on this matter, is consistent with his opinion later (DH Alma) that "Chibas ha'Kodesh" does not cause an item to become Tamei mid'Oraisa. Etzim or Levonah become Tamei only mid'Rabanan as a result of "Chibas ha'Kodesh." Therefore, Rashi here says that there cannot be a Revi'i mid'Oraisa since the chain of Tum'ah must include liquids. (See Insights to Sotah 30:1.)
2) A NEEDLE FOUND IN THE FLESH OF AN ANIMAL
QUESTION: The Gemara cites the Mishnah in Eduyos (2:3) that states that if a Kohen finds a Tamei needle when he cuts open the flesh of a Korban, his hands and the knife remain Tahor. There is no concern that they became Tamei through contact with the needle.
The Gemara attempts to determine what type of Tum'ah this needle has, such that it could have made the Kohen's hands and the knife Tamei. The Gemara cites two explanations. Rav Yehudah in the name of Rav says that the needle is recognized as one that was lost, and it is known to be Tamei with Tum'as Mes (Av ha'Tum'ah). Rebbi Yosi b'Rebbi Avin says that an unidentified needle was found, but it is assumed to be Tamei because the animal was muzzled when it was brought from outside of Yerushalayim. The animal must have swallowed the needle outside of the city, and the Chachamim decreed that all utensils found outside of Yerushalayim are Tamei out of doubt.
From the Gemara earlier (beginning of 19b) it is clear that according to the second answer, the needle is only a Rishon l'Tum'ah out of doubt (RASHI DH Ela Amar Rava), and such a Tum'ah cannot be Metamei the knife, because a utensil cannot become a Sheni. Why, then, does Rav Yehudah in the name of Rav need to say that the needle was Tamei with Tum'as Mes, an Av ha'Tum'ah? He could have said that the needle that was found was a Rishon l'Tum'ah!
ANSWER: If the needle that is found could be identified and proven to be a Rishon l'Tum'ah, why does the Mishnah need to teach that the knife that cut the meat is Tahor? It is obvious that it is Tahor, because even if it touched the needle, a Rishon cannot make the knife (a utensil) Tamei! It must be that the needle is an Av ha'Tum'ah, and the only reason the knife is Tahor is because there is no certainty that it touched the needle, and the rule is that when a question arises in Reshus ha'Rabim concerning the Tum'ah of an item (Safek Tum'ah b'Reshus ha'Rabim), the item is deemed Tahor.
However, according to the second answer of the Gemara, that one found an unknown needle in the animal that was Tamei as a Rishon, the Mishnah teaches an important point. The Mishnah teaches that if one finds a utensil (such as a needle) and is unsure of its Tum'ah status, the level of Tum'ah that he assumes it to have is that of a Rishon, and not an Av. Therefore, it is not Metamei the knife. The Mishnah's Chidush is that the needle is deemed to be a Rishon and not an Av when there is a doubt about its status.
3) PROOF THAT THE "AZARAH" IS CONSIDERED A RESHUS HA'RABIM
QUESTION: According to Rav, if a Kohen cuts open an animal and finds a needle that is known to be Tamei with Av ha'Tum'ah, his hands and the knife remain Tahor. The Gemara asks why they are Tahor -- perhaps his hands or the knife touched the needle, and the needle is able to be Metamei them since it is an Av ha'Tum'ah. Rav Ashi answers that Rav's ruling proves that the Azarah of the Beis ha'Mikdash, where the animal was slaughtered and cut, is considered a Reshus ha'Rabim. There is a rule that "Safek Tum'ah b'Reshus ha'Rabim Tahor" -- when a doubt arises with regard to Tum'ah in Reshus ha'Rabim, it is deemed Tahor.
The Gemara asks that even if the Azarah is considered a Reshus ha'Yachid (in which case a Safek Tum'ah is deemed Tamei), the knife cannot be Tamei because it is an inanimate object and "Ein Bo Da'as li'Sha'el," and thus out of doubt it cannot be deemed Tamei. How, then, can Rav Ashi prove that the Azarah is a Reshus ha'Rabim?
What is the Gemara's objection to Rav Ashi's proof? Rav Ashi proves that the Azarah is a Reshus ha'Rabim from the fact that the Kohen is Tahor, even though he may have touched the needle, and not from the fact that the knife is Tahor. If the Azarah would have the status of a Reshus ha'Yachid, the Kohen certainly would be Tamei out of doubt ("Safek Tum'ah b'Reshus ha'Yachid Tamei"), and therefore Rav Ashi's proof that the Azarah is a Reshus ha'Rabim is valid. (TOSFOS DH Ha)
(a) The TOSFOS HA'ROSH answers that Rav Ashi's wording implies that he is giving a reason why both the knife and the person are Tahor. Although the Gemara asks why Rav Ashi gives that reason to explain why the knife is Tahor, it can still be proven that the Azarah is a Reshus ha'Rabim from the fact that the person himself is Tahor.
(b) Perhaps there is no question at all whether the Kohen touched the needle or not; he knows that he definitely did not touch the needle. The only question was whether the knife touched the needle. Consequently, it cannot be proven from here that the Azarah is a Reshus ha'Rabim, since both the knife and the person would be Tahor even if the Azarah is a Reshus ha'Yachid, as the Gemara asks.
However, if there is no question whether the person touched the needle, why does the Mishnah need to teach that he is Tahor? It is obvious that he is Tahor! What could have made him Tamei if he did not touch the needle?
The answer is that we might have thought that the Kohen's hands are Tamei because they touched a Rishon l'Tum'ah -- the flesh of the animal that he cut, since the Rabanan decreed that, normally, hands that touch a Rishon are Tamei. The reason why his hands are not Tamei in this case even though they touched the flesh is because in the Beis ha'Mikdash, the Rabanan suspended their decree that hands that touch a Rishon become Tamei, as Rebbi Akiva explains in the Mishnah. Although the hands definitely touched the meat, the Rabanan did not decree that they should become Tamei in the Beis ha'Mikdash. (Since Rebbi Akiva makes this assertion in the Mishnah, all interpretations of the Mishnah must be consistent with this point; see TOSFOS RABEINU PERETZ here.)
According to this understanding, when the Gemara asks that if the Azarah is a Reshus ha'Yachid then the person and knife should be Tamei, its intention is to ask only with regard to the knife. It mentions the person only because the Mishnah mentions the person. (M. KORNFELD)