1) CHAGAI'S QUIZ TO THE KOHANIM
QUESTION: The Gemara relates that Chagai tested the Kohanim who returned from the exile in the time of the second Beis ha'Mikdash. He asked them what the status of an object would be in the case of a Sheretz which touched one object, which in turn touched another object, until a total of four levels of objects had been touched. "If one carried affected meat (Rashi: a Sheretz) in the hem of his garment, and with his garment he touched bread, and cooked food, and wine, and oil, and any other food, does it become Tamei? The Kohanim answered and said, 'No'" (Chagai 2:12).
The Gemara understands from the verse that Chagai's question involved an object that was four levels of Tum'ah below the Tum'ah of a Sheretz (or a "Revi'i b'Kodesh"). Why, though, does the verse mention seven items, and not five (a Sheretz and four levels below it)? The seven items that the verse mentions are:
1. affected meat ("Basar Kodesh," or Sheretz),
2. the garment,
4. cooked food,
7. "any other food."
(a) RASHI explains that "affected meat" ("Basar Kodesh") refers to a Sheretz, and the word "Kodesh" means "Tamei," defiled. The word "garment" also refers to the Sheretz itself, since the Sheretz is held in the garment. It does not refer to a garment which merely touched a Sheretz. The last two items in the verse, oil and "any other food" are counted as one item (as "either/or") and are not separate levels.
According to Rashi, the second and seventh items in the verse are not counted, and only five items are left.
(b) TOSFOS strongly disagrees with Rashi's explanation. He asserts that there is no reason to suggest that "any other food" is counted together with oil more than any of the other items mentioned in the verse are counted together. Furthermore, there is no reason to omit the garment from the count and assume that it is not a separate level touched by the Sheretz. (This question is especially difficult on Rashi's explanation, because the Gemara looks for one more level according to Shmuel and it does not simply answer that the garment is separate from the Sheretz. Moreover, according to Rashi's explanation, Ravina later in the Gemara declares that the bread mentioned in the verse is an Av ha'Tum'ah, as Rashi mentions (in DH Ravina). Food, though, can never be more than a Rishon l'Tum'ah!)
Tosfos explains that everyone was aware that a food can be Metamei only a liquid and that a liquid can be Metamei only a food, but food cannot be Metamei another food and a liquid cannot be Metamei another liquid (see Insights to Pesachim 14:1:I:d). Consequently, when Chagai said that the garment touched bread and a cooked food, he meant that it touched either bread or a cooked food, and not that the bread touched a cooked food, because bread and cooked food are both solids and one cannot be Metamei the other. Similarly, wine and oil cannot be Metamei one another because one liquid cannot be Metamei another liquid. Therefore, those two pairs are mentioned as "either/or" -- either one or the other.
According to Tosfos, the fourth and sixth items in the verse are not counted, and only five items are left.
(c) The METZUDAS DAVID writes that perhaps everyone was aware that a liquid cannot be Metamei mid'Oraisa (in accordance with the Tana in the Beraisa who is of that opinion). Accordingly, it is clear that Chagai did not mean that the wine or oil touched something else and made it Tamei. Rather, wine or oil must have been the last step in the chain of Tum'ah, because they cannot make anything after them Tamei. Therefore, the last three items -- wine, oil, or any food -- are counted as one (as "either/or").
According to this approach, the sixth and seventh items in the verse are not counted.
(d) The RADAK says that "affected meat" ("Basar Kodesh") does not refer to the Sheretz itself. The Sheretz is not mentioned in the verse, but it is implicitly understood to be the source of the Tum'ah which made the meat Tamei. The chain of transfer of Tum'ah started with the Sheretz and ended with bread: a Sheretz touched the meat (and made it a Rishon l'Tum'ah), the meat ("Basar Kodesh") touched a garment (and made it a Sheni), and then the garment touched bread (and made it a Shelishi). All of the other items listed in the verse after bread until "any other food" are alternatives to bread ("either bread or..."). "Any other food" is a separate step (it becomes Tamei as a Revi'i), and it is the fourth and final step in the chain of Tum'ah.
According to the Radak, the fourth, fifth, and sixth items in the verse are not counted, but the first item mentioned is counted, and the source of the Tum'ah, the Sheretz, is not mentioned in the verse.
2) REBBI YEHUDAH'S OPINION THAT LIQUIDS ARE "METAMEI" UTENSILS MID'ORAISA
QUESTION: The Gemara discusses the opinion of Rebbi Yehudah who says that when there is a question whether or not a liquid touched a utensil, the utensil is deemed Tamei out of doubt. The Gemara says that it must be that Rebbi Yehudah maintains that mid'Oraisa a liquid can be Metamei a utensil, because if it could not be Metamei a utensil, Rebbi Yehudah would not rule that the utensil is Tamei in a case of a doubt.
How does the Gemara know that Rebbi Yehudah's reason in a case of doubt is because he maintains that liquids can be Metamei utensils mid'Oraisa? Perhaps there is another reason why he is stringent.
One possible reason is that Rebbi Yehudah maintains that a Safek d'Rabanan in Reshus ha'Yachid is like a Safek d'Oraisa in Reshus ha'Yachid (which is Tamei), and that is why he says that a utensil is Tamei out of doubt. The RAN in Nedarim (19a) mentions such an opinion. (See SEFAS EMES.)
Another possible reason for why Rebbi Yehudah is stringent in the case of a doubt whether a Tamei liquid touched a utensil is because he is discussing utensils other than earthenware which can become Tahor by immersion in the Mikvah. Since they can become Tahor, they have the status of a "Davar she'Yesh Lo Matirin," which, in a case of doubt, is treated stringently, even with regard to laws that are d'Rabanan, as the Gemara says in Beitzah (4a). (See TZELACH.)
ANSWER: The SEFAS EMES, based on the PNEI YEHOSHUA, answers that there indeed is no evidence from the words of Rebbi Yehudah here that he maintains that liquids can be Metamei utensils mid'Oraisa. Nevertheless, the Gemara bases its assertion on the words of a Beraisa in the Toras Kohanim and in the Yalkut (Parshas Shemini), in which Rebbi Yehudah derives from a verse that liquids are Metamei utensils.
The Pnei Yehoshua adds that the Beraisa in the Toras Kohanim resolves another difficult point in the Gemara. The Gemara here concludes that Rebbi Yehudah changed his mind and rescinded his opinion that liquids are Metamei utensils mid'Oraisa. Normally, the Gemara does not simply drop a topic and say that a Tana "changed his mind." Why does the Gemara here not explain the view of Rebbi Yehudah further?
This question is resolved by the Toras Kohanim. In the Toras Kohanim, Rebbi Yehoshua ben Korchah questions the opinion of Rebbi Yehudah, and Rebbi Yehudah does not give an answer. The Toras Kohanim implies that Rebbi Yehudah conceded to the validity of the question posed to his opinion. This is the basis for the Gemara's conclusion that Rebbi Yehudah rescinded his opinion.