1) COMBINING TWO TYPES OF "NEVEILOS"
QUESTIONS: The Mishnah (15b) states that "all types of Neveilos join with each other." RASHI there (DH Kol ha'Neveilos) explains that half of a k'Zayis of Neveilah of a Kosher animal combines with half of a k'Zayis of a Neveilah of a non-Kosher animal to make a full Shi'ur of a k'Zayis. With regard to what law do they combine? Rav maintains that they combine only with regard to making a Shi'ur to be Metamei, but not with regard to the Isur of eating Neveilah. Levi argues and maintains that they combine even with regard to the Isur of eating Neveilah. Rav Asi maintains that Neveilah of Kosher animals combine, and Neveilah of non-Kosher animals combine. (The Gemara understands that Rav Asi either agrees entirely with Rav, or that he argues and maintains that even with regard to Tum'ah they do not combine.)
Rashi explains that Levi is of the opinion that "Isur Chal Al Isur" (a second prohibition cannot take effect on something that is already forbidden by another prohibition), and that is why the Isur of Neveilah takes effect on the meat of a non-Kosher animal even though that meat is already forbidden because of the Isur of eating a non-Kosher animal.
There are two basic questions on this Gemara:
(a) If the argument between Rav and Levi is based on whether "Isur Chal Al Isur" or not, then why does the Gemara attempt to prove from various Beraisos which opinion is the correct one? The argument between Rav and Levi depends on the argument between Tana'im about whether "Isur Chal Al Isur" or not, which is cited in a number of places throughout the Gemara!
(b) When the Gemara challenges the opinion of Rav Asi (or Rav, if Rav Asi's opinion is the same as Rav's) from a Beraisa, the Gemara answers that the Beraisa follows the view that "Isur Chal Al Isur," implying that all of the Amora'im here are arguing according to the opinion that maintains "Ein Isur Chal Al Isur"! (TOSFOS DH Amar)
ANSWER: TOSFOS explains that Levi agrees that in general "Ein Isur Chal Al Isur." However, Levi maintains that the verse, "v'Chelev Neveilah" (Vayikra 7:24), teaches that the prohibition of Neveilah is an exception to the rule and it does take effect even on an object that is already forbidden because of a different prohibition, such as Chelev.
Rav argues and understands that this verse teaches only that the Isur of Neveilah can take effect on meat that is already forbidden because of the Isur of Chelev; it does not teach that Neveilah takes effect on other prohibitions. Levi disagrees and asserts that the verse implies that the Isur of Neveilah takes effect on top of any other prohibition, including the prohibition of eating non-Kosher animals.
2) THE "SHI'UR" OF "SHERATZIM" FOR WHICH ONE IS LIABLE
OPINIONS: Rav Yehudah in the name of Rav says that one is punished with Malkus for eating a k'Zayis of forbidden Sheratzim. This is because the Torah forbids eating Sheratzim with the word "Achilah" (Vayikra 11:42), and "Achilah" always refers to an amount of a k'Zayis.
The Gemara challenges this from a Beraisa taught by Rebbi Yosi b'Rebbi Chanina and praised by Rebbi Yochanan. The Beraisa derives from a verse (Vayikra 20:25) that just as Sheratzim are Metamei with a Shi'ur of a k'Adashah, one is punished with Malkus for eating a k'Adashah of Sheratzim. The Gemara answers that the Beraisa is referring to the Shi'ur for eating a dead Sheretz, while Rav is referring to the Shi'ur for eating a live Sheretz.
The Gemara then questions how Rebbi Yochanan could have praised the Beraisa, which says that one receives Malkus for eating a k'Adashah of [dead] Sheratzim, when Rebbi Yochanan himself said elsewhere that one receives Malkus only for eating a k'Zayis of [dead] Sheratzim. Rava answers that "b'Muvdalin Diber ha'Kasuv."
What is the meaning of Rava's answer?
(a) RASHI (DH Amar) explains that when Rebbi Yochanan praised the statement that one is liable for eating a k'Adashah of Sheratzim, he was referring only to the eight Sheratzim that are "Muvdalin" -- the Sheratzim mentioned explicitly in the Torah (Vayikra 11:29-36), which are the only Sheratzim that are Metamei. One who eats any other type of Sheratzim is not Chayav Malkus until he eats a k'Zayis.
(b) TOSFOS (DH Amar) explains that "Muvdalin" refers to all Sheratzim and not only to the eight mentioned explicitly in the Torah. The Shi'ur for eating all Sheratzim is a k'Adashah, while the Shi'ur for eating forbidden animals is a k'Zayis.
According to Tosfos, "Muvdalin" means that when the Torah sets apart some Sheratzim, it implies that all Sheratzim can be distinguished from animals, and, therefore, the Shi'ur for all Sheratzim is different from the Shi'ur for animals.
RASHI and TOSFOS in Zevachim (106b) both give the opposite explanations from what they write here. The Gemara there states that the Isur of ordinary Sheratzim (that are not Metamei) applies even in the most minimal quantity (a "Mah'shehu"). Rashi there (DH she'Ken) explains that the Shi'ur for the Isur of eating Sheratzim that are not Metamei is a k'Adashah (that is, when the Gemara says a "Mah'shehu," it does not mean that there is no minimum amount, but rather that the minimum amount is very small). Rashi there cites the Gemara here and explains that it means that the Shi'ur of Tum'ah of the eight Sheratzim that are Metamei is the same as the Shi'ur for eating all other Sheratzim. Since the Shi'ur for Tum'as Sheratzim is a k'Adashah, the Shi'ur for the Isur of eating all Sheratzim is also a k'Adashah.
Tosfos there (DH Mah) disagrees with Rashi. He asserts that the Gemara here is referring only to the eight types of Sheratzim that are Metamei. Tosfos there maintains that the Shi'ur for eating the eight Sheratzim is a k'Adashah, while the Shi'ur for eating all other Sheratzim is a k'Zayis. (D. BLOOM)