1) THE SOURCE FOR THE PRINCIPLES OF "TA'AM K'IKAR" AND "HETER MITZTAREF L'ISUR"
QUESTION: Abaye challenges Rav Dimi's view and asks that perhaps "Heter Mitztaref l'Isur" does not apply to the Isurim of Nazir at all. If the only source for "Heter Mitztaref l'Isur" is the verse of "Mishras" (Bamidbar 6:3), perhaps the verse teaches the principle of "Ta'am k'Ikar" and not "Heter Mitztaref l'Isur."
The Gemara asks why Abaye asks such a question on the view of Rav Dimi. Earlier, Abaye attempted to prove to Rav Dimi that "Heter Mitztaref l'Isur" applies to all Isurim, and now Abaye asks that it should not apply even to Nazir! The Gemara answers that after Rav Dimi answered all of Abaye's questions and showed that "Heter Mitztaref l'Isur" does not apply in all cases, Abaye responds by asking that perhaps it applies nowhere.
When Abaye originally asked that perhaps "Heter Mitztaref l'Isur" applies for all Isurim, Tosfos repeatedly emphasizes that Abaye's proposal was only according to Rebbi Eliezer, who derives the rule of "Heter Mitztaref l'Isur" from "Kol Machmetzes" (Shemos 12:20). According to the Rabanan who derive "Heter Mitztaref l'Isur" from "Mishras," Abaye was willing to accept that it applies only to Nazir (TOSFOS 36b, DH Iy Hachi and DH Is Sefarim.) (This explanation of Tosfos depends on the Girsa in the Gemara on 36b. According to some texts, the Gemara challenges Abaye's question from the case of "Shtei Maduchos" by saying "ul'Didach," as Tosfos in Pesachim (44a, DH Ela Mai) points out.) Why, then, is the Gemara perplexed with Abaye's question that "Mishras" might teach "Ta'am k'Ikar" and not "Heter Mitztaref l'Isur"? Abaye's question does not contradict his earlier statement! His earlier statement was that according to Rebbi Eliezer "Heter Mitztaref l'Isur" applies to all Isurim, but perhaps Abaye understood that according to the Rabanan "Heter Mitztaref l'Isur" does not apply to all Isurim because "Mishras" teaches "Ta'am k'Ikar"! (ARZEI HA'LEVANON, fn. 109)
ANSWER: TOSFOS (DH Ta'am k'Ikar) explains that Abaye's question -- that "Mishras" might teach "Ta'am k'Ikar" -- is based on the premise that the Halachah of "Ta'am k'Ikar" is a more rational Halachah than "Heter Mitztaref l'Isur." If there is only one verse available to teach either "Heter Mitztaref l'Isur" or "Ta'am k'Ikar," the verse should be used to teach "Ta'am k'Ikar" and not "Heter Mitztaref l'Isur." Since Abaye apparently had no other source for "Ta'am k'Ikar" other than the verse of "Mishras," he asked that perhaps the verse teaches "Ta'am k'Ikar" and not "Heter Mitztaref l'Isur."
If this is Abaye's intent, the Gemara's question on Abaye is as follows. When Rebbi Eliezer states (in a Beraisa, Pesachim 43a) that he derives from "Kol Machmetzes" that "Kutach ha'Bavli" is considered Chametz and is forbidden, perhaps he means that it is forbidden because of "Ta'am k'Ikar," but not because of "Heter Mitztaref l'Isur." If that is Rebbi Eliezer's intention, there remains no source for the principle of "Heter Mitztaref l'Isur." This is problematic according to Abaye, who says earlier that "Heter Mitztaref l'Isur" applies to every Isur according to Rebbi Eliezer! (Rather, Abaye should have said that according to Rebbi Eliezer, "Ta'am k'Ikar" is derived from the verse of "Kol Machmetzes," and "Heter Mitztaref l'Isur" is derived from the verse of Nazir ("Mishras"). "Heter Mitztaref l'Isur" for all other Isurim cannot be derived from Nazir, either because the concept of Nazir is a Chidush or because of "Shnei Kesuvin ha'Ba'in k'Echad," as the Gemara later (37b) says, and as Abaye himself maintains when he admits that according to the Rabanan "Heter Mitztaref l'Isur" does not apply to other Isurim.) (See Insights to Pesachim 44:1.)

37b----------------------------------------37b

2) DERIVING THE PRINCIPLES OF "TA'AM K'IKAR" AND "HETER MITZTAREF L'ISUR" FROM KORBAN CHATAS
QUESTION: The Gemara derives from the laws of the Korban Chatas either the principle of "Ta'am k'Ikar" or the principle of "Heter Mitztaref l'Isur."
How can the Gemara learn these Halachos from the laws of the Korban Chatas?
The verse teaches that when the meat of a Chatas touches another Korban and the other Korban absorbs the taste of the Chatas, the other Korban becomes forbidden like the Chatas. However, the taste of both Korbanos is the taste of meat, and thus when one absorbs the taste of the other the mixture is "Min b'Mino" -- to which the laws of "Ta'am k'Ikar" and "Heter Mitztaref l'Isur" do not apply! (KEREN ORAH; ARZEI HA'LEVANON citing RAV SHMUEL ROZOVSKY, Chidushim to Pesachim)
ANSWERS:
(a) The KEREN ORAH suggests that according to the opinion that Chelev and other fats are considered different Minim with different tastes, the answer is clear: one Korban's meat absorbs the taste of the other Korban's Chelev. This opinion, however, is not unanimous (see SHACH YD 98:1).
(b) Perhaps a simple answer may be suggested. The verse teaches that when any two Korbanos of different types (such as a Chatas and a Shelamim) touch each other, each one absorbs the taste of the other, even when one Korban is a goat and the other is a sheep or cow. The Gemara in Pesachim (88b) states that different animals have different tastes, as the BEIS YOSEF (YD 98) rules. Consequently, the absorbed taste is considered "Min b'she'Eino Mino."

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