1) THE SOURCE THAT "NESACHIM" MUST BE BROUGHT FROM "CHULIN"
OPINIONS: The Mishnah states that all obligatory Korbanos must be brought only from Chulin. A free-will Korban, or Nedavah, may be brought from Ma'aser. The Mishnah adds that this does not apply to Nesachim. All forms of Nesachim must be brought from Chulin, even when the Korban that the Nesachim accompanies is a Nedavah. RASHI (Kesav Yad, DH b'Chol Makom), the BARTENURA, and others explain that this applies even when one specifically pledged to bring a Korban and its Nesachim from Ma'aser.
Why must Nesachim be brought only from Chulin?
(a) RASHI (ibid.) and TOSFOS (DH u'Nesachim) explain that only Korbanos that are partially eaten may be brought from Ma'aser. Tosfos explains that the source for this is the Sifri (that he quotes earlier in DH Eino b'Toras Shelamim). The Sifri says that one may not buy a Korban Olah with money of Ma'aser, because the verse says about Ma'aser Sheni, "And you may spend the money for anything that your soul desires... and you shall eat there before Hash-m, your G-d, and you shall rejoice" (Devarim 14:26), which implies that one may use money of Ma'aser Sheni only to buy food that gives Simchah to a person ("Achilah she'Yesh Bah Simchah"). Tosfos explains that just as the Sifri excludes buying an Olah with money of Ma'aser, it also excludes bringing Nesachim from Ma'aser.
(b) The RAMBAM (Hilchos Ma'aseh ha'Korbanos 16:17) writes that the source to bring Nesachim only from Chulin is the verse, "The one who is bringing a Korban shall bring his Korban to Hash-m" (Bamidbar 15:4). The Rambam explains that the verse (which is discussing Nesachim) implies that the Nesachim must come entirely from the funds of the one bringing the Korban, and they may not have any connection in any way to funds that are already Kadosh.
According to Tosfos, the reason why Nesachim must be brought from Chulin is the Halachah written with regard to Ma'aser Sheni that says that Ma'aser Sheni may not be used for Nesachim. According to the Rambam, it is a Halachah in the laws of Korbanos that says that Nesachim may not be brought from Ma'aser Sheni.
Why does the Rambam not give the source that is given in the Sifri? Why does he find it necessary to present his own Derashah in order to explain the law of the Mishnah?
The CHAZON YECHEZKEL (8:14) explains that it must be that the Rambam maintains that money of Ma'aser Sheni technically may be used for Nesachim, if not for the verse written with regard to Korbanos. Why does the Rambam not learn from the verse cited by the Sifri that just as one may not buy an Olah with money of Ma'aser, one also may not bring Nesachim from Ma'aser? He answers that the Rambam understands that the Sifri specifically excludes bringing an Olah from Ma'aser, and not Nesachim, because no part of a Korban Olah is eaten, and, therefore, an Olah is not an "Achilah she'Yesh Bo Simchah" (as described above). While Nesachim, too, are not consumed at all, they are considered subordinate to the Korban with which they are brought and do not have their own identity. Since the Korban with which they are brought may sometimes be eaten (such as a Korban Todah or Korban Shelamim), Nesachim in general are considered to be an "Achilah she'Yesh Bo Simchah." The Rambam understands that this difference between an Olah and Nesachim makes the Derashah of the Sifri an insufficient source to teach that all Nesachim must be brought only from Chulin. (Y. MONTROSE)
2) WHY ONE CANNOT REFUTE A "HEKESH"
OPINIONS: The Gemara records the dispute between Rebbi Akiva and Rebbi Eliezer, who disagree about the source for the Mishnah's statement that a Korban Pesach must be brought from Chulin. Rebbi Eliezer compares the Korban Pesach brought throughout the generations ("Pesach Doros") with the original Korban Pesach that was brought in Mitzrayim ("Pesach Mitzrayim"). Just as the Korban Pesach brought in Mitzrayim was from Chulin and not from Ma'aser, every Korban Pesach for generations must be brought from Chulin.
Rebbi Akiva questions Rebbi Eliezer's logic. "Can we judge a case that is possible (bringing the Pesach Doros from Ma'aser) from a case that is not possible (bringing the Pesach Mitzrayim from Ma'aser)?" The Korban Pesach in Mitzrayim was brought from Chulin because there was no such thing as Ma'aser at that time! How can we learn from there that the Korban Pesach can never be brought from Ma'aser?
Rebbi Eliezer asserts that his proof is valid, but Rebbi Akiva asks a different question. The blood of the Korban Pesach in Mitzrayim did not need to be sprinkled on the Mizbe'ach, and its limbs did not need to be offered on the Mizbe'ach (since there was no Mizbe'ach). In contrast, these Avodos, Zerikas ha'Dam and Haktaras Eimurin, are required for the Pesach Doros. Since the Pesach Doros is more similar to a Korban Shelamim (which also requires Zerikah and Haktaras Eimurin), one should be allowed to bring it from Ma'aser, just as one brings a Korban Shelamim from Ma'aser.
Rebbi Eliezer answers that the verse says, "You shall perform this Avodah in this month" (Shemos 13:5), which implies that the Avodah of the Pesach Doros should be similar to the Korban Pesach of Mitzrayim. Just as the Korban Pesach in Mitzrayim was brought from Chulin, so, too, the Pesach Doros should be brought from Chulin.
The Gemara later asks why Rebbi Akiva did not question this teaching the same way he questioned Rebbi Eliezer's first teaching, "Can we judge a case that is possible (bringing the Pesach Doros from Ma'aser) from a case that is not possible (bringing the Pesach Mitzrayim from Ma'aser)?" The Gemara answers with a general principle, "Ein Meshivin Al ha'Hekesh" -- we cannot refute a Hekesh.
Why can a Hekesh not be refuted? (See end of Insights to Nedarim 17:1.)
(a) RASHI (DH Zos Omeres) and RABEINU GERSHOM explain that the reason why a Hekesh is irrefutable is that one may not formulate a Hekesh on one's own accord. A Hekesh must be received through tradition from one's teacher, who received it from his teacher, and so on back to Moshe Rabeinu at Sinai. Since a Hekesh comes from Sinai, it is irrefutable. This is also the explanation of Rashi in Sukah (31a, DH Lo Makshinan) and in Sanhedrin (73a, DH Hekeisha).
However, Rashi in Rosh Hashanah (34a, DH Hachi Ka'amar) implies that one may
formulate a Hekesh on his own. This is also apparent from Rashi in Gitin (41b, DH d'Chulei Alma). The Gemara there discusses which is more powerful, a Hekesh or a Gezeirah Shavah. Rashi explains that when the Gemara says that everyone agrees that a Gezeirah Shavah is more powerful because it cannot be refuted, this is "because it is taught from Sinai, for a person cannot teach a Gezeirah Shavah by himself, and thus it is stronger than a Hekesh." This is also the view of the RITVA
in Rosh Hashanah (34a) and TOSFOS
in Sukah (31a, DH v'Rebbi Yehudah Savar). Tosfos there writes that all of the thirteen Midos sheha'Torah Nidreshes ba'Hen (the thirteen exegetical principles of expounding Torah law) may be utilized without an oral tradition from Sinai, except for a Gezeirah Shavah. (See Insights to Rosh Hashanah 34:1
According to this opinion, why is a Hekesh irrefutable?
The HALICHOS OLAM (4:2) explains that a Hekesh is tantamount to a law written explicitly in the Torah, and therefore it cannot be refuted.
How, though, are we to understand the apparent contradiction in the words of Rashi?
1. The MISHNEH L'MELECH (Hilchos Korban Pesach 2:13) suggests in his first answer that Rashi in fact maintains that one may formulate a Hekesh by himself. The Gemara in Sukah is expressing only the view of Rebbi Yehudah.
However, the IGROS MOSHE (OC 1:16) points out that this explanation is problematic, because Rashi here and Rashi in Sanhedrin also state that one may not teach a Hekesh on his own accord.
2. The Mishneh l'Melech suggests a second explanation. Rashi indeed maintains that one may not formulate a Hekesh on his own. When Rashi in Gitin writes that a Gezeirah Shavah is stronger than a Hekesh because it is taught from Sinai, he means that a Gezeirah Shavah is also from Sinai and cannot be formulated on one's own accord.
How, though, are we to understand the words of Rashi in Rosh Hashanah? The Igros Moshe answers that when Rashi mentions in Rosh Hashanah that a Hekesh may be taught on one's own, he is not referring to the type of Hekesh transmitted from Sinai, which is irrefutable. Rather, he is referring to a Hekesh that one makes on his own. Such a Hekesh does not carry the same weight as a Hekesh from Har Sinai. (See also ARUCH LA'NER to Sukah 31a.) (Y. MONTROSE)