Gemara

(Rami bar Chama) Question: If a Chulin piece was next to a piece of Shelamim after Zerikah (throwing the blood on the Mizbe'ach), and he said "this (the Chulin) is like this", what is the law?

Is he Matfis in the initial Isur (before Zerikah), or in its (current) Heter?

Answer #1 (Rava - Mishnah): (If one said...) 'Nosar', or 'Pigul' (he forbids.)

These are after Zerikah, and it is still binding. This shows that one is Matfis in the initial Isur.

Rejection (Rav Huna brei d'Rav Noson): Perhaps the Mishnah discusses Nosar of an Olah (it is always forbidden). This is a bigger Chidush than meat of an Olah:

Not only Hatfasah in an Olah is forbidden, for surely he is Matfis in Davar ha'Nadur (something that became forbidden through a vow);

Rather, it forbids Nosar and Pigul of an Olah. One might have thought that he is Matfis in the Isur of Nosar or Pigul, i.e. Hatfasah in Davar ha'Asur (something forbidden, but not due to a vow), which does not forbid.

13b (Mishnah): The following Nedarim are permitted:

He said 'what I will eat of yours is like Chulin', or 'it is like pork, idolatry...'

If one told his wife "you are (forbidden) to me like my mother," we permit it through a different Pesach, lest he take the matter lightly.

14a: (Ravina): The Mishnah means that the following are permitted like Chulin: 'like pork, idolatry...' Had it not said 'like Chulin', one might have thought that She'elah (asking a Chacham to permit the vow) is required.

Rejection: The Seifa requires a Pesach for one who forbade his wife. This implies that in the Reisha, we do not require She'elah!

Conclusion: We must say that Chulin was taught needlessly.

"Ish Ki Yidor Neder" teaches that Hatfasah must be in Davar ha'Nadur, not in Davar ha'Asur.

(Mishnah): If one told his wife "you are forbidden to me like my mother" (we permit it through a different Pesach).

Contradiction (Beraisa): If a man told his wife "you are forbidden to me like my mother", or "my sister, like Orlah, or Kilayim", this has no effect.

Answer #1 (Abaye): This has no effect mid'Oraisa, but mid'Rabanan we require She'elah.

Answer #2 (Rava): The Beraisa discusses a Chacham. Our Mishnah discusses an Am Ha'aretz (ignoramus).

Support: Also elsewhere we find that an Am ha'Aretz must permit his vow (lest he take vows lightly), but a Talmid Chacham need not:

(Beraisa): If one vowed (really, swore) b'Torah (by the Torah), it does not take effect.

(R. Yochanan): He must ask a Chacham to permit it.

(R. Nachman): If a Chacham swore b'Torah, he need not permit it.

Rishonim

The Rif and Rosh (2:1) bring the Gemara on 14a.

Rosh: "Ki Yidor Neder" teaches that Hatfasah must be in Davar ha'Nadur, i.e. something that became Kadosh through speech. Hatfasah in a vow is like vowing (Nazir 20b). One can forbid meat to himself like he did on a previous day. An Am ha'Aretz needs She'elah only when he forbade his wife like Davar ha'Asur. If he forbade food and drink like Davar ha'Asur, he does not need She'elah. This stringency was taught only in the Seifa (forbidding a wife), not in the Reisha.

Rambam (Hilchos Nedarim 1:8): If one said 'this produce, or this species, or what I will eat with Ploni, is like pork, idolatry, Treifos, or similar matters, it is permitted. It is not a Neder, because something that is not pork cannot become pork.

Rambam (9): The rule is, if one says that something permitted should be like something forbidden, this forbids only if the latter can be forbidden through a vow.

Rambam (2:13): If one told his wife 'you are (forbidden) to me like my mother', or said 'this produce is to me like pork', this is not a vow. If the Noder is a Chacham, he does not She'elah. If he is an Am ha'Aretz, he must ask a Chacham to permit it, and the Chacham makes him think that his wife or the produce is forbidden. We permit him through a different Pesach, lest he be careless about Nedarim.

Rebuttal (Kesef Mishneh): We could say that the Tana waited until the Seifa to teach that an Am ha'Aretz needs She'elah. However, the Gemara inferred that She'elah is required only in the Seifa, but not in the Reisha! I do not know why the Rosh 'inferred' this himself, without citing the Gemara.

Answer #1 (Lechem Mishneh): The Rambam explains that without 'Chulin', one might have thought that everyone needs She'elah for any vow. Regarding a wife, this inhibits fulfilling his marital obligations, so we exempt a Chacham from She'elah. This is rejected, for surely the Seifa (only an Am ha'Aretz needs She'elah) applies also to the Reisha.

Answer #2 (Gra YD 205): The Rambam's text said only 'the Seifa requires a Pesach' (and we learn to the Reisha, therefore 'Chulin' is unnecessary).

Perush ha'Rosh (13b DH Harei): We require a Pesach other than mere regret.

Rosh (14a DH veha'Tanya): The reason an Am ha'Aretz must ask is not lest he take other vows lightly. If so, he should need to ask even when he forbade food!

Poskim

Shulchan Aruch (YD 204:1): Examples of Hatfasah in Davar ha'Nadur are 'this loaf is forbidden like a Korban' or 'it will be forbidden to me' or 'it is Isar to me.' The same applies if he forbade a loaf and made it like Davar ha'Nadur, and said 'this is like this.' Even if there are 100, all are forbidden. Or, if he vowed to fast one day, or not to eat meat that day, and he said that another day should be like that day, it is forbidden.

Beis Yosef (DH u'Mah she'Chosav Oh she'Nodar): The Rosh says that he need not mention what he forbade that day. This is like a Tosefta (1:7), which says that 'this is upon me' forbids, even though he did not mention Korban.

Gra (3 DH v'Chen): This is unlike Rashi (Shevu'os 20b DH Taritz), who says that Rava holds that Hatfasah does not work even in Nedarim. He defines Hatfasah to be when he did not specify the Isur (I will not eat), rather, he said only 'this is like this.'

Rema: There are distinctions regarding the wording of the vow. The Shulchan Aruch did not write them. See inside (the Beis Yosef).

Shulchan Aruch (205:1): The following are permitted. If one said 'this produce, or this species, or what I will eat of yours, is like pork, idolatry, or similar matters. The same applies to one who told his wife 'you are forbidden to me like my mother or like Orlah.' If the Noder was an Am ha'Aretz, he needs She'elah. The Chacham makes him think that his wife and the produce is forbidden. We are stringent on him, and do not permit through regret. Rather, he must find a different Pesach and we permit the vow, lest he be take Nedarim lightly.

Rema: Some say that nowadays, everyone is considered an Am ha'Aretz, for most people are not Bnei Torah. However, some say that even an Am ha'Aretz need ask only when he forbade his wife, but not for other things.

Source (Ran 13b DH ha'Omer): We decree about a wife. Such vows are common, because people are exacting, and perhaps he will come to forbid her like Davar ha'Nadur. We do not decree about other Nedarim, for they are not common. We are not Pose'ach with the honor of his mother, i.e. had you known that it is dishonorable for your mother would you have vowed.

Prishah (3): We are more stringent about a wife, for such vows could inhibit Peru u'Rvu.

Shach (3): Even regret from the beginning does not suffice.

Shach (4): The Bach says that nowadays we are all Amei ha'Aretz regarding all vows. The Rema's opinion is primary. He is supported by the Gemara, the Rosh, Ran and R. Yerucham.