Gemara

(Beraisa) Question: What is Isar mentioned in the Torah ("Lesor Isar Al Nafsho" - Bamidbar 30:3)? One said 'I will not eat meat or drink wine like the day my father died... or the day that Gedalyah ben Achikam was killed...'

Nedarim 14a: We learn from "Ish Ki Yidor Neder" - Hatfasah must be in Davar ha'Nadur, not in Davar ha'Asur.

Question: We should expound "Lesor Isar" to include Hatfasah in Davar ha'Asur!

Answer: That is used to teach about Isar.

Shevu'os 20a (Beraisa): "Isar" is a Shevu'ah. If you will say that Isar is a Shevu'ah, he is liable for it. If not, he is exempt.

Objection: The Tana just said that Isar is a Shevu'ah!

Answer #1 (Abaye): The Beraisa means that Isar is Hatfasah in a Shevu'ah (forbidding something like something else forbidden by a Shevu'ah);

The prohibition of Isar: if you will say that being Matfis in a Shevu'ah is like saying a Shevu'ah, he is liable (if he transgresses it). If not, he is exempt.

Answer #2 (Rava): Really, Hatfasah in a Shevu'ah is not like saying a Shevu'ah. The Beraisa means that "Isar" is a Shevu'ah, but not always. The Torah wrote it between Neder and Shevu'ah to teach that it can be either;

If it was said like a Shevu'ah, it is a Shevu'ah. If it was said like a Neder (Alai, forbidding an object to himself), it is a Neder.

Question (against Rava - Beraisa): Examples of the Isar that the Torah discusses are 'eating meat ... is forbidden to me like the day that my father died... or the day that Gedalyah was killed...' He is forbidden.

(Shmuel): The case is, he had already taken a Neder never to eat meat or drink wine on that day.

This is not difficult for Abaye. The Beraisa explains Hatfasah in a Neder, for this takes effect just like (and is learned from) Hatfasah in a Shevu'ah.

This refutes Rava (who says that Isar is not through Hatfasah)!

Answer: The Beraisa means, the Neder that the Torah discusses is 'eating meat... are forbidden like the day...'

R. Yochanan also holds like Rava.

(Ravin citing R. Yochanan): 'Isar I will not eat your food' is a Shevu'ah.

36a (R. Avahu): It says "Va'Yavei Oso b'Alah (Nebuchadnetzar imposed an Alah on Chizkiyahu)" and it says "Asher Hishbi'o bei'Lokim" - this teaches that Alah is a Shevu'ah.

Rishonim

Rif and Rosh (Shevu'os 7b and 3:1): Abaye explains the Beraisa to say that Isar is Hatfasah in a Shevu'ah. He is liable for it only if being Matfis in a Shevu'ah is like saying a Shevu'ah. Rava explains that "Isar" is a Shevu'ah if it was said in the text of a Shevu'ah. If it was said in the text of a Neder, it is a Neder. The Halachah follows Rava.

Ran (8a, second column): Initially we thought that the Beraisa says that Isar is Hatfasah. Rava answered that really, it discusses the primary Neder of the Torah, which is Hatfasah in Davar ha'Nadur. Ultimately, the source of Hatfasah is Hekdesh, which can transfer its Kedushah. A Neder can work without Hatfasah, but only due to Yados. If one said 'this is forbidden to me', we complete his words 'like a Korban.'

Gra (YD 239:23): If "Isar" of the Torah was Hatfasah, we would be forced to say that Hatfasah in a Shevu'ah takes effect. Isar was written primarily regarding Shevu'ah - "Oh Osrah Isar Al Nafshah bi'Shevu'ah." From there, we learn to Nedarim. We conclude that Hatfasah is not learned from Isar, rather, from "Ki Yidor Neder."

Rif (ibid.): Also a Beraisa teaches that the Isar Neder mentioned in the Torah is 'I will not eat meat...like the day my father died. Shmuel explained, the case is that he previously vowed not to eat meat on that day.

Question (Rosh): Why did the Rif say that the Beraisa refers to Hatfasah in a Neder? Abaye brought the Beraisa to prove that just like Hatfasah in a Neder is a Neder, Hatfasah in a Shevu'ah is a Shevu'ah. Rava answered that the Beraisa discusses the Isar Neder; it does not discuss Hatfasah!

Answer (Rosh): The Rif explains that Abaye holds that the Beraisa explains Isar written in the Torah. It itself is not a Shevu'ah, nor a Neder. Rava explains that the Beraisa does not discuss "Isar" written in the Torah, for that is a Shevu'ah or Neder (depending on how he said it). Rather, it discusses a Neder that is not truly a Neder, i.e. Hatfasah.

Rambam (Hilchos Shevu'os 2:6): If one said 'no, no' or 'yes, yes' or 'Mivta I will do so and so' and mentioned Hash-m's name or a Kinuy, this is a Shevu'ah.

Rambam (7): If one said 'Isar to Hashem' or 'to the One whose name is Chanun (gracious)' that I will or will not do, since he said it like a Shevu'ah, it is a Shevu'ah.

Kesef Mishneh: Isar in the text of a Shevu'ah is saying 'I will do (a certain action) or 'I will not do.' It requires Hash-m's name or a Kinuy, like the Rambam said regarding 'Mivta'.

Rosh (4:24): From R. Avahu we learn that Alah and Shevu'ah require Hash-m's name. The Ramban infers that we do not. We learned that 'no, no' is a Shevu'ah. This cannot be with Hash-m's name, for then it is a Shevu'ah even without 'no, no'! R. Yochanan said 'lEi'lokei Yisrael I will not reveal'; this was considered a Shevu'ah. Perhaps without Hash-m's name is a Yad for Shevu'ah. The Ra'avad requires Hash-m's name only for lashes, but not for Korban and "Lo Yachel Devaro."

Maharshal (cited in Tif'eres Shmuel 2): Perhaps they do not argue. The Ramban requires Hash-m's name only for lashes and Korban, but not for Isur. This is unlike the Rambam and SMaG, who require an expression denoting a Shevu'ah in addition to Hash-m's name.

Radvaz (on Rambam 7): The Rambam holds that Hash-m's name without an expression denoting a Shevu'ah creates an Isur, but there are no lashes or Korban.

Rambam (Hilchos Nedarim 1:1): One kind of Neder forbids to oneself things permitted to him. E.g. 'produce of this country are forbidden to me for 30 days' or 'forever'. This is in any language, even though he did not mention Shevu'ah, Hash-m's name or a Kinuy (another name that refers to Hash-m). This is the meaning of "Lesor Isar Al Nafsho." The same applies if he said 'it is to me Isar'. This is called Nidrei Isar.

Poskim

Shulchan Aruch (YD 204:1): The primary vow the Torah discusses is Hatfasah in Davar ha'Nadur.

Beis Yosef (DH u'Mah she'Chosav Rabeinu): The Tur says so. He means that also this is the primary vow. The primary vow is when he directly forbids it on himself. Hatfasah does not always work, e.g. when it is in Davar ha'Asur.

Rebuttal (Bach DH v'Rabbeinu): The Tur holds like the Ran (above) that the primary Neder is Hatfasah. When one says 'is forbidden to me this loaf', this is only like a Yad (for 'like a Korban'). Even if he begins with the item, and says 'this loaf is forbidden to me', this is not a Neder, rather, Isar.

Shulchan Aruch (ibid.): E.g. he says '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.'

Tur (237): The Rambam requires Hash-m's name or a Kinuy with 'no, no', Mivta and Isar. The Ramban does not require it, and this is the Rosh's conclusion.

Bach (DH u'Mah she'Chosav veha'Isar): Isar requires Hash-m's name only when it is said like a Shevu'ah. The Tur (agrees, just he) wrote briefly.