Gemara

(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 Hatfasah 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 a Shevu'ah (If it was said in the text of a Shevu'ah, forbidding an action), or a Neder, if it was said in the text of a Neder (Alai, forbidding an object to himself).

Rishonim

Rif and Rosh (3:1): Abaye explains the Beraisa to say that Isar is Hatfasah in a Shevu'ah. He is liable for it only if Hatfasah 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. There is no Hatfasah in a Shevu'ah. Abaye and Rava explain according to what they taught elsewhere. Abaye taught that Hatfasah in a Shevu'ah is like saying a Shevu'ah, and Rava said that it is not. The Halachah follows Rava.

Ran (8a, second column): Rava holds that Hatfasah in a Neder works, for it forbids the item. A Shevu'ah forbids the person, not the item, so Hatfasah in the item does not work. However, if Reuven swore not to eat meat and Shimon said 'also I am like him', Hatfasah in a person could work. The Rif says that even this does not work.

Question (Ran): Why don't we learn from Nedarim, just like we learn Yados (partial statements; one who says a Yad is like one who said the entire vow) from Nedarim to Shevuos? Even though we cannot learn regarding Hatfasah in an object, we can learn regarding Hatfasah in a person!

Answer (Ran): Rather, initially we thought that the Beraisa teaches that Isar is Hatfasah. Rava answered that really, it discusses the primary Neder of the Torah, which is Hatfasah in Davar ha'Nadur (something forbidden through a vow). 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.' Since all Nedarim are based on Hatfasah (in Hekdesh), we cannot learn to Shevuos. There is no source about which we could say that all Shevuos are Matfis in that source. It says "Oh Hishava Shevu'ah" for parallel structure with "Ki Yidor Neder." Perhaps Hatfasah in a person who swore is not applicable, because this does not give to him a new status, the way that a Neder makes something Kodesh.

Question: Why doesn't it forbid like a Yad for Shevu'ah?

Answer (Ran): It is not a Yad because he said all that he wanted to say. Nevertheless, I am stringent about Hatfasah in a person who swore. Abaye and Rava argue in a case in which one swore not to eat meat, and then said 'this bread is to me like this meat.'

Rosh: 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. We learn from "Ki Yidor Neder" that Hatfasah in Davar ha'Nadur works, but not in Davar ha'Asur (something forbidden not due to a vow), and not Hatfasah in a Shevu'ah.

Pilpulei Charifta (6): This is from the fact that the Torah taught this only regarding Neder.

Rosh: R. Yosef ha'Levi says that Hatfasah in a Shevu'ah is not like a Shevu'ah regarding Korban, but it forbids. Also the Ra'avad says that "Oh Hishava Shevu'ah" includes Hatfasah in a Shevu'ah regarding "Lo Yachel Devaro", but not regarding Korban.

Ramban (Nedarim 4a): If Rava agreed that Hatfasah in a Shevu'ah forbids, he and Abaye would have argued about whether or not one is liable for it. 'It is not like saying a Shevu'ah' connotes that it is not a Shevu'ah at all. (Even if it would connote otherwise,) perhaps Rava said this to parallel Abaye's words.

Ran (Sof 7b and Reish 8b): It seems that the Rif holds like the Ramban. I say that the Gemara did not say 'it is not a Shevu'ah at all' because sometimes it takes effect, i.e. Hatfasah in a person, which works due to Hatfasah or Yados. Hatfasah in an object does not work at all. Perhaps we forbid Hatfasah in an object mid'Rabanan, due to Hatfasah in a person, which is mid'Oraisa.

Rosh (10 and Nedarim 2:1): Abaye holds that Hatfasah in a Shevu'ah is like swearing, but Rava disagrees. It does not help, because a Shevu'ah forbids the person to an item, and not the item. A Neder forbids an item to himself. Hfh in a Neder is a Neder, for he forbade a loaf to himself, and makes another loaf like the first. Hfh in a Shevu'ah is not a Shevu'ah, for if he swore not to eat a loaf, and makes another loaf like it, there is no Isur in the first loaf. If he said 'I swear about this loaf just like I swore about that one', that is a new Shevu'ah. It is not hfh. The first oath merely spares him from explicitly saying that the oath is not to eat it. If one swore to fast one day, and later said 'today is like that day', it has no effect, for the day has no Kedushah.

Rambam (Hilchos Shevuos 2:8): If Levi heard David swear, and said 'I am like you', since he did not say 'Shevu'ah', and David did not impose an oath on him, he is exempt (if he transgresses). This is Hatfasah in a Shevu'ah, which is exempt.

Rambam (9): Similarly, if swore not to eat this meat, then said 'this loaf is like this meat', he is exempt (if he eats the loaf), for he did not say a Shevu'ah. He was only Matfis. Even though he is exempt from lashes and Korban, it is forbidden to eat the loaf that he was Matfis in a Shevu'ah.

Hagahos Ba'al ha'Hashlamah: Some say that Rava exempts only from a Korban, but there are lashes for "Lo Yachel Devaro."

Poskim

Shulchan Aruch (YD 239:9): One who is Matfis in a Shevu'ah is exempt. E.g., if he swore not to eat a loaf, and said about another 'this is like the forbidden loaf', he is exempt from lashes and Korban.

Gra (YD 239:23): If "Isar" of the Torah were 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."

Shulchan Aruch (ibid.): Nevertheless, there is an Isur on the latter loaf.

Gra (10): The Gemara said 'it is not like saying a Shevu'ah.' It did not say 'it is nothing.'

Shulchan Aruch (10): If Levi heard David swear, and said 'I am like you', he is forbidden.

Question (Simlas Binyamin): Why did the Shulchan Aruch teach this? Even Hatfasah in an object forbids. Nowadays, in any case there are no lashes!

Answer (Etzei Levonah): This is a proper Shevu'ah, with lashes. One who transgresses a Lav with lashes is disqualified from testimony (CM 34:12).