Gemara

25b (Mishnah): We are Pose'ach with Shabbos and Yom Tov (he overlooked that his vow will include these days).

At first, Chachamim permitted those days and forbade other days. Later, R. Akiva taught that a vow that was partially permitted is totally permitted.

(Rabah): (A man saw people eating his fruits, and forbade them through a vow. Later, he saw that his father was among them.) If he says 'had I known that father was among them, I would have said 'they are all forbidden except for father'', Beis Shamai and Beis Hillel agree that are all forbidden except for his father;

They argue about when he says 'had I known, I would have said that Ploni and Almoni are forbidden, and father is permitted'.

(Rava): Beis Shamai and Beis Hillel agree that if he says 'had I known, I would have said 'Ploni and Almoni are forbidden, and father is permitted'', they are all permitted;

They argue about when he says 'had I known, I would have said 'they are all forbidden except for father.''

(R. Meir and R. Yosi argue about a self-contradictory statement, e.g. 'this animal is Temuras (in place of) a Korban Olah (it is) Temuras Shelamim.') Beis Shamai hold like R. Meir, who says that that his first words are primary. (Therefore, we his latter words do not nullify his first words;)

Beis Hillel hold like R. Yosi, who says that his last words are primary. (This is like a partially permitted Neder, which is totally permitted.)

Question (Mishnah): In what case did R. Akiva say that a partially permitted vow is entirely permitted? If one said 'Konam, none of you may benefit from me', and he became permitted to one of them, he is permitted to all of them;

If he said 'may not benefit from me - not this one, nor this...', and the first was permitted, all are permitted. If the last was permitted, the others remain forbidden.

According to Rava, why is the Seifa only R. Akiva's opinion? Rava holds that when he says 'had I known, I would have said 'they are forbidden to this one and to this one...'', all agree!

Suggestion: Rabah can establish the Seifa to be when he retracted and said 'had I known, I would have said 'they are forbidden to all of you, except for...''

Rejection: If he said 'to all of you', it is inappropriate to discuss permitting the first or last!

Answer (for Rabah and Rava): In the Seifa, he made each dependent on the previous - 'they are forbidden to Shimon. Levi is (forbidden) like Shimon; Yehudah is like Levi...'

Support (Beraisa): If the middle one was permitted, the later ones are also permitted, but the earlier ones are forbidden.

(Beraisa): If Reuven took one vow not to benefit from five people, and one of them was permitted, all five are permitted. If he said '(had I known, I would have said) 'all are forbidden except for Ploni'', only Ploni is permitted.

According to Rabah, the Reisha is like R. Akiva, and all agree to the Seifa. According to Rava, the Seifa is like Chachamim, and all agree to the Reisha.

Rishonim

The Rif brings the opinions of Rabah and Rava, and how we answer the questions against them.

Rambam (Hilchos Nedarim 8:6): Any vow or oath that was partially permitted is totally permitted. E.g., a man saw people eating his fruits and said 'they are forbidden to them.' Later, he saw that his father was among them. All are permitted, even if he says 'had I known that father was among them, I would have said 'Ploni and Almoni are forbidden, and father is permitted.'' However, if he says 'I would have said 'they are all forbidden except for father'', the others are forbidden. He revealed that he did not permit part of the Neder. Rather, he would have taken the same vow, and stipulated about his father.

Rosh (3:8): The Halachah follows R. Akiva and Beis Hillel. Rava holds that in every case, R. Akiva totally permits a partially permitted vow. Rabah holds that this is only if he says that he would have reversed his words (initially he forbade 'all of them', and he would have forbidden each individually). It is not clear whether the Halachah follows Rava, who is Basra, or his Rebbi, Rabah. Therefore, we are stringent.

Birkei Yosef (YD 322:1, Shirei Berachah 1): The Halachah does not follow a Talmid against his Rebbi when they argued face to face. The Safek is whether or not Rabah and Rava argued about this face to face.

Rosh (ibid.): According to Rabah, if a man forbade two things that were not in front of him, e.g. meat and wine, and later he regretted one of them, it is not clear whether or not the other is permitted. Perhaps Rabah requires reversal only where it applies. Or, perhaps we do not distinguish: the entire vow is permitted only if he would have reversed it.

Rosh (ibid.): The Rambam totally permits a partially permitted vow only regarding Nidrei Shegegos of our Mishnah, like Beis Hillel, being Pose'ach with Shabbos and Yom Tov, and everything similar. Any vow permitted like this is like a mistake, since he says 'had I known...' If a Neder was totally uprooted through regret, only what he regrets is permitted, and the rest is forbidden.

Ran (26b DH ul'Inyan): Some forbid Ma'amid (when he later says that he would have vowed similar to his first vow) only in a case like the Mishnah, when he did not know that his father was there. The Neder was not partially permitted, for his father was never included. However, if all were included and he requested (a Chacham) to permit one of them, like the Mishnah 'we are Pose'ach...', since one was permitted, all are permitted. The Ramban disagrees. We permit all only when he changed the vow; I agree.

Poskim

Shulchan Aruch (232:7): If a man saw people eating his fruits from afar, and said 'they are forbidden to them', and later he saw that they are his father or brothers, whom he did not want to forbid, it is not a vow. Even though he did not specify, it is known that one does not forbid his fruits on his father or brothers. Even if strangers were with them, since the vow is Batel regarding his father and brothers, it is Batel also regarding the strangers.

Ran (25b DH u'Veis Hillel): He need not permit the vow, like we say about the four vows. This is because had he known that his father or brothers were among them, he would have excluded them from the Kelal (grouping). There is a mistake in the vow itself. His mouth and heart were not united, so the vow is Batel. This is unlike 'I forbid my wife to benefit from me.' If he does not say 'because she stole my wallet', it takes effect, even though he vowed only because he (mistakenly) thought that she did. There, he intended to forbid her; the mistake is not in the Neder itself.

Taz (13): 'He did not specify' means that he did not say that he forbids them because they are strangers. Once, someone vowed to fast tomorrow because he thought that tomorrow is Thursday, and he found out that it is Wednesday. This is like one who did not realize that his father is eating. He had no intent to forbid Wednesday; this is a mistake in the Neder itself. However, 'it is known that one does not forbid his father' connotes that it must be known to all. If not, this is like one who vowed because he thought that his wife stole his wallet. If the vow did not say so explicitly, he must permit it. When one realizes that he erred, his vow is permitted. He must reverse his words only when he still wants a vow.

Shulchan Aruch (8): This is when he reversed his words. At first he said 'all of you are forbidden.' Later, he said 'had I known that father was among you, I would have said 'Ploni and Almoni are forbidden, and father is permitted.'' Or, if he first said 'they are forbidden to this one and this one...', and later he said 'had I known, I would have said 'all of you are forbidden, except for father.''

Ran (26a DH Eisivei): In the first case he retracted from 'all of you', for he does not want to include his father even for a moment, before concluding 'except for father.' In the latter case he retracted from 'Ploni and Almoni are forbidden...', lest one think that he will forbid also his father.

Shulchan Aruch (ibid.): However, if he said 'all of you are forbidden', and later he also said 'had I known, I would have said that all of you are forbidden, except for father', the others are forbidden. The same applies if he first said 'they are forbidden to this one and this one...', and later said 'had I known, I would have said 'Ploni and Almoni are forbidden, and father is permitted.''