1) "MA'AMID" OR "MESHANEH"
QUESTION: Rabah (26a) states that the dispute between the Rabanan and Rebbi Akiva about whether or not a Neder that was partially annulled becomes entirely annulled ("Hutar Miktzaso Hutar Kulo") applies only in a case of "Meshaneh." In a case of "Ma'amid," however, everyone agrees that the rest if the Neder remains in force. Rava disagrees with Rabah and says that the Rabanan and Rebbi Akiva argue only in a case of "Ma'amid," in which case Rebbi Akiva says that the entire Neder is annulled. In a case of "Meshaneh," however, everyone agrees that the entire Neder is annulled. (See Chart to Nedarim 26a for the definition of "Meshaneh" and "Ma'amid.")
Rav Ada bar Ahavah challenges both opinions from the Mishnah later (66a) which discusses a case of a person who makes a Neder to prohibit onions. The person makes a Neder not to eat onions because they are unhealthy for the heart, and then he is informed that the Kufri species of onion is beneficial for the heart. Rebbi Meir in the Mishnah rules that since his Neder becomes partially annulled (with regard to Kufri opinions) the entire Neder becomes annulled. The Gemara assumes that the case of the Mishnah is a case of "Ma'amid," and thus the Mishnah is a clear refutation of Rava's opinion (and certainly of Rabah's). (The Gemara rejects the question by saying that the case there is a case of "Meshaneh.")
Ravina then challenges Rava's explanation from a Beraisa in which Rebbi Nasan says that when a person makes a Neder that he will not eat the contents of a basket (because the fruit inside is of inferior quality) and then he discovers that the basket also contains fruit of superior quality, part of his Neder remains in force (and he may not eat the inferior fruit) while part of it is annulled (and he may eat the superior fruit). Rebbi Akiva argues and says that the entire Neder is annulled because "Hutar Miktzaso Hutar Kulo." The Gemara assumes that the Beraisa discusses a case of "Meshaneh" and thus it is a clear refutation of the opinion of Rava, because it says that Rebbi Akiva argues with the Rabanan even in a case of "Meshaneh."
Why does the Gemara assume in its challenge (that of Rav Ada bar Ahavah) from the Mishnah that the case of the Mishnah is a case of "Ma'amid," while in its challenge (that of Ravina) from the Beraisa it assumes that the case of the Beraisa is a case of "Meshaneh"?
(a) TOSFOS (27a) asks this question. He answers that when a person makes a Neder to prohibit himself from "onions," the word "onions" is a general term which refers to the vegetable called "onion," regardless of the species. It is logical, therefore, that even when he discovers the benefits of Kufri onions, he will uphold ("Ma'amid") his original expression in his Neder and will not change his Neder and list all of the species of onions which he intended to prohibit. When he realizes that he does not want Kufri onions to be included in his Neder, he says that had he known about Kufri onions he would have said, "All onions are prohibited to me except for Kufri onions."
In contrast, when he makes a Neder not to eat "the basket" without specifying "figs," he obviously does not mean literally that he is not going to eat the basket; the basket is made of wood and is not edible. Rather, he intends to prohibit the contents of the basket. Therefore, when he discovers that part of the contents is something that he does not want to be included in his Neder, he does not says that had he known about the superior fruit he would have phrased his Neder as, "The basket is prohibited to me, except for superior figs," because the two parts of that statement are not uniform -- "basket" and "figs" are two different categories of items altogether. Rather, he says that he would have specified each species and then said, "Except for superior figs (B'nos Shu'ach)" (or, alternatively, he says that he would have prohibited "the basket, except for the smaller container within the basket"). When he articulates an exception to his Neder, he must articulate it in terms consistent with the subject of his Neder.
(Tosfos also points out that there is a different Girsa in the Gemara, according to which the question is resolved.)
(b) The PARASHAS NEDARIM answers that the case of onions in the Mishnah later, which Rav Ada bar Ahavah quotes, is the Seifa of the same Mishnah that discusses the case of a person who made a Neder not to eat meat and then realized that he did not want to include Shabbos and Yom Tov in his Neder. Since the latter case (of Shabbos and Yom Tov) is a case of "Ma'amid" according to Rava, the Gemara logically assumes that the other case in the Mishnah (the case of onions) is also a case of "Ma'amid."
Once the Gemara answers that the case of onions there is actually a case of "Meshaneh," it is logical to assume that the cases in the Tosefta -- which discusses the case of onions together with the case of "Kalkalah" (the basket of figs) -- are also cases of "Meshaneh." The Gemara answers that the case of onions in the Tosefta is a case of "Ma'amid" while the case of onions in the Mishnah is a case of "Meshaneh."