(Mishnah): If one swore not to fulfill a Mitzvah and fulfilled it, he is exempt. If he swore to fulfill a Mitzvah and did not fulfill it, he is exempt;

One might have thought to obligate him from a Kal va'Chomer, like R. Yehudah ben Beseira does;

R. Yehudah ben Beseira: Reshus, one is not bound to do it due to the oath of Sinai, and he is liable for it [for Shevu'as Bituy]. A Mitzvah, for which he is already bound by the oath of Sinai, all the more so he should be liable for it [for Bituy]!

Chachamim: No - if you say [that he is liable for Bituy] for SHEVUOS of Reshus, which apply in the negative just like in the positive, will you say so about SHEVUOS about Mitzvos, for which the negative is unlike the positive?! (One may not swear to be Mevatel a Mitzvah.)

(Gemara - R. Mani): The text of our Mishnah must say [if he swore to fulfill a Mitzvah and did not fulfill it, he is] liable; R. Yehudah ben Beseira says so. (According to our text, which says exempt, how can we say that this is R. Yehudah ben Beseira's words?!)

Rebuttal: (The text can say that he is exempt.) R. Yehudah ben Beseira said about a Shevu'ah of Isurim, e.g. he swore "I will eat Neveilah" and he did not eat, he is exempt. [The same applies] if he said "I will not eat [Neveilah]" and he ate;

If you will say that [R. Yehudah] obligates, Chachamim should have challenged him "no - if you will say that [one is liable for Shevu'as Bituy] for Reshus, for which the negative is like the positive and the positive is like the negative - will you say so about Shevu'ah of a Mitzvah, in which the negative is unlike the positive?!"

Question: What is the warning [not to transgress] a Shevu'ah of Reshus?

Answer (R. Yosi bei R. Bun): "Lo Yachel Devaro" - he may not make his words profane (false).

(Mishnah): If one said 'I swear that I will not eat this loaf, I swear that I will not eat it, I swear that I will not eat it', if he eats it, he is liable only once;

This is the oath of Bituy for which one is lashed if he transgressed b'Mezid, and brings an Oleh v'Yored if he was Shogeg.

(Gemara): If one said 'I swear that I will not eat this loaf today', and he did not eat it, he is not lashed, R. Yochanan and Reish Lakish both exempt, but for different reasons;

R. Yochanan exempts because he cannot receive [definite] warning (perhaps he will eat the loaf later today). Reish Lakish exempts because he transgress a Lav without an action.

What do they argue about? If he burned [the loaf] or cast it to the sea - if you will say that [above he is exempt] because he cannot receive warning, [also here] he is exempt. If you will say that [above he is exempt] because there is no action, [here] there is an action.

Question (R. Pinchas): If he was an Istnis (very finicky, and he swore about something he cannot eat), since the Shevu'ah does not apply to him for the future (he cannot fulfill it), does it apply to him for the past? (KORBAN HA'EDAH explains similar to this.)

Question (R. Yosi): If he said "Shevu'ah Shevu'ah Shevu'ah that I will not eat", and he ate, what is the law?

Answer (R. Yosi bei R. Bun): We learn from our Mishnah. If one said 'I swear that I will not eat this loaf, I swear that I will not eat it, I swear that I will not eat it', if he eats it, he is liable once, for he mentioned "this";

Inference: Had he not mentioned "this", he would be liable for each one!