Gemara

(Mishnah): There are two primary kinds of Shevuos (of Bituy). There are four in all. The two primary kinds are 'I swear that I will eat' and 'I swear that I will not eat.' The other two are 'I swear that I ate' and 'I swear that I did not eat.'

25a (Mishnah): Oaths of Bituy apply to one's own things, and to other people's. They apply to tangible and intangible things.

If Reuven swore 'I will give (or not give) to Ploni', 'I gave (or did not give) to Ploni', 'I will (or will not) sleep', 'I slept (or did not sleep)', 'I will (or will not) throw a rock into the sea', 'I threw (or did not throw) a rock into the sea.'

R. Yishmael says, one is liable (a Korban) only for oaths about the future - "Lehara Oh Leheitiv."

R. Akiva: If so, he should only be liable for doing good or evil. What is the source to include neutral things (e.g. throwing a rock into the sea)?

R. Yishmael: The Torah includes this ("l'Chol Asher Yivatei").

R. Akiva: This also includes oaths about the past!

(Rav): If one said 'I swear that Ploni threw (or did not throw) a rock into the sea', he is liable (for an oath of Bituy if this is false);

(Shmuel): He is exempt.

Rav obligates. Since the oath applies in the positive and negative, it is Bituy. Shmuel exempts. Since it does not apply to the future, it is not Bituy.

Nazir 20b (Mishnah): If Reuven said 'I am a Nazir'; another (heard him and) said 'and I'; (and another said) 'and I', they are all Nezirim.

Rishonim

Rambam (Hilchos Shevuos 1:2): Shevu'as Bituy applies only to things that one could do in the future or in the past. Oaths of the past are 'I ate', 'I threw a rock into the sea' or 'Ploni spoke with Almoni'. Oaths of the future are 'I will eat', or 'I will not eat', 'I will (or will not) throw a rock into the sea.'

Ramban (Bava Basra 126 DH v'Ika Lemeidak): Why does a Tanai help for Nezirus? Nezirus cannot be through a Shali'ach, so Tanai does not help for it, like we say about Chalitzah (Kesuvos 74a)! Some say that we learn from Bnei Gad and Bnei Reuven only Tanayim between people, but any Tanai that pertains only to himself is always valid. Some say that Nezirus can be through a Shali'ach. One can say 'when you want, vow that I am a Nazir.'

Noda b'Yehudah (1 YD 67): I did not find explicitly whether Shlichus helps for Shevu'ah. It seems that it must, for Tana'im help for Shevuos.

Beis Meir (EH 120:4 DH Emnam): The Ran (Pesachim 3b) says that one cannot make a Shali'ach for Bitul Chametz, for a Shali'ach cannot make Hefker. I do not know why this is obvious. The Beis Yosef (OC 434) says that Hefker works through a Neder, and one cannot vow through a Shali'ach. I say that this is because "Motza'ei Sefasecha Tishmor" is a Mitzvah on his body, which cannot be done through a Shali'ach. The Ramban says that any Tanai that pertains only to himself is always valid. Since he himself stipulated and he wants to fulfill his Tanai, it need not be Kaful. His initial reasoning was that Shlichus does not help for Nezirus; the same applies to Hekdesh and Nedarim. Likewise, a Shevu'ah cannot be through a Shali'ach, for it says "Levatei bi'Sfasayim." The Noda b'Yehudah overlooked the Ramban. Surely, Shevu'ah is like Nezirus. The Ramban's answer explains also Shevu'ah. It follows that if one made a Tanai in a Shevu'ah to another person, he needs Mishpatei Tanayim. Since Shevu'ah cannot be through a Shali'ach, Mishpatei Tanayim are not fulfilled, so the Tanai is Batel and the Shevu'ah is valid.

Tzlach (Berachos 21b DH R. Tam): The Ramban holds that Shome'a k'Oneh (one who listens is like one who answers) to be Yotzei a Berachah, but it is not considered a Hefsek in Tefilah. If so, hearing a Shevu'ah is not like saying a Shevu'ah. Nazir 20b proves this. If it suffices to hear, why must each say 'and I'? However, perhaps Berachos are different. All Yisrael say the same text of Berachos, but the first Nazir's words apply only to making himself a Nazir. Also, he did not intend for the listener; Shome'a k'Oneh requires the speaker to intend for the listener.

Poskim

Shulchan Aruch (YD 236:1): "Nefesh Ki Sishava Levatei bi'Sfasayim" includes (false) Shevuos about the past, e.g. 'that I ate' but (really) he did not eat, or 'I did not eat' and really he ate, and for the future, e.g. 'I will eat' and he does not eat, or 'I will not eat' and he eats.

Gra (1): The entire Sugya is like R. Akiva.

Be'er ha'Golah (232, after Os 25): The Ri mi'Gash said that if one swore through writing but did not say the oath, he is obligated to fulfill it. If he does not, Shamayim punishes him, but Beis Din does not.

Chavos Ya'ir (194): A document said 'Levi accepted with a handshake and Shevu'ah b'Po'el Mamash.' Levi claimed that it was Asmachta (an exaggerated promise). The Shulchan Aruch (207:19) and SMA (73:18) say that an oath helps for Asmachta. If the document said that Levi admits, if he knows that it is false, he is not liable. However, if he wrote 'I swear that I wil pay', or 'I accept with an oath and handshake...', it is as if he swore, and he must pay. One who vowed not to speak with Ploni may write to him, for vows depends on the way people speak. For monetary laws, writing is stronger than speech; one cannot say 'I was joking' or 'I did not want people to think that I am rich.' Likewise, one who writes 'I accept upon myself to fast on Monday... or to give Tzedakah', it is as if he said it. We hold that one must bless Birkas ha'Torah for writing, for it is like saying the words. I suspect that even if he did not write exactly what he accepts, rather 'I accept to continue the stringency I adopted for another half year' or 'I accept to fulfill what I dreamed', it takes effect, like one who vowed Stam and explained later what he meant. In any case, if he specified, it is a vow. For testimony, the Torah excluded writing. (This implies that it helps for other matters.) A written command to write a Get is invalid, for the husband must write it, and a Shali'ach is like him only if he commanded him in front of him. Or, this is a stringency of Eshes Ish. A written oath obligates b'Yedei Adam, but not b'Yedei Shamayim.

Chasam Sofer (YD 220): If one wrote a Shevu'ah without saying it, if he admitted that he will pay by a certain day, the Terumas ha'Deshen says that it takes effect. The Chavos Ya'ir obligates only b'Yedei Adam, but not b'Yedei Shamayim. I.e. if Beis Din does not force him, and he knows that he did not swear and he admitted falsely merely to placate his opponent, he need not fulfill it. Beis Din would accept a good excuse for swearing falsely. If he swore 'I accept upon myself', then the oath takes effect from when he writes it. The Ri mi'Gash obligates b'Yedei Shamayim and exempts b'Yedei Adam. Shev Yakov (49) disagreed with the Chavos Ya'ir. I defend the Chavos Ya'ir. Speech is stronger for interpersonal matters such as testimony, for one understands the intent better. For what one will fulfill himself, writing is better. It is more of an action. The Torah said 'to utter' to exclude thought, but Kal va'Chomer writing works.

Pischei Teshuvah (1): R. Mordechai Bana'at said that a Cherem written in a Sefer that no one else may publish it is invalid, for a written Shevu'ah is invalid. Tiferes Tzvi (YD 62) disagrees. However, if the first publisher already sold his Seforim, he cannot stop others from publishing, for this inhibits Shamayim's work, and he does not lose. R. Efrayim Margoliyos says that he keeps he rights (to stop others for publishing it) until the time that Chachamim fixed. The Noda b'Yehudah (1 YD 66) says that even if a written Shevu'ah is invalid, surely a written Cherem is valid. He also wrote (67) that Shevu'as Heses through a Shali'ach is valid, for one can stipulate about Shevuos. Shemen Roke'ach (2:27) disagrees.

Shulchan Aruch (ibid): It applies also to matters that do not entail harm or benefit, e.g. 'I threw a rock into the sea'or 'I did not throw' or 'I will throw' or 'I will not throw.' It applies even to intangible things, e.g. 'I slept', 'I did not sleep', 'I will sleep' or 'I will not sleep.'

Shach (1): This refers to an amount of time that he could survive without sleeping or eating. If he said that he will not sleep for three days, or Stam, it is a Shevu'as Shav (a vain oath), for it is impossible. The Bach says that if he swore Stam, he is believed to say that he meant one day. If so it is Shav when he intended for whatever his words connote, or when he forgot what he meant.

Atzei Levonah: Even though this is Devarim sheb'Lev (unspoken intents), the Bach (232) says that we can testify that he meant today, for one cannot live three days without sleeping. Simlas Binyamin questions this, for one can explain his intent (unlike the simple meaning) only when an extortionist forced him to swear. When (there was no Ones and) his words were clear, this is Devarim sheb'Lev (to say that he meant today).

Shulchan Aruch (ibid): It applies even to things that can be done only through others, e.g. 'I gave or did not give to Ploni', 'I will or I will not give.' It applies even to things that others did, e.g. 'Ploni threw a rock into the sea' or 'he did not throw.' All of these are Bituy. If his oath was false, he brings a Korban.

Gra (4): The Halachah follows Rav regarding Isurim.