Gemara

Vows are interpreted according to the way people speak.

19a (Mishnah): If one said "the weight of my Yad is Alai"... (we fill a barrel with water, and he inserts his arm until his elbow...)

Question (Tana d'Vei Menasheh): "Al Yadecha" refers to Kibores (where the muscle bulges in the upper arm).

Answer: Mid'Oraisa, it is until the Kibores. We explain Nedarim based on the way people speak.

Shevu'os 21b (Mishnah - R. Akiva): If one swore 'I will not eat' and he ate Mashehu, he is liable...

Chachamim: We never find that someone is liable for eating Mashehu!

22a (Rava): R. Akiva and Chachamim argue in a case that he swears 'I will not eat';

However, if he swore 'I will not taste', he is liable for Mashehu.

Objection: This is obvious!

Answer: One might have thought that 'tasting' refers to eating (i.e. a k'Zayis), as people commonly say. Rava teaches that this is not so

(Rav Papa): R. Akiva and Chachamim argue only about Shevu'os, but all agree that for Konamos, one is liable for Mashehu.

Question: What is the reason?

Answer: Also Konamos are like specifying Mashehu, since he does not mention eating.

22b - Question (Rava): If one swore 'I will not eat earth', how much is he liable for?

Question (Rava): If one swore 'I will not eat Chartzan (grape skins or pits), how much is he liable for?

Since they are eaten along with the grape, he refers to the quantity of eating, a k'Zayis;

Or, since they are not normally eaten by themselves, he means Mashehu!

These questions are not resolved.

Rishonim

Rambam (Hilchos Shevu'os 4:1): If one swore not to eat today, and ate less than a k'Zayis, he is exempt. Eating less than a k'Zayis is not eating. It is as if he ate a Chetzi Shi'ur of Nevelos, Trefos or similar matters. If he swore 'I will not eat this' and he ate it he is liable, even if he swore falsely about a seed.

Chidushei ha'Ran (Shevu'os 22a DH ul'Inyan): We hold like Rabanan, who say that there is a Shi'ur for a Shevu'ah. One is exempt for Chetzi Shi'ur, and presumably there is not even an Isur. L'Chatchilah he may eat Chetzi Shi'ur. This is unlike other Isurim, for which the Torah forbids Chetzi Shi'ur according to R. Yochanan; the Halachah follows him. Even Reish Lakish (who argues) agrees that Chetzi Shi'ur is Asur mid'Rabanan. Here, he made the Isur himself. He intended only for a k'Zayis. the same applies to Konamos, when he mentioned eating, e.g. 'eating this is Alai b'Konam.' He may eat Chetzi Shi'ur. Why does the Rambam forbid Chetzi Shi'ur?

Mishneh l'Melech: I can understand the Rambam (and the Ran, after he retracted to agree with the Rambam) if eating connotes a k'Zayis, and a Gezeras ha'Kasuv forbids Chetzi Shi'ur, because it can join. However, the Re'em holds that Stam eating is Mashehu, and a tradition from Sinai uproots this and says that one is punished only for a Shi'ur. If so, one who swears not to eat should be liable for Mashehu, like 'eating' connotes! This requires investigation.

Note: Perhaps the Re'em means that in the Torah, eating connotes Mashehu, but Shevuos depend on Lashon Bnei Adam (the way people speak), and they use eating to connote a k'Zayis.

Mishneh l'Melech (DH v'Da): R. Akiva and Chachamim argue about the Shi'ur of liability for a Shevu'ah. Shevuos depend on Lashon Bnei Adam. If eating connotes a k'Zayis, all should agree that one is liable only for a k'Zayis! If eating connotes Mashehu, all should agree that one is liable for Mashehu! One cannot say that they argue about Lashon Bnei Adam! Rashi connotes that in Lashon Bnei Adam, it connotes Mashehu. This is why R. Akiva obligates for Mashehu. Chachamim follow Lashon Torah, in which 'eating' connotes a k'Zayis. Perhaps Shevuos and Nedarim follow Lashon Bnei Adam only when it is not unlike Lashon Torah, but when they differ, we follow Lashon Torah. In the Yerushalmi, R. Yochanan says that we follow Lashon Bnei Adam, and R. Yoshiyah says that we follow Lashon Torah. Surely, the Halachah follows R. Yochanan. In Erchin (19b) we say that in Lashon Torah, "Yad" is until the Kibores. Nedarim are according to Lashon Bnei Adam, even to be lenient. Even R. Yoshiyah follows Lashon Torah only to be stringent. How can Chachamim follow Lashon Torah to be lenient? Rather, they hold that one forbade to himself only like the Shi'ur of Isurei Torah. This is only when he mentioned eating, but not for Konamos if he did not mention eating, so it is as if he specified any amount.

Mishneh l'Melech: The Re'em holds that Stam eating in the Torah is any amount. If so, why do Chachamim say that one is liable only for a k'Zayis? Do not say that surely, he forbade to himself only the amount for which the Torah punishes for Isurei Torah. If so, we should say the same for Konamos! With difficulty, we can say that they hold that one who swears that he will not eat refers to Stam eating, for which one is liable for a Shi'ur and there is an Isur of any amount. This is a poor answer.

Rambam (Hilchos Nedarim 1:5): If one forbids to himself kinds of food, e.g. 'figs are forbidden to me' or 'these figs are forbidden to me', and he ate any amount, he is lashed mid'Oraisa for "Lo Yachel Devaro." There is no Shi'ur to Nedarim. One who vows from a matter is as if he specified 'any amount.' If he said 'eating Peros of country Ploni is forbidden to me', he is not lashed until he eats a k'Zayis.

Question (Tosfos 22a DH Aval): Why are Konamos more stringent than Hekdesh? Konamos are forbidden only because they are like Hekdesh. One is not liable for any amount of Hekdesh, rather, only for a k'Zayis!

Answer #1 (Radvaz): I say that since Hekdesh is forbidden to the entire world, it does not take effect on less than Shaveh Perutah. One forbids Konam only to himself, therefore he forbids any amount. It is as if he specified.

Answer #2 (Torah Chayim 22a DH Aval): When one does not intend to forbid a matter, rather, to be Makdish it for the Mizbe'ach or Bedek ha'Bayis, and automatically it is forbidden due to Me'ilah, the Torah decreed that one is liable only for a Shi'ur, i.e. Shaveh Perutah?. When one says 'this loaf is Konam to me' or 'it is like Hekdesh', even though it is forbidden only because it is like Hekdesh, since he intends to forbid it, it is as if he specified 'it is like Hekdesh, for any amount', since he did not mention eating. He transgressed "Lo Yachel Devaro" for any amount, like the Rambam says.

Rosh (Shevu'os 3:5): Rava asked, if you will say that one is liable for any amount of earth because it is not edible, since Chartzan can be eaten in a mixture, perhaps he intends for a k'Zayis. Or, since people do not eat it by itself, he is liable for any amount. These questions were not answered. In practice, nowadays (that we do not have lashes or Korban) it makes no difference. Even if he intends for a k'Zayis, the Torah forbids Chetzi Shi'ur.

Poskim

Shulchan Aruch (YD 238:1): If one swore Stam 'I will not eat', he may not eat Mashehu, but he is liable only for a k'Zayis. If he specified Mashehu, or said 'I will not taste', he is liable for Mashehu.

Beis Yosef (DH Shevu'ah): The Poskim rule like Chachamim. It seems that Chachamim argue only about Chiyuv Korban and lashes, but all agree that Mashehu is forbidden, just like the Torah forbids a partial Shi'ur of all Isurim.

Gra (1): Even though this is an Isur that he put on himself, and one might think that he did not intend to forbid less than a k'Zayis, since we say in Yoma (74z) that Chetzi Shi'ur is forbidden because it can join (to a full Shi'ur), the same applies here. This is unlike Chidushei ha'Ran.

Erech Lechem (Maharikash): Likewise, one who swore not to drink is liable for a Revi'is.