GEMARA

(R. Yanai): One who curses himself transgresses "Hishamer Lecha u'Shmor Nafshecha M'od";

(R. Avin): 'Hishamer', 'Pen', or 'Al' always denotes a Lav.

Bava Kama 91a (Mishnah - R. Akiva): A person may not injure himself.

Contradiction (Beraisa - R. Akiva): A person may injure himself.

Answer #1 (Rava): A person may not bodily injure himself, but he may embarrass himself.

Question (Beraisa): "(If one swore) to harm or to benefit..." - just like the benefit discussed is optional (not a Mitzvah), also the harm. This includes one who swore to harm himself (he is liable if he does not do so).

Answer #1 (Shmuel): The Beraisa discusses an oath to fast.

Objection (Beraisa): What is the case of (swearing to) harm others? 'I will hit Ploni, I will bruise his brain'.

Answer #2 (to both questions): Tana'im argue about this (each Tana says that R. Akiva agrees with him).

Question: Which Tana forbids a person to damage himself?

Answer #1: It is R. Elazar, who says that one who tears too much clothing out of grief over a death transgresses 'Bal Tashchis'. All the more so, he holds that one who damages his body transgresses!

Rejection: Perhaps damaging one's body is less severe, for the damage can heal, unlike tearing clothing.

Answer #2: It is R. Elazar Hakapar b'Ribi:

(Beraisa - R. Elazar Hakapar): "Me'Asher Chota Al ha'Nefesh" - he sinned against his own soul, by denying himself wine. If one is called a sinner for denying himself wine, all the more so one who (fasts and) denies himself all food is called a sinner!

Ta'anis 11a (Shmuel): One who fasts is called a sinner.

He holds like R. Elazar Hakapar.

(R. Elazar): A Nazir pained himself only from one thing, and he is called Kadosh - "Kadosh Yihyeh Gadel Pera Se'ar Rosho". All the more so, one who fasts and pains himself from everything is called Kadosh!

Elsewhere, R. Elazar expounded that one should consider that his innards are Kodesh (and he may not weaken them)! Rather, if one can bear the fast, he is praiseworthy. If not, he sins.

(Reish Lakish): He is called Kadosh - "Gomel Nafsho Ish Chased v'Ocher She'ero Achzari".

RISHONIM

Rashi (Ta'anis 11b DH Nikra): Reish Lakish says that one who fasts and can bear it is called Ish Chased". If he weakens himself he is called Achzari.

Tosfos (Ta'anis 11b DH Gomel): Reish Lakish says that one who does not fast is called Ish Chased".

Rambam (Hilchos De'os 3:1): Since Ta'avah is so bad, one might think that he should go to the other extreme and not eat meat and wine, get married, have a nice house and clothing... One who does so is called a sinner. A Nazir who denies himself wine needs atonement. All the more so, one who denies himself everything needs atonement! Therefore, Chachamim commanded to refrain only from what the Torah forbids, and not to add Isurim through vows and oaths.

Question (Tosfos Shevuos 36a DH u'Shmor): "U'Shmor Nafshecha Me'od" forbids cursing oneself. We should also learn that it is Asur to harm oneself from here, or from "v'Hishamru Me'od l'Nafshoseichem"! Why is there disagreement about whether or not one may harm himself? We should learn from "u'Shmor Nafshecha Me'od" or "v'Hishamru Me'od l'Nafshoseichem"!

Answer (Tumim 27:1): Indeed, the one who permits to harm oneself also permits cursing oneself. The Gemara did not learn the Isur to curse parents from the Isur to curse oneself, for this would not answer according to the opinion that one may harm himself.

Question (Tosfos Ta'anis 11a DH Omar): Here, Shmuel says that one who fasts sins. In Bava Kama, Shmuel said that the Heter to harm oneself refers to fasting!

Answer #1 (Tosfos): There is a small sin to fast, but the reward outweighs it. This is why we say that one who sees a Sotah in her disgrace should become a Nazir, even though he sins by abstaining from wine. It is good to fast to annul a bad dream on Shabbos, but he must fast again during the week this is only atone for fasting on Shabbos.

Rebuttal and Answer #2 (Gvuras Ari Ta'anis 11a DH Omar and DH Ela): If the reward outweighs the punishment it is totally permitted, just like an Aseh override a Lav because an Aseh is stronger! Shmuel merely explained the Beraisa, but perhaps he himself disagrees.

Answer #3 (Lechem Mishneh): The Rambam explains that mid'Oraisa the verse applies only to a Tamei Nazir. Even though the Gemara says that the verse applies even to a Tahor Nazir and it was written regarding a Tamei Nazir since he sins twice, this is an Asmachta. Really, it applies to a Tahor Nazir only mid'Rabanan. Similarly, the Isurim to fast or harm oneself, which are learned from a Nazir Tahor, are mid'Rabanan. Even though an oath takes effect on an Isur mid'Rabanan, the Beraisa connotes that the Tana permits harming oneself even mid'Rabanan. Shmuel established the Beraisa to discuss fasting. This is not like R. Elazar Hakapar, rather, like R. Elazar who argues with him. However, all agree to the law of the Beraisa, that the oath takes effect.

Chasam Sofer (8a DH ul'Rebbi and 21a DH Chutz): R. Elazar Hakapar says that a Nazir Tamei sinned during the days that do not count, for he pained himself for naught. We learn from R. Elazar Hakapar that paining oneself, even passively, is considered an action for which one brings a Korban.

Aruch l'Ner (Rosh Hashanah 9a DH Kol): If one benefits from this world more than the Torah allows, he sins against his Neshamah. Likewise, if he does not give to his body the food and drink it requires, he sins against his body - "me'Asher Chota Al ha'Nefesh". The command to eat on Erev Yom Kipur and fast on Yom Kipur are to atone for "Nafshoseichem" (plural, as opposed to Nafshechem), these two parts of the soul.

Igros Moshe (CM 1:103): Perhaps donating blood (when there is no patient who needs it right now) is forbidden, even if one is paid for this, because it harms oneself. Tosfos (Bava Kama 91b DH Ela) forbids harming oneself even for profit. However, we learned from a Nazir, who is called a sinner for paining himself. If he does so willingly for profit, he is not pained! Perhaps we can distinguish. The pain of refraining from wine is only if he desires it. If one desires the profit more, he feels no pain. Bodily harm is always painful. However, the Gemara says that bloodletting is a Refu'ah. Nature changed after the time of the Gemara; the Rambam (Hilchos De'os 4:18) counsels to let blood less often than the Gemara says. Even though we do not let blood nowadays for Refu'ah, presumably nature has not changed totally and even nowadays it helps a bit. Therefore, we do not protest if one donates blood.

POSKIM

Shulchan Aruch (CM 337:19): A worker may not starve or afflict himself, for this decreases the work he will do for his employer.

Taz: If he will be working in a vineyard he may starve himself beforehand, for he may eat while working and he will work properly.

Shulchan Aruch (CM 420:31): Even though one may not wound himself, if he did, he is exempt.

Tur (CM 420:31): One may not wound himself. The Ramah says that the Halachah is that one may

Beis Yosef (DH u'Mah she'Chosav b'Shem): It seems that the Tur holds that the Halachah never follows R. Elazar Hakapar.

Bach (21): It seems that the Tur holds that the Sugya in Shevuos is like Chachamim who argue with R. Elazar Hakapar, and the Halachah follows them.

Prishah (21 DH Kosav): Even though one may not tear too many garments due to a Mes, all agree that there is more Heter to wound the body, for it heals, unlike garments.