Gemara

27b (Rav Chisda) Question: David and Levi were partners in a pit of water. Each day, one would water his field from it. Once, Levi was using it on David's day. David protested, to no avail. He hit Levi with the handle of a shovel. How much must he pay for this?

Answer (Rav Nachman): He can hit him 100 times, and he is exempt! All agree that one may take the law into one's own hands to avoid a loss.

(Rav Yehudah): One may not take the law into his own hands;

(Rav Nachman): One may do so.

All permit when needed to avoid a loss. They argue about when there is no loss. Rav Yehudah forbids, for he can go to Beis Din. Rav Nachman permits. Since he acts properly, he need not exert himself to go to Beis Din.

Question (Beraisa): If Reuven's ox came upon Levi's ox to kill it, and Levi removed his ox from underneath, and Reuven's ox fell and died, Levi is exempt.

Suggestion: The case is, Reuven's ox is an established gorer. Levi would not have lost money, for Reuven would have been liable to pay full damage!

Answer: No, Reuven's ox is Tam, so Levi would have received only half damage.

Question (Seifa): If Levi knocked Reuven's ox off Levi's ox, and it died, Levi must pay. (If the ox is Tam, why must Levi pay? He would have lost!)

Answer: Levi should have removed his ox without knocking off Reuven's ox.

Question (Beraisa): If Reuven filled Shimon's courtyard with jugs of wine and oil, Shimon may break jugs on his way out and on his way in.

Answer (Rav Nachman bar Yitzchak): He may break jugs on his way to Beis Din, and when he goes to obtain proofs that he will need in Beis Din.

Question (Beraisa): "You will cut off her hand (of one who grabbed a man by his private parts to save her husband)" means that she must pay.

Suggestion: The case is, she had no other way to save her husband.

Rejection: No, she could have saved in another way.

Bava Metzia 6a (Abaye): When two are holding a Talis, they swear because we are concerned lest one of them grabbed the Talis in order to collect an old debt.

Question: If this is the concern, each should get half without an oath!

Correction: Rather, perhaps one grabbed it to collect a Safek old debt.

Question: If so, we should be concerned lest he swear mi'Safek!

Rishonim

Rif: In monetary laws, the Halachah follows Rav Nachman, who says here that one may take the law into his own hands.

Nimukei Yosef (DH Me'ah): We learn from here that also a Shali'ach Beis Din may hit one who does not comply, if necessary. Surely, one of the parties has no greater rights than a Shali'ach Beis Din!

Nimukei Yosef (DH v'Ha): If he cannot prove that it is his (and his opponent does not admit), if he hit him he is liable.

Nimukei Yosef (DH mid'Nokat): The Beraisa discusses filling his Chatzer (or Reshus ha'Rabim) with jugs. If there was room to walk, one who broke them is liable. Reuven had no right to put them in Shimon's Chatzer, and Shimon may take them out, but he may not break them.

Nimukei Yosef (DH Lo): We learn from the woman who saved her husband that even when there is a loss, if one could have saved without damaging, but he saved by damaging or embarrassing, he is liable.

Rambam (Hilchos Sanhedrin 2:12): One may take the law into his own hands if he has the power. Since he acts according to Halachah, he need not toil to come to Beis Din, even if he would not lose by doing so. Therefore, if his opponent complained and brought him to Beis Din and it is found that he did properly, we do not undo what he did.

Rosh (3:3): If Reuven sought to steal Shimon's item and Shimon hit him to stop him, or if Shimon saw his item with Reuven and hit him until he let Shimon have it, the Amora'im do not argue. If there is a loss, e.g. perhaps the water in the pit will be finished and the field will be ruined, or a woman who has no other way to save her husband, one may take the law into his own hands and he is exempt. He is exempt for hitting the other person if he had no other way to save his property. They argue about a case when he could get his money through Beis Din, and merely saves himself toil. Rav Yehudah holds that one may not hit another to save himself toil. Rav Nachman holds that since he can prove that he acts properly, he may save even through hitting if there is no other way. If he cannot prove that it is his, he cannot say 'it is mine, and I hit to save my money.' The Halachah follows Rav Nachman in monetary laws. Also, we challenged Rav Yehudah and gave poor answers.

Hagahos Ashri: If Reuven's ox came upon Levi's ox to kill it, and Levi knocked Reuven's ox off and it died, Levi must pay, even if it is Tam. Levi should have removed his own ox, for it is not so much harder than knocking off Reuven's ox. If Reuven filled Reshus ha'Rabim or Shimon's courtyard with jugs, Shimon may break them. It is a toil to stack them; he need not go around.

Rosh (Teshuvah 64:1): One may take the law into his own hands only regarding a particular item that is his, like Rabbeinu Meir explained. However, if one seized property due to a claim of another debt, he is not called a thief, like we say in Bava Metzia (6a). If one grabbed Ploni's Talis because Ploni owes him a loan, he is not suspected of stealing. If he swore that the Talis is his, it is not a false oath.

Poskim

Shulchan Aruch (CM 4:1): One may take the law into his own hands. If Shimon saw that Reuven has Shimon's item that he stole, he may take it from him. He may hit him until he lets Shimon have it (Rema - if he cannot save any other way), even if there is no loss if he would wait until he takes him to Beis Din. This is if he can prove that he takes according to Halachah.

SMA (2 and Shach 3): If he cannot prove that it is his, he may not seize it in front of witnesses. He may seize it not in front of witnesses, and he will be believed in Beis Din through a Migo (Teshuvas Rosh 64:1, 106:1).