Gemara

99b (Mishnah): If a public road passed through Reuven's field, and he set aside the end of his field to be a new path, intending to take the area of the old path, the new path also becomes a public road, and Reuven may not take the old path.

(Gemara) Question: Why didn't Chachamim enact that Reuven gets the old path? He should be allowed to take a stick and hit anyone who walks on the old path!

Answer: This teaches that one may not take the law into his own hands.

Rejection #1 (Rav Zvid): Chachamim decreed that he does not get the old path, lest he give a crooked path in its stead.

Answer #2 (Rav Mesharshiya): The case is, he gave them a crooked path in its stead.

Answer #3 (Rav Ashi): Any replacement path he gives is considered a crooked path, for it is closer for some people, but further for others.

Bava Kama 27b (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.

Bava Kama 117a: Reuven and Shimon each claimed to own a certain trap. Reuven handed it over to the king's officer.

Abaye: We cannot make him pay. He claims that it was his!

Rava: We don't allow him to do such a thing! Rather, he is excommunicated until he brings the trap to Beis Din so they can judge the case.

Berachos 5b: Four hundred barrels of Rav Huna's wine spoiled. Rabanan suggested that it is because he did not give to his sharecropper his share of vine branches. Rav Huna countered 'he stole much more than that from me!'

Rabanan: This is like people say, that one who steals from a thief tastes theft.

Rishonim

Rif (Bava Kama 12b): In monetary laws, the Halachah follows Rav Nachman, who says here that one may take the law into his own hands.

Rambam (Hilchos Sanhedrin 2:12): One may take the law into his own hands if he has the power.

Rosh (Bava Kama 3:3): One may take the law into his own hands only if the other comes to steal from him, or is holding his item. He may not (forcibly) take a security for an existing debt. That must be done in Beis Din (113a). Therefore, if he hit him, he is liable like anyone who hit him.

Rivash (396): If Reuven has property of Levi, who owes him, he may keep it for a security, for one may take the law into his own hands, even if he would not lose by going to Beis Din. However, he may sell it only after 30 days, in Beis Din. Surely he cannot keep it for himself! If a widow assessed property for herself, this is void (Kesuvos 98a), even if it was in front of a Beis Din of Hedyotos who know the value. She can sell to others, for then she tries to get the highest price for it; this helps her and the orphans. When she keeps it herself, she seeks to buy as cheaply as possible, which harms the orphans. Also, she cannot be the buyer and seller! A Beis Din of experts can sell on behalf of the orphans. Rav Safra was a partner with Isar, and took half the merchandise in front of witnesses who knew the value. It was invalid, for it was not in front of Beis Din (Bava Metzi'a 31a).

Poskim

Rema (CM 4:1): Some permit taking the law into his own hands only for a particular item of his that Reuven stole or seeks to steal or damage it, but not if Reuven owes him for a previous theft or other reason.

Source (Beis Yosef DH Kosav, citing Mordechai 30): Rabbeinu Meir says that one may take the law into his own hands only regarding a particular item that Reuven refuses to give back. He bring a proof from Berachos 5b. One may take back only what was stolen, but not something else in place of it.

Darchei Moshe (1): The Rosh and Nimukei Yosef agree. The Rivash (above) disagrees, and allows seizing for anything except for a loan.

Rema: He may do so only by himself, but not through Nochrim. If he acted through Nochrim, if he had no other recourse, what he did is done (he is exempt).

Source (Gra 14): Reuven handed over a disputed trap to an officer. Rava said that he cannot do so only because he could not prove that it was his.

Yam Shel Shlomo (3:6): A case occurred in which Levi denied having Reuven's deposit, and Reuven made accusations against him to Nochrim. Nochrim broke the door, Reuven took his deposit and Levi suffered great losses. Chachamim ruled that if Reuven had no other way to get it, he is exempt. If he had another way, he is liable, like R. Meir, who obligates for Garmi. I say that even the Maharam and Rosh, who permit taking the law into his own hands only when one can prove that he took properly, permit here. Even though he had no witnesses that Levi had it, or Levi said that he returned it, since he knew that Levi has it, it is like seeing one's object in another's hands. This is unlike one who cannot collect through Yisrael law and went to Nochri courts. The other party can take him to Beis Din and force him to return it. Here is different, for he knew that he will get his deposit, and all knew that it is his. If Reuven could have collected through Beis Din, he must pay all the damage to Levi. Even though in such a case he is exempt if he hit Levi, he is liable for going to Nochri courts instead of Beis Din. The Mordechai (Bava Kama 193) says that if one went to Nochri courts to save his money without intent to damage, he does not pay like a Moser, for it is only Gerama. He did improperly and should be tied to a post, but he is exempt. However, if he could have saved through Beis Din, he is a Moser who intends to damage.

Rema: Some say that taking the law into his own hands is only when he hurt him; one may do so only if he can prove that it is his. However, one may always seize property for security, and then go to Din with him.

Darchei Moshe (3): The Mordechai (Bava Basra 569) allows a Tzibur to seize a security before going to Din, for they are a Rabim and they will go to Din.

SMA (4): This opinion permits taking a security even when he cannot prove that he is owed, even in front of witnesses. We need not teach about without witnesses, for then he has a Migo. One may not seize a stolen object in front of witnesses, for surely the other party denies it. One may seize security for debts, for perhaps the other party would admit. The Rema concludes 'and he goes to Din with him', i.e. if he will deny owing, he will return the security.

Rebuttal (Nesivos ha'Mishpat 3): This is from the Maharik (161), and he says oppositely (witnesses are needed to seize a stolen object, but not for a loan)!

Questions: The Gemara sought to prove from the Mishnah of a path that one may not take the law into his own hands. Tosfos (Bava Kama 28a DH v'Linkot) says that we thought that he does not get back his path because he cannot summons everyone to Din. This is difficult. If one can prove what was stolen, he can collect in the thief's absence. Surely, he can clarify what was his path. (If not, obviously he gets nothing!) If so, a Shali'ach Beis Din can enforce that he get back his path, or Beis Din can authorize him to be their Shali'ach! Also, if all permit seizure without hitting, he should be allowed to take back his path, just without force!

Answer (Nesivos ha'Mishpat): Since he caused the situation (by trying to take their path), he may not collect in their absence. Alternatively, he may not collect in their absence because he could gather them, just it is toilsome. We permit seizure without hitting only if he will later go to Din.