Gemara

(Mishnah): A Chazakah without a claim is invalid.

Chazakah works because when a Machazik (one who took possession) claims that he bought a field and lost the document, and the Muchzak denies this, the Machazik's claim is more credible, since an owner is not silent when a thief benefits from his land for three years;

If the Machazik has no claim of ownership, should we claim for him?!

33b: Levi claimed that he bought David's land, and ate the years of Chazakah. He brought witnesses that he ate the Peros for two years.

Rav Nachman: He returns the land and two years worth of Peros.

34b (Rav Yehudah): If David and Levi each claimed to own a ship, and Levi asked Beis Din 'seize the ship until I bring witnesses', we do not seize it. If Beis Din seized it, and Levi asked 'release it, and whoever overpowers will get it', we do not release it.

35b (Chachamim of Neharde'a): If two were fighting over property, and a third person grabbed the contested property, he keeps it;

(R. Chiya - Beraisa): One who steals from many people is not a thief.

Rejection (Rav Ashi): No, he is a thief. (Since he has no claim to it, we take it from him). R. Chiya meant that one who steals from many people cannot (simply) return the theft. (He is Yotzei only if the real owner gets it back.)

41a (Mishnah): A Chazakah without a claim is invalid. If Reuven said 'what are you doing in my field?', and Shimon said '(I have been here three years.) No one told me 'this is my field'', his Chazakah is invalid.

If he answered 'you (or your father) gave or sold it to me', his Chazakah is valid.

Objection: Obviously, if Shimon has no claim why the land is his, this is not a Chazakah!

Answer: One might have thought that Shimon really bought it and lost the document. He did not say that, for he fears lest Reuven demand to see the document. One might have thought that we ask Shimon if this is so, due to "Psach Picha l'Ilem" (speak on behalf of a mute, who cannot speak for himself)! The Mishnah teaches that we do not.

Rishonim

The Rif brings the Mishnah.

Nimukei Yosef (DH Gemara): The Rashbam says that if before Shimon left Beis Din, he says that he had a document and lost it, but he was afraid to say so lest they ask him where the document is, he is believed. It seems that R. Yonah agrees.

Rambam (Hilchos To'en 14:12): Any Chazakah without a claim is invalid. If Shimon ate Peros of a field for several years, and Reuven said 'what are you doing in this field? It is mine!', and Shimon said 'I do not know whose it is. Since no one said anything to me, I entered it', his Chazakah is invalid, for he did not say that he bought it, received it for a gfit, or inherited it. Even though he did not claim, we do not remove it from him until Reuven brings witnesses that it is his. If Reuven brings witnesses, he gets the field, and Shimon pays for the Peros he ate. We do not suggest to Shimon 'perhaps you had a document and lost it', unless he himself claims this. If he does not claim, he returns all the Peros he ate. Similarly, if one ate for years of Chazakah due to a document, and we find that the document is Pasul, he has no Chazakah. He returns the field and all the Peros he ate.

Magid Mishneh: The Tosefta (2:1) says that if one ate Peros due to a document, and it was found to be Pasul, his Chazakah is invalid.

Hagahos Maimoniyos (30): The same applies if Shimon says 'I bought it from Ploni, who bought it from you.' The Gemara teaches about one who makes no claim, to teach that we do not ask if he lost his document.

Rosh (3:35): The Gemara suggested that we ask Shimon if he really bought it and lost the document. Even though it concludes that we do not, we infer that if he himself said so, he explains his initial words. He is not like one who retracts and makes a new claim. (One may not retract.)

Mordechai (534, brought in Darchei Moshe CM 146:2): If Shimon was Machazik in land, and Reuven said 'it is mine', and Shimon said 'no one said anything to me. I will not leave until you bring a proof', R. Yakir was unsure whether he must bring a proof. The Rashbat said that Shimon's claim is valid. If two were fighting about a ship, and Beis Din seized it, they do not release it unless one of them brings a proof. He brought other proofs that if money came to one's hand and there is a Safek, no one else may take it without a proof.

Question (Gra CM 146:20): How can he learn from the ship? Beis Din holds the ship, and there they have Vadai claims! If one steals contested property, we conclude that he cannot (simply) return it. To be Yotzei, he must pay everyone to ensure that the real owner gets it back.

Poskim

Shulchan Aruch (CM 146:9): Any Chazakah without a claim is invalid. If Shimon ate Peros of a field for several years, and Reuven said 'what are you doing in this field? It is mine!', and Shimon said 'I do not know whose it is. Since no one said anything to me, I entered it', his Chazakah is invalid, for he did not say that he bought it, received it for a gfit, or inherited it. Even though he did not claim, we do not remove it from him until Reuven brings witnesses that it is his. If Reuven brings witnesses that it is his, he gets the field, and Shimon must pay for the Peros he ate. We do not suggest to Shimon 'perhaps you had a document and lost it', unless he himself claims this.

Bedek ha'Bayis: The Tur says that when Reuven brings witnesses that it is his, he must swear to get the field. I do not know why an oath is needed.

SMA (18): Here, the Shulchan Aruch cites the Rambam, who does not mention an oath. In Siman 140 (Sa'if 1), the Shulchan Aruch says that if Shimon does not have three years of Chazakah, the original owner swears and gets back his field, even though the Rambam does not mention an oath there, either. Here, the Shulchan Aruch relies on what he wrote there. However, we can distinguish. There, Shimon has a claim how he received the field. Here, he does not.

Prishah (14): The Gemara suggested a claim to make for Shimon. Even though we do not claim for him, if he claims we accept it Due to this concern, Reuven must swear.

Bach (DH v'Yesh): Why should he swear? No one claims it! Do not say that there is supporting evidence that he sold it to Moshe, and Moshe went abroad. This is only a Safek, for which we merely make a Stam Cherem, but not an oath.

Shach (5): I say that he must swear because he comes to take from one who was Muchzak for three years, so he is like one who receives, who swears even when the opposing side is unsure (Rema 91:3, Shulchan Aruch 357:1). Had the SMA seen Bedek ha'Bayis, he would not have suggested that the Shulchan Aruch relies on what he wrote above.

Gra (21): He must pay for Peros he ate, like one who ate for only two years.

Darchei Moshe (2): Teshuvas ha'Rashba says that Reuven gets it even if he was Muchzak for only one day before Shimon came.

Rema: If he does claim this, we heed him. This is not like one who retracts and makes a new claim.

Shulchan Aruch (ibid): Similarly, if one ate for years of Chazakah due to a document, and the document was found to be Pasul, his Chazakah is invalid. He returns the field and all the Peros he ate.

Rema (222:4): If David says 'I sold my field, but I do not know to whom', and Ploni says 'you sold to me', he is believed.

SMA (10): Ir Shushan says that David can make Ploni swear. This is difficult, for one cannot make another swear due to a Safek claim! Perhaps here is different, for Ploni comes to take.

Shach (8): It seems that the Magid Mishneh (Hilchos Ishus 9:12), citing the Rashba (Kidushin 63b DH Tanan, citing the Yerushalmi Kidushin 3:7) argues with the Rema. It seems that the Mechaber (146:9) disagrees. Hagahos Ashri (Bava Metzia 3:14) explicitly disagrees.