Gemara

Rav Yehudah asked Rav Shmuel bar Yehudah to be one of five judges for Chalitzah, to publicize the matter.

Rav Shmuel bar Yehudah: "B'Yisrael" requires a Beis Din of Yisrael, not of converts, and I am a convert!

Rav Yehudah: I would force someone to pay money on the word of someone as honest as Rav Shmuel bar Yehudah.

Objection: This cannot be! "According to two witnesses a matter will be established"!

Correction: Rather, I would not allow collection of a document if he said that the document was paid.

Kesuvos 85a: Rava judged a case, and ruled that the defendant must swear.

Rava's wife: I know that she is suspected to swear falsely!

Rava changed his ruling, and said that the claimant will swear.

Rava judged a case involving a document. Rav Papa told Rava that he knew that the document had been paid.

Rava: Does anyone else know that it was paid? If not, you are only one witness. Your testimony does not help.

Rav Ada bar Masnah: Is Rav Papa less credible than your wife?!

Rava: I am certain that my wife would not lie. I am not certain about Rav Papa.

Rav Papa: Since you say that a judge may rely on one whom he knows does not lie, I would tear a document on the word of someone like my son Aba Mar.

Objection: This cannot be!

Correction: Rather, I would not allow one to collect with it.

Rosh Hashanah 21b (Rav or Shmuel): "Bikesh Koheles Limtzo Divrei Chefetz" - Shlomo wanted to be able to judge through understanding, without witnesses or warning. A Bas Kol said to him "v'Chasuv Yosher Divrei Emes" - the Torah says "Al Pi Shnayim Edim..."

Sanhedrin 93b (Rava): "Va'Haricho b'Yir'as Hash-m" teaches that Moshi'ach is Morach (smells, i.e. senses who is guilty) and judges - "V'Lo l'Mar'e Einav Yishpot...v'Shofat Tzedek".

Bar Koziva ruled for two and a half years, and told Chachamim that he is Moshi'ach. Chachamim tested him, they saw that he cannot judge by 'smell', they killed him.

Rishonim

The Rif (Kesuvos 43b) brings the Gemara in Kesuvos and concludes that if someone like Aba Mar discredited a document, Rav Papa would not allow one to collect with the document without an oath. Alternatively, through Shuda (judges' discretion) he would give to the one whose document was not discredited.

Rosh (Kesuvos 9:6): If discrediting a document means that one cannot collect it without an oath, what is so special about Aba Mar? Whenever one witness says that a document was paid, one must swear to collect! Perhaps Rav Papa means that even if Aba Mar was a relative, he would rely on him to discredit a document. However, 'does someone else know?' connotes that they would be two witnesses. R. Tam says that Rav Ada held that reversing the oath is like tearing the document. Rava answered that we do not tear the document, and that Rav Papa is not even believed like Rava's wife to reverse the oath. R. Chananel explains that the document had Stam Ne'emanos (it said that the lender will be believed to collect), It did not specify 'like two witnesses'. A regular witness could not obligate an oath against Ne'emanos, only a witness whom the judge believed absolutely. The Rif says that through Shuda the judge gives to the one whose document was not discredited. Shuda applies to two sale documents, not to loan documents! The case is, there are two loan documents and the borrower does not have enough to pay both of them.

Ran (DH u'Mistabra): A Mishnah teaches that when someone is suspected to swear falsely, the other party swears and collects. We could not say that Rav Papa would tear a document based in one witness, for that is taking money from a person. It requires two witnesses.

Rif and Rosh (ibid.): A Gaon says that nowadays a judge cannot say 'I know about him' (that he would not lie), for we do not know the criteria for this. Therefore, one cannot discredit a document or reverse an oath without clear testimony. Nevertheless, based on testimony of a trustworthy person we delay the verdict and investigate until the matter becomes clear.

Rambam (Hilchos Sanhedrin 24:1): A judge can judge monetary cases according to what he is firmly convinced to be the truth, even if there is not a clear proof. We need not say that if he surely knows the truth that he acts according to his knowledge. If Levi was obligated to swear, and a trustworthy person whom the judge relies on said that Levi is suspected about oaths, the judge can reverse the oath and tell David (Levi's opponent) to swear and receive. Even the trustworthy person was a woman or slave, he may rely on him, and all the more so if he himself knows that Levi is suspected. Similarly, if a trustworthy person whom the judge relies on, even a woman or relative, said that Levi's document was paid, the judge can say that Levi cannot collect without swearing. Or, if David also has a document, the judged can award the property to David and ignore Levi's document.

Rebuttal (Ra'avad): Whenever the Gemara discusses 'discrediting a document', it means that one cannot collect, even with an oath.

Defense (Kesef Mishneh): In Kesuvos 36b, the Gemara says 'we do not tear the document, nor do we collect with it'. Tosfos infers that since it does not say so here, one may collect with an oath. We can ask why the Rambam explains that in Shevu'os 42a the words mean that one cannot collect with it at all. In Kesuvos 36b, since we know that he sought to forge a document, he cannot collect. In Shevu'os he does not collect because he admits that he was paid. Here there is no reason not to collect against one witness, only the Ne'emanos does not exempt from the oath.

Rambam (ibid.): A judge rules according to what he believes to be the truth. The Torah requires two witnesses for a judge to rule according to them even if he does not know whether or not the matter is true.

Rambam (2): This is letter of the law. After the spread of Batei Din with judges of improper deeds or insufficient Chachmah, most Batei Din in Yisrael agreed not to reverse an oath (to the other party) without clear testimony. We do not mar a document and destroy its Chazakah through a woman or Pasul witness. In all judgments a judge may not rely on what he believes or knows, lest every commoner say that he believes or relies on Ploni. Nevertheless, if one trustworthy man testified about any matter and the judge believes that it is the truth, we delay the verdict and do not reject the testimony. We discuss the matter with the parties until they admit to the witness, compromise, or the judge withdraws from the case.

Poskim

Shulchan Aruch (15:5): A judge can judge monetary cases according to what he is firmly convinced to be the truth, even if there is not a clear proof. After the spread of judges that were not proper or wise enough, they agreed not to reverse an oath without clear testimony. We do not mar a document and destroy its Chazakah through a woman or Pasul witness. Nevertheless, if one trustworthy man testified about any matter and the judge believes that it is the truth, we delay the verdict and do not reject the testimony. We discuss the matter until the parties admit to the witness, compromise, or the judge withdraws from the case.

Beis Yosef (DH Kosav Maharik): Maharik (14:2,3) says that a judge may do what seems improper if he sees that a party is lying or not cooperating. He must act according to the truth, not according to the claims if they are false.

Rema: If it seems to Beis Din that one party stole from the other and they cannot make him pay through Din, they can decree on others not to do business with him. If the party is a woman, they can decree that no one marry her or engage in Shiduchim for her until she returns what is not hers.