The Torah says (Deuteronomy 19:16): "One witness shall not arise against a man for any sin or guilt that he may commit; according to two witnesses or according to three witnesses a matter shall stand." Thus, two witnesses provide conclusive proof of reality, but one witness does not. (However, the testimony of one witness can require a defendant to swear to his innocence or else pay the debt alleged against him.)
In monetary law, two witnesses may absolutely require someone to pay a debt or absolve him from that obligation. In capital cases, two witnesses may testify that a person has committed a crime punishable by the death penalty in Jewish law, and the Sanhedrin may execute the person on their word. (This scenario is not practically applicable today.)
The testimony of two witnesses is equal in its force to the testimony of three witnesses or any larger number of witnesses. Therefore, if two witnesses say an event occurred, and one hundred witnesses say it did not occur, the groups of witnesses are considered to contradict one another, but no more weight is given to the larger group by merit of its numerical advantage. There would need to be new evidence in order to resolve the judgment. If one of the witnesses is disqualified, his entire group is disqualified, even if the other witnesses are themselves qualified and could present a valid testimony without his assistance. (Source: Makkot chapter 1.)
In monetary law, examination of witnesses is less stringent than in capital law, and testimony is accepted even despite minor contradictions that may exist in the testimony of two separate witnesses. (If one witness says a defendant owes $100, and the other says he owes $200, the judges accept that both witnesses agree to the existence of a $100 lien, even though only one witness testifies to each individual lien. Similarly, if one witness says a defendant owes $100 based on a loan granted on Monday, and another witness says he owes $100 based on a loan granted on Thursday, the defendant is considered to owe $100 by the combined testimony of the two witnesses even though they disagree as to the source of the obligation.) In contrast, for capital cases, the judges threaten the witnesses by warning of the consequences of perjury (source: Mishnah Sanhedrin chapter 4), and they ask many questions and will invalidate testimony even for minor inconsistencies, even if the contradiction seems substantively irrelevant to the case at hand. The purpose of these stringencies is to prevent the killing of an innocent defendant.
Witnesses in monetary cases are evidentiary: even if they do not witness a robbery, the robber is still obligated to compensate the owner for damages (although there may be no way to prove this fact). Evidentiary witnesses are known in Hebrew as eidei beirur.
A pair of witnesses may be invalidated if:
A lone witness cannot give testimony in capital cases. His testimony is useless unless there is a second witness to join him. In monetary cases a lone witness has limited powers. He can require a defendant to take an oath stating that the defendant is in the right, and if the defendant refuses to take the oath he must pay instead. (In most cases it is at the defendant's discretion whether to swear, but in rare instances the court will not allow the defendant to swear and will instead require him to pay [Shevu'ot chapter 7].)
In the case of a classical Agunah, a woman whose husband has disappeared and it is unknown whether the husband is still alive, a single witness (and even a woman or slave, who are normally invalid as witnesses) may testify that the husband has died, and on that basis the woman may remarry.
Testimony of a deaf, mentally incompetent or young person (before Bar Mitzvah is excluded. Testimony from women is also generally excluded.
Anyone who is caught guilty of a sin which demonstrates greed, i.e. who sins in order to acquire money, is also disqualified.
Other witnesses are also disqualifed, as listed in Tractate Sanhedrin.
The Talmud in the third chapter of Sanhedrin delineates the rules governing who may provide written or oral testimony in Jewish law. A valid witness in a Jewish Beit Din must be an adult (see Bar Mitzvah) man, not a woman nor a slave, and not be related by family connections to any of the other witnesses or judges. In addition, the witness must be an honest person who can be trusted not to lie.
The Mishnah (Sanhedrin 24b) states: "The following people are disqualified: a gambler with dice, a lender who collects interest, a chaser of doves, and a merchant who profits from produce of Shemittah." The Talmud explains that each of these four activities falls within an expanded definition of theft because people who violate Torah laws or social norms in pursuit of money cannot be trusted to tell the truth.
- Gamblers with dice...when are they reinstated? When they destroy their dice and completely reverse themselves, so that they do not play even for free.
- Lenders who collect interest...when are they reinstated? When they tear up their promissory notes and completely reverse themselves, so that they do not collect interest even from a non-Jewish lender.
- Chasers of doves...when are they reinstated? When they destroy their tools for chasing and completely reverse themselves, so that they do not chase doves even in the desert.
- Merchants who profit from produce of Shemittah...when are they reinstated? When the next Shemittah year arrives [seven years later] and they withdraw.