The Torah in Parashat Shoftim (Debarim 19:15) commands that a court may not reach a decision about a case on the basis of the testimony of a single witness. At least two witnesses must testify about the act or event in question for Bet Din to reach a decision. This verse forbids a court from punishing a defendant or awarding money to a plaintiff based on a lone witness’ testimony.
The Sefer Ha’hinuch explains that even an outstanding and honest person could be tempted to falsely testify about somebody because of a tinge of hostility or animosity that he feels toward the defendant. No matter how righteous and trustworthy somebody is, he is not free from feelings of resentment, and so even a generally honest person might speak dishonestly to cause harm to somebody towards whom he harbors feelings of bitterness. When two upstanding witnesses are cross examined and testify to the same event, it can be assumed that they speak the truth. This cannot necessarily be assumed, however, when just a lone witness testifies.
Although a court may not punish or extract money based on a single witness’ testimony, it will demand an oath from the litigant against whom a lone witness testifies. Furthermore, a single witness is believed in matters of “Issur Ve’heter” (where a status of permissible or forbidden must be determined), such as to testify that a piece of food is permissible for consumption. Another instance where a lone witness’ testimony is accepted is in the case of a Sota, where a husband warned his wife not to go into seclusion with a certain man, and she violated the warning. If even a single witness testifies that she and the man in question committed adultery, he is believed, and the woman does not then drink the special waters to determine her guilt or innocence. Likewise, in a case where a murder victim is discovered, and it is unknown who committed the crime, if even a single witness testifies about who murdered the victim, the Egla Arufa ceremony is not performed.
The Sages enacted a provision that a woman may remarry based on the testimony of a lone witness who claims to have seen her husband die. In order to avoid a situation of an Aguna (“chained” woman), a lone witness’ testimony is accepted in such a case.
This prohibition applies in all times and in all places, and is binding upon judges of a Bet Din. A judge who ordered a defendant to pay money based on the testimony of a single witness has violated this command. He is not liable to Malkut, because the wrongdoing can be rectified through the return of the money in question, and Malkut are not administered in the case of a violation that can be corrected.