“Today we will learn about oaths,” Rabbi Dayan announced to his shiur (class).

“I heard that you’re not supposed to swear,” Avrum said.

“Certainly, a person must be extremely careful when uttering an oath, for the prohibition against swearing falsely is extremely severe,” replied Rabbi Dayan. “As a rule, the phrase ‘I swear’ should be expunged from your vocabulary. However, there are three cases in which the Torah imposes an oath in the context of beis din.”

“For example?” asked Avrum.

“Let’s say you claim that your friend borrowed $100 and he denies the loan,” answered Rabbi Dayan. “A single witness testifies that you lent him the money. To contradict the single witness, your friend would have to take an oath that he did not borrow.”

“This makes me think about a case that recently occurred,” Avrum said. “The class went to a pizza store for lunch. Most brought money, but a few didn’t, and I laid it out for them.”

“So what happened?” asked Rabbi Dayan.

“It was a bit hectic with 30 people all paying at the same time, so I don’t know exactly for whom I paid,” said Avrum. “My cousin, though, says that he saw me lay out the money for Dov.”

“What do you say about this?” Rabbi Dayan asked Dov.

“I paid by myself. Avrum did not pay for me,” said Dov. “Furthermore, Avrum admits that he doesn’t know whom he laid out the money for. He cannot make a claim based on his cousin’s testimony.”

“But my cousin is a single witness,” said Avrum. “Wouldn’t Dov have to swear to contradict my cousin?”

“A cousin is a relative who is disqualified from serving as a witness,” said Rabbi Dayan. “We cannot impose an oath on his word (C.M. 33:2).”

“What if I had a valid witness?” said Avrum. “Would Dov have to take an oath to contradict the single witness, even though I don’t know for sure whether he owes me?”

“Generally, a person only needs to swear when there is a definite claim against him,” explained Rabbi Dayan. “There are some cases, though, in which our Sages required an oath to disqualify even a possible claim (C.M. 75:17, 93:1).”

“But I’m not claiming that Dov might owe me,” argued Avrum. “I am making a definite claim that he owes me, based on the witness! Does that count?”

“There is a dispute among the Rishonim whether the plaintiff must come with a definite claim when a single witness testifies,” answered Rabbi Dayan. “Some say that even when there is a witness, a definite claim by the plaintiff himself is required. However, many maintain that a single witness suffices to impose an oath even if the plaintiff himself is unsure of the facts and makes a claim based on the witness’s testimony, just as two witnesses obligate the defendant even if the plaintiff knows about the debt only based on their testimony (see Rosh, Shavuos 6:5).”

“What does the Shulchan Aruch rule?” asked Avrum.

“The Shulchan Aruch rules that a claim based on a single witness is considered a definite claim that warrants an oath only if the witness actually testifies before us,” answered Rabbi Dayan. “However, if the witness is not present to testify, but just told the plaintiff what happened, it is considered a doubtful claim that does not warrant an oath (C.M. 75:21, 23).”

“And what my cousin says is meaningless?” asked Avrum. “I know him well and trust him completely, so there’s no doubt in my mind that he is truthful!”

“That is insufficient basis on which to impose a Torah oath,” replied Rabbi Dayan. “However, if you trust him absolutely, some say that this suffices to impose a Rabbinic oath, shevuas hesses, provided that your relative doesn’t have a vested monetary interest in the case. Others require that he come before the beis din or that there also be some circumstantial evidence against the defendant in order to impose this oath (see C.M. 75:23; Shach 75:82-83).

“Nowadays, beis din usually avoids imposing an oath regardless,” concluded Rabbi Dayan, “and works toward seeking a compromise instead.”