“Today we will learn about oaths,” Rabbi Dayan announced to his shiur. “There are many sugyos [passages] in the Gemara that deal with imposing an oath.”
Avrumi raised his hand. “I heard that you’re supposed to avoid swearing,” he said. “Then why does the Gemara talk about it a lot?”
“Certainly, a person must take the utmost care when uttering an oath, as the prohibition of 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 itself imposes an oath in the context of beis din in order to ascertain the truth.”
“For example?” asked Avrumi.
“One case is where there is a single witness,” answered Rabbi Dayan. “Let’s say you claim that your friend borrowed $100 from you and he denies the loan. There is a single witness who saw you lend him the money. To contradict the witness and exonerate himself, your friend would have to take an oath that he did not borrow.”
“This makes me think about a case that recently occurred,” Avrumi said. “I wonder what the halacha would be.”
“What was the case?” asked Rabbi Dayan.
“We recently took a class trip to the park to play ball,” said Avrumi. “Afterward, we went to the pizza store for lunch. Most of the class brought money and paid for themselves, but a few didn’t have money and I laid it out for them.”
“OK, so what happened?” asked Rabbi Dayan.
“You can imagine that it was a bit hectic with thirty people all paying at the same time, so I don’t know exactly whom I paid for,” said Avrumi. “My cousin, though, says that he saw me lay out the money for Dov.”
Rabbi Dayan turned to Dov. “What do you say about this, Dov?” he asked.
“I paid by myself; Avrumi did not pay for me,” said Dov. “Furthermore, Avrumi acknowledges that he doesn’t know whom he laid out the money for. He has no right to claim based on what his cousin says.”
“But my cousin is a single witness,” said Avrumi. “Wouldn’t Dov have to swear to contradict my cousin’s testimony?”
“A cousin is a relative who is disqualified from serving as a witness,” said Rabbi Dayan. “Thus, we cannot impose an oath upon his word.” (C.M. 33:2)
“And if it weren’t my cousin, but someone else who is a valid witness?” said Avrumi. “Would Dov then have to take an oath to contradict the single witness, even though I myself don’t know for sure whether he owes me?”
“Generally, a person only needs to take an oath when there is a definite claim against him,” explained Rabbi Dayan. “There are some cases, though, in which our Sages required an oath even on a possible claim.” (C.M. 75:17, 93:1)
“But I’m not claiming Dov might owe me,” argued Avrumi. “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 unaware of the facts and claims based on the witness, just as two witnesses obligate the defendant even the plaintiff knows about the debt only based on the testimony.” (See Rosh, Shavuos 6:5)
“What does the Shulchan Aruch rule?” asked Avrumi.
“The Shulchan Aruch rules that a claim based on a 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 Avrumi. “I know him well and trust him completely, so there’s no doubt in my mind!”
“That is insufficient basis to impose a Torah oath,” replied Rabbi Dayan. “However, if you trust him absolutely, some say this is sufficient basis to impose a rabbinic oath, shevuas heses, 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)