Benjy Braun was learning Parshas Matos with his father.

“This parsha contains a fascinating story about the tribes of Gad and Reuven,” said Mr. Braun. “It emphasizes Am Yisrael’s responsibility for each other, and especially for those in the Land of Israel.”

Benjy read about the tribes of Gad and Reuven, who asked to settle the eastern bank of the Jordan River. Moshe consented to their request only on condition that they first cross the Jordan with their brethren and help complete the conquest of Canaan. He then instructed Elazar, Yehoshua, and the tribal leaders:

“If the children of Gad and the children of Reuven will cross the Jordan with you… give them the land of Gilad as a heritage. But if they do not cross over, armed, with you, then they will settle with you in the land of Canaan (Bamidbar 32:29-30).”

“Why was it necessary for Moshe to explicitly state the flip side of the condition?” Benjy asked. “Isn’t it obvious that if they don’t fulfill the condition, they don’t get the Transjordan?”

Benjy’s father looked in Rashi, but couldn’t find an explanation. “That’s a tough question,” he finally acknowledged. “Let’s ask Rabbi Tzedek on Shabbos.”

On Shabbos, they approached Rabbi Tzedek.

“Benjy had a question on Parshas Mattos,” Mr. Braun said.

Rabbi Tzedek looked at Benjy fondly. “What was your question?” he said.

“Why did Moshe have to state both sides of the stipulation with Gad and Reuven?” Benjy asked. “Isn’t the flip side obvious?”

“Perhaps, but the Gemara (Kiddushin 61a; Gittin 75a-b) derives from these verses important principles about the proper formulation of legal stipulations,” answered Rabbi Tzedek. “These are referred to in halacha as ‘the stipulation of bnei Gad and bnei Reuven.’ For this reason, the Torah was very specific in its wording of Moshe’s stipulation.”

“Oh,” said Mr. Braun. “I wasn’t aware that there was a specific formulation.”

“Yes, a stipulation must be formulated with four elements to be legally binding,” explained Rabbi Tzedek. “Otherwise the stipulation is invalid and the transaction is upheld as if there were no stipulation (E.H. 38:2).

“The first, and most obvious from the verse, is the need to spell out both sides of the stipulation (tenai kaful),” continued Rabbi Tzedek. “That is, if the condition is fulfilled, the transaction will carry through; if the condition is not fulfilled, the transaction is nullified. This is parallel to the case of bnei Gad and Reuven, in which Moshe spelled out that if they would go forth in battle, they would be granted the eastern bank of the Jordan; if not, they would be settled on the western side.”

“What is the second criterion?” asked Mr. Braun.

“The order of these two clauses: the positive side of the condition must be stated first, and then the negative (hen kodem l’lav),” replied Rabbi Tzedek. “In this case: ‘If they will cross…,’ and afterwards, ‘If they will not cross....’ The stipulation should not be formulated in the opposite order: ‘If they do not cross…; If they will cross....’”

“And the third criterion?” asked Benjy.

“The conditional ‘if’ clause must precede the transaction statement, not the other way around (tenai kodem l’maaseh),” explained Rabbi Tzedek. “In this case: ‘If they will cross the Jordan, they will receive…,’ and not: ‘They will receive… if they cross.’

“And finally, the condition must be something possible to fulfill (davar she’efshar l’kayemo),” added Rabbi Tzedek. “In this case, it was possible for them to cross the Jordan and do battle. However, if a person stipulates a certain transaction if the person jumps to the moon, the stipulation is disregarded and the transaction is upheld.”

“Does that mean,” asked Mr. Braun, deep in thought, “that if I sold something on condition, but the precise formulation was not followed — the sale is binding without fulfillment of the condition!”

“This formulation is required for stipulations in marriage and divorce,” explained Rabbi Tzedek. “There is extensive discussion whether this formulation is also needed in monetary law (C.M. 207:1).”