Joseph escalates the pressure with a legal framing. Targum Pseudo-Jonathan on Genesis 42:16 preserves the formula: send one to fetch your youngest brother while the rest remain bound, "and if not, (by) the life of Pharoh you are spies."
An Egyptian oath in a Hebrew mouth
The oath — "by the life of Pharaoh" — was standard Egyptian court rhetoric. Swearing by the reigning monarch's life gave a statement legal weight equivalent to a sworn affidavit. Joseph uses it twice in this scene, here and in verse 15. The rabbinic tradition in the Tosefta Sotah 10:9, a tannaitic legal collection compiled in the Land of Israel around the third century CE, reads Joseph's oath not as assimilation but as strategic performance. He is playing the role of an Egyptian vizier fully, down to the oath formula, because the brothers must believe he is exactly that.
The design of the test
The offer — one brother goes home, the rest wait in custody — is a trap with a purpose. The brothers had once returned home without Joseph. Will they now leave another brother behind when it would be easier to walk away? The Aramaic paraphrase, which took its final form in the Land of Israel around the seventh or eighth century CE, preserves the full stakes: either the story checks out or they are executed as spies. There is no neutral outcome.
The takeaway
Joseph does not test his brothers with words. He tests them with stakes. The right question, asked with real consequences behind it, reveals what a person is actually made of.