14 And Joseph said unto them, That is it that I spake unto you, saying, Ye are spies: 15 Hereby ye shall be proved: By the life of Pharaoh ye shall not go forth hence, except your youngest brother come hither. 16 Send one of you, and let him fetch your brother, and ye shall be kept in prison, that your words may be proved, whether there be any truth in you: or else by the life of Pharaoh surely ye are spies.
Joseph repeats his accusation that they are spies. He suggests that their story is clearly false, and that he cannot believe it unless they bring the one brother out of eleven that is absent. In other words, “if you expect me to believe that all ten of you are brothers in one family, then bring along another member of your kin to confirm it.”
The account from earlier in Joseph’s life seem to suggest that Benjamin was born before Joseph was sold into Egypt, so this was presumably not a ploy to meet the lad for the first time. Several have speculated that Joseph’s chief concern might have been instead to ascertain that Benjamin was well taken care of. After all, he had very personal reasons to distrust the sons of Leah, Bilhah, and Zilpah, and the way they behaved towards the sons of Rachel.
The solution that Joseph suggests is extreme. Keep all of the brothers in imprisonment, except for one, who will be permitted to go and retrieve the youngest brother, Benjamin. Only if the storied son is revealed will all the others be allowed to go free.
What isn’t clear is whether Joseph is giving the brothers any alternative. If they didn’t agree to this exchange, would they have been free to leave, just without any of the grain they needed? Or were they locked into the situation now, whether they liked it or not?