Joseph sets a strict condition for his brothers to secure their release and clear their names of espionage: they must bring their youngest brother to Egypt. The primary approach among commentators explains how this demand directly addresses the accusation. Spies do not typically operate as a unified family unit of brothers. The brothers had previously explained that their suspicious behavior, such as entering the city through multiple different gates, was merely a search effort to find their lost brother. By verifying their family structure, Joseph could confirm their honesty and dispel the suspicion of treason [ברכת אשר על התורה].
Other commentators offer additional reasons for this specific test. Joseph assumed that a young boy would not be able to maintain a lie under intense interrogation [אברבנאל]. Furthermore, if the brothers were indeed lying about their identity, the youngest brother would never agree to accompany them into a dangerous situation where he might be killed [ספורנו, ביאור יש״ר]. Joseph was also unconcerned that the brothers might simply purchase a random child from the market and present him as their sibling. Jacob’s sons possessed a highly unique and impressive physical appearance, making it extremely difficult to pass off a stranger as family [הטור הארוך]. The brothers themselves would be too afraid to attempt such a deception, knowing the governor could easily expose the lie by examining the child's facial features or native language [פענח רזא, בכור שור].
Beyond the immediate interrogation, Joseph harbored hidden, personal motives for this demand. He sought to fulfill his early prophetic dreams, which foretold eleven stars bowing down to him [הכתב והקבלה, ביאור יש״ר]. Additionally, he needed to test his brothers to see if they had harmed Benjamin, or if they had changed and would now risk their own lives to save him out of genuine brotherly love [אברבנאל].
To enforce his demand and instill fear, Joseph swears a severe oath by the life of Pharaoh. Linguistically, biblical oaths often use a phrasing where a positive condition implies a negative outcome, meaning his declaration functioned as a binding vow that they would physically not be allowed to leave their current guardhouse without fulfilling the condition [שד״ל, מזרחי, גור אריה, רש״י, דברי דוד]. Some suggest this was an oath sworn by an Egyptian idol [אם למקרא]. This raises a significant debate among commentators regarding whether the righteous Joseph swore a false oath, knowing he eventually planned to let most of them go. One approach suggests that because he was forced into a deceptive situation, he deliberately swore by Pharaoh's life rather than by God, with some adding that he secretly directed his vow toward the first Pharaoh, a man known for his deceit [רש״י, מזרחי, חומת אנך].
Conversely, many strictly reject the notion that Joseph would utter a false oath, arguing it contradicts his high moral standing. They maintain that his vow was perfectly fulfilled: he swore that the group as a whole would not leave, and indeed, Simeon was left behind in prison [אברבנאל, הכתב והקבלה]. Moreover, since Joseph already knew they were honest men and not spies, the underlying premise of the oath was rooted entirely in truth [הכתב והקבלה].
On a deeper, allegorical level, Joseph’s demand was not merely a political interrogation but a profound spiritual trial. Egypt was notorious as a land of extreme impurity and immorality. Through his harsh testing, Joseph was subtly indicating that their time in this land would challenge their ability to maintain their holiness without falling into sin, testing whether they could successfully elevate the hidden sparks of holiness trapped within Egypt [פני דוד].