What's the whole story behind the cup in the Miketz saga? What did Joseph aim to achieve by concealing the goblet in Benjamin's sack? Since we already know the end of the story, we might assume that Joseph wanted to test his brothers and see if they were willing to sacrifice themselves to save Benjamin. However, that's a hindsight perspective after reading the continuation of the biblical story.
What was Joseph thinking at that moment?
In my opinion, the solution lies at the beginning of the passage when Joseph's two sons, Manasseh and Ephraim, are born in Egypt. The text explains the names: "And Joseph named the firstborn Manasseh, for 'God has caused me to forget all my toil and all my father's house.' And the second one he named Ephraim, for 'God has made me fruitful in the land of my affliction.'" (Genesis 41:51-52). Now, it becomes clear objectively. Joseph seems to want to disconnect from his father's house, forget his father, and succeed in Egypt.
To his surprise, his ten brothers arrive during the famine, something he did not expect. When he meets them, it's mentioned: "And Joseph remembered the dreams that he had dreamed about them" (Genesis 42:9).
What did he remember in the dreams? That there were not just ten stars but 11 stars! He recalls Benjamin. Now, he fears that Benjamin might be in danger, just as Joseph, Rachel's son, was cast away from the family. Maybe Benjamin is also cast away from the father's family.
Joseph wants to save Benjamin, and the whole scheme of hiding the goblet is an attempt to separate Benjamin from his brothers.
If Joseph's idea had succeeded, and his proposal had been accepted ("The man in whose possession the (silver) goblet was found shall be my slave, but as for you, go up in peace to your father" - Genesis 43:17), he might have thought that Benjamin would stay with him in Egypt. His brothers would never return to him.
He did not expect that Judah would not allow it.
Joseph was surprised to find out that Judah already understood that there was a bond among the brothers (as he declared to his father in Genesis 42:9 - "I will guarantee him").
All of Israel is responsible for one another.
The dimension of the collective has already been born. This, in essence, will reveal the failure of Joseph's attempt to separate Benjamin from his brothers, as we will see in the next part of the narrative.
You can also join the Hebrew people on your daily behavior, as mentioned in Isaiah [54:6-7]
"And the foreigners who join with GOD to serve Him and... to be His servants... I will bring them to My holy mount, and I will cause them to rejoice in My house of prayer..." - Click here.