The brothers are innocent of the cup, and they know it. Their defense, preserved in Targum Pseudo-Jonathan, is an argument from character.
"Behold, the money which we found in the mouth of our bags we brought to thee again from the land of Kenaan; how then should we steal from thy lord's house vessels of silver, or vessels of gold?" (Genesis 44:8). The logic is airtight. We returned silver we were not asked to return. Why on earth would we now steal silver we were not asked to take?
It is the classic Talmudic argument from kal vachomer, from light to heavy. If we were honest in the small thing, we are surely honest in the greater. A man who returns a found coin does not then pocket a goblet.
The brothers are telling the truth — about the cup. They did not steal it. But the Targum invites us to hear the irony crackling under their words. These same men, twenty-two years earlier, had sold their own brother for twenty pieces of silver and told their father a wolf had done it. Their moral ledger is not as clean as their argument suggests.
The sages teach that the mouth which once lied about Joseph is now telling the truth about a cup. That is the shape of teshuvah. You do not become a new person all at once. You start by being honest about small things. Eventually the honesty grows until it can bear the weight of the big things — the brother in the pit, the father you deceived, the years you stole.
The brothers' defense is valid. It is also a trial run for the greater confession Judah is about to deliver.