Facing the severe accusation of theft, the brothers mount a defense built on simple, unbreakable logic drawn from their own past behavior. The primary approach among commentators is that their response represents a classic logical deduction, standing as one of ten such arguments found throughout the Bible [רש״י, מזרחי].
They break their defense down into several compounding steps to highlight their innocence. First, they point out that the silver they previously took back to Canaan came into their possession by mere chance, not through any active theft. Furthermore, because it was found inside their own sacks, they could have easily claimed legal ownership of the property [מלבי״ם, ביאור יש״ר, שפתי כהן]. Even though they had previously been cleared of any debt, they held themselves to a stricter standard out of doubt and returned the money of their own free will [ביאור שטיינזלץ]. They brought it all the way back from Canaan, a distant land entirely outside Egyptian control, long after the original owner would have given up hope of ever seeing it again [מלבי״ם, ביאור יש״ר, שפתי כהן]. The conclusion naturally follows: if they went to such lengths to return money they could have kept, how could they possibly commit an active robbery right inside the ruler's own home? [מלבי״ם, ביאור יש״ר].
However, a potential weakness exists in this logical defense. While a person might not steal ordinary money, they could still be tempted to take a rare, highly desirable goblet, especially one rumored to possess magical powers of divination [משכיל לדוד, פרדס יוסף, שפתי כהן, ברטנורא]. To resolve this and strengthen the brothers' claim, several explanations are offered. One view clarifies that the money they voluntarily returned was actually worth more than the goblet they were accused of taking [ברטנורא]. Another perspective notes that their defense naturally extended to vessels of silver and gold, directly addressing the specific type of object that went missing [נתינה לגר].
Furthermore, the brothers implicitly rejected the suspicion that they stole the goblet for witchcraft. Since all forms of sorcery were strictly forbidden to them, a divining cup held no mystical appeal; they would only view it for its basic material value. Having already proven they had no desire for wealth that did not belong to them, the accusation loses its foundation [פרדס יוסף, שפתי כהן]. This is why they carefully avoided mentioning the goblet itself in their argument, focusing instead on the sheer absurdity of being labeled greedy [שד״ל, ביאור יש״ר]. Ultimately, by going beyond the basic requirements of the law to return a foreigner's lost property, they proved their complete separation from even the slightest hint of theft [פרדס יוסף].