In our Parsha, when Yosef accuses his brothers of stealing Paroh's cup, they defend themselves with a common example of Talmudic reasoning, a Kal VaChomer. Literally meaning "lenient and strict," it is a proof from a more minor case applied to a major case or vice-versa. The Kal VaChomer they used was based on the fact that they had come to return the money which they themselves had originally brought to pay for their food, but which was given back to them; having been honest enough to do this, why would they then steal from the house of Paroh (בראשית מ"ד:ח')?
There is, however, a flaw in the reasoning of the brothers' argument, as pointed out by the Nachal Kedumim. We can understand this flaw by examining Yosef's accusation more closely. Yosef confronts the brothers saying that they had taken the cup which Paroh usually uses to attempt to divine future events, and that they had done an evil thing (שם פסוק ה'). If this cup had some sort of mystical power, then surely one could not deny having taken it with an argument about the monetary aspect of the theft, since the cup had more than monetary value. The Kal VaChomer focusing on the financial issue is thus not relevant. How then was the brothers' argument at all logical? Again we must look more closely at Yosef's words, specifically his calling the theft an "evil thing" (שם). Why this statement calling the theft an "evil thing"? The Nachal Kedumim explains this as being due to the fact that divination and magic was permitted for Paroh, a non-Jew, but the brothers, who kept the Mitzvos of the Torah, were forbidden from trying to make predictions. Since it was something prohibited to them, taking the cup used for these activities must have been a wanton act of theft for theft's sake and was therefore an "evil thing".
This answers our problem about the brothers' logical reasoning as well. By calling the theft an "evil thing," Yosef is recognizing that the brothers could not be stealing it for its mystical value, for this was of no use to them. Their only possible motivation must have been financial, and therefore their Kal VaChomer remains valid.