The nameplate on the gold box: "The portrait is in this box" (that is, the portrait is in the gold box) and the nameplate on the lead box "The portrait is not in the gold box" are two propositions, one of which negates the other. According to simple logical knowledge, we can know that a sentence must be true or false, so we can draw a conclusion that these two sentences must be true or false.

Because only one of the three sentences is true, the phrase "The portrait is not in this box" said on the nameplate of the silver box must be deceptive, so the suitor decided that the portrait of Portia was in the silver box.