During the trial established by Portia's father, Bassanio sifts the choices by acting upon the premise that appearances mislead.
When musing over the gold and silver, he compares their flashy exterior to the sophistic arts that are applied in the fields of legal and religious debate:
In law, what plea so tainted and corrupt / But, being seasoned with a gracious voice, / Obscures the show of evil? In religion, / What damned error but some sober brow / Will bless it, and approve it with a text, / Hiding the grossness with fair ornament? (3.2.75-80)
Bassanio will go on to extend this meditation on pretexts by considering the beard men grow to appear wise.
Thus, he takes as his rationale the proposition that all flashy displays are deceitful. From this premise he is able to conclude that he ought to choose the lead casket. This reasoning shows him to be a man skeptical of appearances and who applies a syllogistic reasoning process when making decisions.