After the Prince of Arragon has made his disastrous choice of caskets and, following Shylock's rules, departs (Act 2, Scene 9), Bassanio arrives in Act 3, Scene 2, to make his choice. Portia's opening lines make her love for Bassanio clear when she tells him that he ought to stay "a day or two" to consider his choice of caskets, and she goes even further:
I would detain you here some month or two/Before you venture for me. I could teach you/How to choose right, but I am then forsworn. (3:2:9-11)
Clearly, Portia's love for Bassanio leads her up to the point of breaking her promise not to assist a suitor, but because she is truly virtuous, she cannot cross the line of duty for the sake of her happiness.
She realizes, of course, that Bassanio might choose the wrong casket, in which case Portia acknowledges that she would then wish that she had violated her promise. Almost out of desperation, she is forced to admit that she has fallen under the spell of Bassanio's eyes:
Beshrew your eyes,/They have o'erlooked me and divided me;/one half of me is yours, the other half yours,/Mine own, I would say; but if mine,/ then yours . . . . (3:2:14-17)
Loosely translated, Portia is saying, "Curse your eyes,/ They have bewitched me and divided me;/ one of me belongs to you, the other half is also yours,/ I would call [the other half] mine, but what is mine is also yours. . . ." In effect, Bassanio's eyes have conquered Portia.
The image of Bassanio's bewitching eyes is Shakespeare's creative way of turning the Renaissance and Elizabethan belief that the eyes mirror the soul into an important plot device in this scene.