Appearance and Prejudice
One of Shakespeare's most frequent themes is appearance versus reality. This theme manifests itself in different ways for different purposes. In Merchant of Venice (2.2.181), Bassanio says to Gratiano:
Thou art too wild, too rude and bold of voice—
Parts that become thee happily enough,
And in such eyes as ours appear not faults.
But where thou art not known—why, there they show
Something too liberal.
Gratiano's antics are perceived by his friends differently than they are by strangers. His friends like him for the very characteristics that strangers find objectionable. Consequently, Bassanio asks Gratiano to tone it down so that Portia will not judge Bassanio harshly by the nature of the company he keeps. Interestingly, Portia later in Merchant (3.4.1) expounds on her rationale for coming to Antonio's defense, though she has never met him, for precisely this reason, but she stands the argument on its head. She argues that Antonio must be an honorable man because he is Bassanio's best friend, whom she now loves.
–for in companions
That do converse and waste the time together,
Whose souls do bear an equal yoke of love,
There must be needs a like proportion
Of lineaments, of manners, and of spirit;
Which makes me think that this Antonio,
Being the bosom lover of my lord,
Must needs be like my lord.
This theme is a subtext to a central conflict of this play, the relationship between Christians and Jews, between Antonio and Shylock. In a sense, it is guilt, or goodness, by association, which is, of course, a type of prejudice.