Though Shylock’s “Hath not a Jew” speech carries tremendous emotional weight, it is clear that ethos—or the power of established authority—rules Venice. The play’s tension culminates in the courtroom, which is managed not only by written law but by the men who know it best: the Duke and Balthazar (Portia’s disguise as a male lawyer).
The Duke speaks for every Christian in the room when he says,
Shylock, the world thinks, and I think so too,
That thou but leadest this fashion of thy malice
To the last hour of act, and then, ’tis thought,
Thou ’lt show thy mercy and remorse more strange
Than is thy strange apparent cruelty;
[...] We all expect a gentle answer, Jew.
He makes it clear that he not only holds authority over Shylock, but that he is part of a dominant collective—the Christians—who control public opinion and who expect mercy from Shylock though they refuse to offer it themselves. The pun on "gentle" ("Gentile) in the last line cements this dynamic, as the ruling class expects outsiders like Shylock to conform with their ideals.
Shylock, of course, refuses to let Antonio off the hook and ultimately faces the wrath of Portia’s legal intellect. She, too, is without mercy but she is both logical and clever and wins the battle because of these strengths. Not only does she render Shylock’s bond impossible (for he cannot collect the flesh without collecting blood, a provision not specified in the contract), but she adds that,
It is enacted in the laws of Venice,
If it be proved against an alien
That by direct or indirect attempts
He seek the life of any citizen,
The party ’gainst the which he doth contrive
Shall seize one half his goods; the other half
Comes to the privy coffer of the state,
And the offender’s life lies in the mercy
Of the Duke only, ’gainst all other voice.
Portia has the power to strip Shylock of not only his wealth but his livelihood and, eventually, his dignity. And like the Duke, she does so through an authority founded on both intellect and the power of public opinion.