How are the themes of mercy and justice explored in the Merchant of Venice?

The central scene of this impressive play is Act IV scene
1, which is of course the famous trial scene. The most pertinent speech in this entire
scene, however, is Portia's reflection on the nature of mercy and justice. Let us remind
ourselves of what is said in this speech that captures these twin themes so
brilliantly:

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The quality of mercy is not
strained.

It droppeth as the gentle rain from
heaven

Upon the place beneath. It is twice
blest:

It blesseth him that gives and him that
takes.

'Tis mightiest in the mightiest; it
becomes

The throned monarch better than his
crown.

His scepter shows the force of temporal
power,

The attribute to awe and
majesty,

Wherein doth sit the dread and fear of
kings.

But mercy is above this sceptered
sway;

It is enthroned in the hearts of
kings...

Note how this speech
shows us what is at issue. Shylock as the right in law to his pound of flesh. He insists
on justice and the law. Ironically, Portia, in her disguise, insists on upholding every
letter of the law, knowing that in the law lies the loophole that will allow Antonio to
be safe. Yet she still tries to encourage Shylock to show
mercy.

However, what is clear in this scene is that
Shylock's concern is not for justice and for the law, but for revenge. He is happy to
use justice and the law as vehicles to achieve his revenge, but they are not important
to him. Therefore, perhaps we can argue that his punishment is fitting because of the
way he uses and manipulates important concepts such as justice. He receives justice
because of the way that he abuses it, and the irony of Portia applying the law fully in
his case is not lost on both him and others. Having just refused to show the slightest
bit of mercy in the case of Antonio, he can hardly expect mercy to be shown towards him.
Yet, having said this, many productions focus on the punishment of Shylock as being too
harsh, and suggest that there is a double standard in Portia's eloquent plea for mercy
and then her own inability to show mercy to Shylock. Either way, justice and mercy is
one of the central themes of this play.