Portia gives a truly profound reply to Shylock. It’s all about “mercy” as a quality, as an attribute. Such a quality is not forced, cannot be forced. Portia explains that her “must” is neither law nor commandment. It’s a different “must”. "The quality [that is] mercy is not [to be con]strain’d......[not to be forced]." It’s a must which is never a must.
Mercy, she says, is “mightiest in the mightiest.” Mercy fits “the enthroned monarch better than his crown”, better than “his sceptre”. That sceptre “shows the force of temporal power,” and is “the attribute to awe and majesty, wherein doth sit the dread and fear of kings.” “But mercy is above this sceptered sway...” Mercy is above and beyond the power and majesty and authority of awesome judges and world leaders. The must which is behind mercy is a must because, if absent, it shows that the person who has this power over others is dominated by that power and is by no means worthy of that power.
No wonder mercy is so very, very rare........ Are any of us worthy of the power we have over others? Shakespeare’s Portia explains that rarity. “Mercy is an attribute of God himself”.
Shylock is seeking pure justice so she says to him, “Therefore, though justice be thy plea, consider this, that in the course of justice none of us should see salvation.....” If pure justice is to be the only rule in our world, we’re all undone; we’re all in an unchangeable mess. None of us can be saved – unless there is mercy.
Portia again: “We do pray for mercy, and that same prayer doth teach us all to render the deeds of mercy.”
That’s the most important reason why Shylock must be merciful. He wanted to know why he was compelled to be merciful. “Tell me that”, he said. He needed to know of his own personal need to receive mercy himself. If only he could have discovered that, he would have seen that he must be merciful because, unless he showed mercy to others he would receive none himself.