In Portia's speech on "the quality of mercy," what is the relationship between justice and mercy?
In Portia’s famous monologue, she attempts to convince Shylock to favor mercy over justice. He insists that the law technically supports his bond with Antonio, even though it requires a pound of Antonio’s flesh. Shylock asks “On what compulsion must” he be merciful. Portia replies that “The quality of mercy is not strain'd.” It is not given under “compulsion” but comes as naturally as “gentle rain from heaven.” She associates mercy with both nature and deity.
Portia notes how impressive it is when those who wield power demonstrate compassion. They could enact justice but instead decide to forgive. It is somewhat reminiscent of Prospero’s comments in The Tempest when he realizes that “the rarer action is / In virtue than in vengeance.” According to Portia, the king’s “sceptre shows the force of temporal power,” whereas grace is a divine quality:
But mercy is above this sceptred sway;
It is enthroned in the hearts of kings,
It is an attribute to God himself…
The speech argues against a limited sense of fairness because “in the course of justice, none of us / Should see salvation.” No one is free from sin. Therefore, Portia argues that, in the grand scheme of things, mercy is superior to justice.