As the play progresses, the question of what constitutes justice becomes more and more prominent. In the past, Shylock has been wronged by Antonio, and in act 3, he insinuates that revenge is a just action. Shylock holds the idea that one receives what one gets, and his intended revenge is one version of this kind of justice. The fact that Antonio loses his ships throughout the play provides even more evidence to Shylock that Antonio deserves his penalty for defaulting on the loan, and Shylock is all too eager to dispense the corresponding punishment. 

Further, justice is a function of the law, which seems to be Shylock’s primary argument in act 4; as long as the contract is honored, justice will be served. In this way, Shylock’s insistence in taking Antonio’s flesh is doubly just: not only does it serve the karmic function of returning the cruelty that Antonio had previously shown Shylock, but it is also legally stipulated that he should take Antonio’s flesh. Throughout the trial, he continues to appeal to justice as his rationale for following through with the contract. 

However, the opposing claim—that mercy is greater than justice—is also argued throughout the play, especially in act 4. The first time mercy is referred to in the play is when Launcelot speaks with Jessica in act 3, telling her that there is no mercy in heaven for her kind. When Portia speaks of the divine qualities of mercy in act 4, it seems as though she is at least partly responding to Launcelot’s claim. 

Mercy, she argues, does not follow the logic of kings, laws, and justice. Were that the case, Shylock might be killed—or at least ruined—by the end of the play. In fact, it is Shylock’s claims to justice that ultimately prevents him from getting his way; that is, while he is legally and justly allowed to take the pound of flesh, Portia asserts that he has no just claim to any blood that may fall during the transaction. Through this outcome, the play privileges mercy over justice.