In her famous "the quality of mercy is not strained" speech in act 4, scene 1 of William Shakespeare's The Merchant of Venice, Portia, disguised as a young Doctor of Laws, appeals to Shylock's humanity to forego his legal right to a pound of Antonio's flesh and instead accept a monetary settlement in payment of Antonio's debt to him.
Portia was wasting her breath, of course, and she very likely knew she was, because Shylock is wholly intransigent throughout the trial—he insists on the letter of the law and the "pound of flesh" he's legally owed—and his hatred of Antonio, and indeed of all Christians, overwhelms any mercy he might have felt or shown to Antonio were he not so adamant about receiving the justice he so stridently demands.
As intelligent and resourceful as Portia is throughout the play, it's unlikely that she didn't have the technicality of the law which she later uses against Shylock already in mind when she made her famous speech, but she nevertheless must make the effort to persuade Shylock to amend his bond by appealing to his basic humanity.
PORTIA. Then must the Jew be merciful.
SHYLOCK. On what compulsion must I? tell me that.
PORTIA. The quality of mercy is not strained.
(act 4, scene 1, lines 185–187)
Portia's argument for Shylock to extend mercy to Antonio fails, as she knew it would, and as Shylock holds his knife ready to cut his pound of flesh from Antonio's body, Portia turns the legal tables on him.
PORTIA. Tarry a little;—there is something else.
(act 4, scene 1, line 314)
By law, Shylock is entitled to his pound of Antonio's flesh, but, by law, he's not entitled to even one drop of Antonio's blood. Not only that but, as it turn out, by threatening to take his "pound of flesh" from Antonio, by law Shylock is guilty of attempted murder.
The irony of Portia's "the quality of mercy is not strained" speech is that not one Christian in the court room, not the Duke, Portia, Antonio, or Gratiano, extends any mercy whatsoever to Shylock. To compound their display of inhumanity towards Shylock, Antonio, to whom Portia entreated Shylock to extend mercy and who did not expect to leave the courtroom alive, demands that in addition to the penalties that the Duke levies against Shylock, that Shylock become one of them, a Christian. Mercy "droppeth, as the gentle rain from heaven" (line 188) on everyone else but not on Shylock.