I assume that you are referring to Act IV, scene i, when Portia appears at the Venetian court, disguised as a young doctor of law, to argue the case brought before the Duke concerning Antonio's inability to repay Shylock the money loaned him. Shylock requires a pound of flesh from the body of Antonio nearest the heart in recompense.
Portia does a few things to solve this problem and save Antonio. First she pleads to Shylock to have mercy upon Antonio and forgive him the bond. In a long speech on the value and quality of mercy, she says:
. . . Therefore, Jew,
Though justice be thy plea, consider...
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