This phrase which is used here is taken from William Shakespeare's "The Merchant of Venice". It occurs where Portia demands Shylock be merciful, stating that “The quality of mercy is not strained, It droppeth as the gentle rain from heaven upon the place beneath” (Act-IV, Scene-I).

Portia's statement is directly making an appeal to Shylock to spare Antonio’s life. She is saying that, as all people pray and plead to God for mercy or to be merciful and kind towards mankind, likewise Shylock should be merciful and kind to him. He will get a reward when he gets to heaven.

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