This encounter between Feste and his mistress, Olivia, occurs in Act I scene 5. Feste, having heard his mistress bid that the "fool" be taken away, says that Olivia should be taken away. When asked to prove that Olivia is mad, he does so by asking her about her dead brother. When asked if her brother is in hell, Olivia angrily responds that he is in heaven. Feste responds by saying he has proven that Olivia is a fool:
The more fool, madonna, to mourn for your brother's soul, being in heaven. Take away the fool, gentlemen.
This scene shows Feste's function as the court fool to Olivia: he is meant to bring amusement with his ridiculous twisting of words, and to brighten even the darkest of moments. He is a topsy-turvy figure, who with his cunning wordplays turns everything on its head and makes one thing seem to be like another. This is something that Viola recognises in her soliloquy about Feste, and this is also something that brings Feste into conflict with Malvolio, who does not appreciate Feste's humour in any sense. It is this conflict that forms such a vital element of the subplot of this play, as Feste goes on to twist the words of Malvolio to such an extent that he doubts his own sanity.