In William Shakespeare's play "Romeo and Juliet," Mercutio speaks of dreams as nothing more than fantasies that are "the children of an idle brain." A dream is a delusion caused by thoughts or desires we had before our brain went idle and was allowed to wander.
In "Romeo and Juliet," Mercutio goes on what seems to be a delusional rant about Queen Mab and the delivery of dreams. He gives some examples of utter fantasy based on how people have been living their lives: "O'er courtiers’ knees, that dream on court'sies straight; O'er lawyers’ fingers, who straight dream on fees; O'er ladies’ lips, who straight on kisses dream."
He says that lovers (courtiers) dream of love, lawyers dream of money and women dream of sweet kisses. These examples continue throughout the dialogue between Mercutio and Romeo, touching on the subjects of various individuals' dreams brought on by a fictional dream bringer, Queen Mab.
Mercutio closes his conversation by stating, "True, I talk of dreams; Which are the children of an idle brain, Begot of nothing but vain fantasy." Mercutio is reiterating that dreams nothing more than fantasies brought on by our own inner desires and the wanderings of the idle brain.