In Shakespeare's Hamlet, there exist instances of madness that play a crucial role in the development and unfolding of the play. The three major illustrations of the madness are when Hamlet feigns his madness at the start of the play, Ophelia's madness after Polonius" death and Hamlet's semi-madness when he becomes violent with his mother towards the middle of the play.
The first case of madness presented is Hamlet's feigned madness at the start of the novel. It is quite apparent that this first madness is feigned because Hamlet himself tells Horatio not to alert anyone if he is seen out of his mind. Hamlet's emulated madness is revealed through his talk and the way he responds to other in the play, such as when he calls Polonius a "fishmonger" or when he so harshly turns Ophelia and her love down when he realizes he is being observed. This artificial madness can be deemed reasonable on accounts that it provides Hamlet with time, it gives him the time needed to decide what action to take, whether to believe the ghost or not, and it provides a means to get close to Claudius without drawing attention to himself. The madness Hamlet puts on is also reasonable because it shows his rationality and his morality. The madness related to the work as a whole is quite important as a plot mover, it progresses the plot in the play and it gives insight to Hamlet's inner feelings.
The next case of madness can be found in Ophelia's role. After her father is killed by Hamlet, she goes completely mad. Her madness is exposed by her singing bawdy drinking songs, wandering around and losing herself in her own thoughts. Her madness is true form the heart, and her madness is quite justifiable. She is justified in going mad because in her soul she realizes the instant that she finds out her father has died that she played a part in this turn of events. It is this thought that drives her crazy; that because she listened to her father's false advice she lost her love, and then she lost her father at her love's hands, so now she has nothing to live for.