Scholars debate whether Macbeth's vision of Banquo is real or a hallucination. Macbeth had already seen a hallucination before murdering Duncan: a knife hovering in the air. Several performances of the play have even ignored the stage direction to have the Ghost of Banquo enter at all, heightening the sense that Macbeth is growing mad, since the audience cannot see what he claims to see. Scholars opposing this view claim that while the dagger is unusual, ghosts of murdered victims are more believable, having a basis in the audience's superstitions. Spirits in other Shakespeare plays—notably "Hamlet" and "Midsummer Night's Dream"—exist in ambiguous forms, occasionally even calling into question their own presence.