In Shakespeare's Macbeth, in Act Two, scene one, Macbeth, who is on his way to murder his King, cousin and houseguest, suffers from a unstable state of mind.
In Act One, scene seven, Macbeth has shared his misgivings about the murder of Duncan with Lady Macbeth, so the audience understands not only that he is torn about committing the deed, but that he is also weak under the hateful scorn of his wife, who calls him a coward. Eventually she convinces him that he can do it and all will be well.
At the beginning of Act Two, Macbeth has just finished speaking to Banquo, worried that his best friend might be a threat as he is the only person besides Lady Macbeth that knows of the witches' predictions.
When he leaves Banquo, Macbeth is either hysterical or experiencing a "supernatural" event. Certainly we know from the end of the previous act that his heart is not in this plot: up until now, Macbeth has not, as far as we know, ever considered murder. He has been a valiant soldier for the crown and a dedicated servant to his King, who he loves. It is safe to assume that he is nervous and conflicted at the thought of what lies before him. Macbeth has to know that this one act will change everything for him, and he is wise enough to realize that when the deed is done, life will be good if he is successful, but certainly not the same. Before anything else, he will look at himself through different eyes. And in worrying about the threat of Banquo and his secret knowledge, there is foreshadowing that Macbeth will become worried, even paranoid, when it comes to protecting his place on the throne.
One of the famous soliloquies of Macbeth is in this scene when Macbeth imagines a dagger floating before him, seeming to lead him to Duncan's chambers. He wonders if it is the result of a "heat-oppressed brain" (a sick mind). He also questions the dependability of his vision:
Mine eyes are made the fools o'the'other senses... (II.i.44)
When he looks again, there is now blood on the blade. Still he thinks he is imagining what he sees. He also allows that at night, strange things happen: the world sleeps and "wicked dreams" fill men's minds; witches are doing their evil work; wolves wake murderers who then silently move to kill their victims. (II.i.49-56)
Macbeth is not only preparing to kill his King and someone dear to him, but he is also preparing to turn his back on his moral compass. He admits his vaulting ambition drives him on, but he has not been a man born to murder. For these reasons, he is torn between believing his mind is playing tricks on him or his eyesight is unreliable; or, he may believe the vision could be some kind of magic—he has seen the witches at work already. Recognizing that evil things occur at night, Macbeth may well now consider the he will become a part of that evil when he kills Duncan.