Macbeth's visions and hallucinations play a pivotal role in his ambition for power and in his development as a character.
From Act I, Scene 3, in which Macbeth encounters the three witches, the "vaulting ambition" of Macbeth takes him into the phantasmagoric realm of visions and things imagined. These visions and hallucinations of Macbeth then become the objective correlatives of both his fears and his guilt.
Prior to his murder of King Duncan, for instance, in his troubled mind Macbeth knows that Duncan, who is his kinsman, is a virtuous man, having "borne his faculties so meek" (1.7.17). And because of his fears and twinges of conscience, Macbeth envisions a dagger before he commits his dastardly act of regicide. This dagger is the objective correlative of the emotions that Macbeth feels as he is about to commit the terrible crime:
Art thou but
A dagger of the mind, a false creation,
Proceeding from the heat-oppressed brain? (1.7.36-39)
Similarly, Macbeth's guilt tortures him as, after Banquo's murder, he envisions the ghost of this noble man. In Act III, Scene 4, in his guilt Macbeth feels confronted by the ghost of Banquo, and with trepidation he speaks to this ghost:
Thou canst not say I did it: never shake
Thy gory locks at me. (3.4.54-55)
After the ghost of Banquo reappears, Macbeth's paranoia grows as "nothing is but what is not" and he begins to lose his hold upon what is real and what is not. In this fear Macbeth tries to eliminate anyone who may be a threat to him. He revisits the witches for assurance that he will remain king. When he demands to know about the future, the witches summon apparitions to inform him. The first apparition is that of an armed head that tells him to be wary of Macduff; the second is a child, covered with blood, that states that no man born of a woman will cause harm to Macbeth; and the third vision is that of a child wearing a crown who is holding a tree. This apparition says,
Macbeth shall never vanquished be until
Great Birnam Wood to high Dunsinane Hill
Shall come against him. (4.1.96-98)
So immersed in this realm of witchcraft and things seen and unseen is Macbeth that he believes himself safe from harm by heeding the words of these apparitions. Ironically, however, it is his increasingly delusional mind and the equivocations of the witches which bring about his demise. For it is only when he is confronted by the reality of these ambiguities that Macbeth realizes the deception of the preternatural world and its visions as well as the devastating consequences of the phantasmagoric realm in which he has dwelt.