What hallucination is Macbeth describing in Act 2, Scene 1? and why?
Shakespeare seems to have taken considerable pains to preserve some audience sympathy for his protagonist, since this is supposed to be Macbeth's tragedy. The playwright tries to shift some of the blame for Macbeth's terrible crime of murdering the loving, trusting, generous, fatherly King Duncan onto Lady Macbeth, the Three Witches, "Fate," and Macbeth's apparent mental disorder. Shakespeare has Macbeth imagine that a floating dagger is leading him towards Duncan's bedchamber. The audience cannot see such a dagger and therefore assumes that poor Macbeth is not in his right mind. Otherwise he could not bring himself to kill the King.
Macbeth talks to the imaginary dagger because otherwise the audience would not understand what was supposedly happening. The hallucination itself is not tangible. It exists only in Macbeth's mind. So he has to inform the audience of what is going on in his mind.
Come, let me clutch thee.
I have thee not, and yet I see thee still.
Art thou not, fatal vision,sensible
To feeling as to sight? Or art thou but
A dagger of the mind, a false creation,
Proceeding from the heat-oppressed brain?
I see thee yet, in form as palpable
As this which now I draw.
Thou marshall'st me the way that I was going,
And such an instrument I was to use.
In addition to using Macbeth's hallucination in an attempt to further palliate Macbeth's guilt, the soliloquy tells the audience where Macbeth is going and what he is going to do. We never see the actual murder committed--and this too may be because it would make Macbeth lose whatever audience sympathy he still retains--but we see what happens immediately before and immediately after that event. We can imagine what happens in between. Macbeth is still in a hallucinatory state when he appears in his own bedchamber holding the two daggers he took from the grooms. Therefore we know he was not in his right mind when he killed the King. He might plead not guilty by reason of temporary insanity. We can at least appreciate the fact that Macbeth displays and expresses extreme remorse for what he has done. Shakespeare maintains a shred of sympathy for his hero up until the time Macbeth has his soldiers slaughter Macduff's wife and little boy while others obviously are engaged in slaughtering everybody else in Macduff's castle.