Is this a dagger which I see before me,
The handle toward my hand? 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(45)
A dagger of the mind, a false creation,
Proceeding from the heat-oppressed brain?
In this section of Macbeth's famous "Is this a dagger which I see before me" soliloquy, Macbeth is questioning his sanity as he deliberates whether or not he should murder King Duncan. He is trying to understand if this dagger that he is envisioning is a figment of his imagination, but the questioning runs deeper than imagination or not; Macbeth is trying to understand if killing the King is a desire of his sane or insane mind.
I see thee yet, in form as palpable
As this which now I draw.
Thou marshall'st me the way that I was going,(50)
And such an instrument I was to use.
Mine eyes are made the fools o’ the other senses,
Or else worth all the rest. I see thee still,
And on thy blade and dudgeon gouts of blood,
Which was not so before. There's no such thing:(55)
It is the bloody business which informs
Thus to mine eyes. Now o'er the one half-world
Nature seems dead, and wicked dreams abuse
The curtain'd sleep; witchcraft celebrates
Pale Hecate's offerings; and wither'd Murder,(60)
Alarum'd by his sentinel, the wolf,
Whose howl's his watch, thus with his stealthy pace,
With Tarquin's ravishing strides, towards his design
Moves like a ghost.
This second section has Macbeth deciding that he knows the dagger is a figment of his mind, but it has become so real to him that he can actually envision the king's blood on the dagger itself. Macbeth decides that he will use the curtain of night to perform the evil deed of killing his king; in the night, Shakespeare uses pathetic fallacy to show how all of nature is in tune with the evilness of Macbeth's plan.
Thou sure and firm-set earth,
Hear not my steps, which way they walk, for fear(65)
Thy very stones prate of my whereabout,
And take the present horror from the time,
Which now suits with it. Whiles I threat, he lives;
Words to the heat of deeds too cold breath gives.
A bell rings.
I go, and it is done: the bell invites me.(70)
Hear it not, Duncan, for it is a knell
That summons thee to heaven, or to hell.
Macbeth recognizes how nature will understand that the deed he will commit is evil, and he is requesting the blanket of night to keep him cover. In this final section of the soliloquy, Macbeth has become self-assured and has finalized his decision to murder Duncan. Nothing will stop him at this point, not his conscience, nor his questionable sanity.