Macbeth's dagger soliloquy in Act II scene i of Macbeth shows his state of mind to be one in which his hold on rationality has abandoned him. The first line reveals that Macbeth is having an hallucination: he sees a dagger that he cannot grasp:
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.
The first fifteen lines elaborate upon the hallucination. Macbeth directly says "I see" four times in the first fifteen lines and indirectly implies that he sees three times (e.g., "I see before me ..."; "yet I see thee still ..."; "fatal vision ..."; "Mine eyes are made the fools ..."). Lines sixteen and seventeen offer his denouncement of the vision:
There's no such thing:
It is the bloody business which informs
Thus to mine eyes.
Macbeth's state of mind is most plainly revealed in the first fifteen lines. First of all, he's hallucinating; never a good sign of a good or sound state of mind. From this we must know that everything that follows is the work of a mind unhinged and deranged (disarranged) and we must know that Macbeth cracked under pressure before Lady Macbeth did, although Macbeth, being a trained man of war, can hold appearances together longer than she and continue to give the appearance of sensible conduct, while she soon retires to a wash basin and "Out, damned spot! out, I say!"
We also know that Macbeth questions himself as to whether the vision is real or an illusion:
Art thou not, fatal vision, sensible
To feeling as to sight? or art thou but
A dagger of the mind, a false creation, ....
We also know that his brain is oppressed with literally overwhelming fear--Macbeth's reason, his rational thought, has been overwhelmed: "Proceeding from the heat-oppressed brain." Shakespeare metaphorically describes and compares his fear to "heat." We also know that Macbeth compares the hallucinatory vision to his own dagger, which he draws from its sheath ("As this which now I draw").
His state of mind at this juncture is separated from reality and operating from a delusional perspective because, as he compares the real dagger to the vision, he regards the vision as an omen that points his way: "Thou marshall'st me the way that I was going; ...." The result of the soliloquy is that we know it is at this point that Macbeth's actions are set because of his state of mind, a state in which the power of his fear-shattered and unhinged mind accept his hallucination as an omen.