Macbeth certainly has a vivid imagination, but his poetic imagination explores metaphors depicting his state of mind. The first time it shows itself is when he imagines the dagger.
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. (Act 2, Scene 1, enotes pdf p. 27)
Macbeth clearly imagines the dagger at about the time he is supposed to kill Duncan. The dagger represents his poetic imagination because it is a metaphor for the act he needs to commit. He sees, yet cannot grasp, the dagger. He feels like he needs to kill Duncan, yet he cannot gain the courage and conviction to do so. By the end of the speech, Macbeth has convinced himself.
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. (Act 2, Scene 1, p. 28)
Macbeth’s imaginative dialogue with the imaginary dagger has strengthened his resolve.
One of Macbeth’s most famous speeches is also a reflective fit of imaginative genius. When he hears about his wife’s death, he first imagines it’s too soon, and comments to himself that “all our yesterdays have lighted fools” (Act 5, Scene 5, p. 84). He realizes that life is a passing trend.
Out, out, brief candle!(25)
Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage
And then is heard no more. It is a tale
Told by an idiot, full of sound and fury,
Signifying nothing.(30) (Act 5, Scene 5, p. 84)
This beautiful metaphor again gives us a window into Macbeth’s state of mind. He is pondering the brevity and fickleness of life, as he accepts his wife’s death and his own.
All quotes taken from: http://www.enotes.com/macbeth-text