One of the great soliloquies of Western Art comes from the imagination of Macbeth, contemplating regicide. This speech is a miracle of eloquent verse, vivid imagery, psychological progression and foreshadowing. If Shakespeare had written no other plays, he’d still be remembered today for this bloody masterpiece:

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

A dagger of the mind, a false creation,

Proceeding from the heat-oppressèd 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.

Mine eyes are made the fools o’ th’ 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.

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 curtained sleep. Witchcraft celebrates

Pale Hecate’s offerings, and withered murder,

Alarumed 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. Thou sure and firm-set earth,

Hear not my steps, which way they walk, for fear

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. 

—Macbeth, Act II Scene i 

Is the dagger real? It is a hallucination? Is Macbeth mad? What a delicious mix of ambition, fear, temptation and eloquence. Honestly–to hell with what it means: just read it aloud and revel in its lyrical brilliance. Then read the play or watch a movie of it, and there, in context, the many meanings of this ruinous foreshadow will open up for you. 

Macbeth knows he plots murder, but hedges when he sees the apparition of a bloody dagger. It causes him pause, until he can–despite the warning of the apparition–steel himself to proceed with his planned murder. For in the end Macbeth cannot discern the meaning of what he sees, and thus its warning is lost on him.   

The image is of Patrick Stewart in his great turn as the cursed Macbeth in the 2010 film.