What is the meaning of [Is this a dagger which I see before me?]

This is one of the best-known lines from Shakespeare’s Macbeth, 1605. Shakespeare used the image of a dagger in many of his plays. In fact there are few of his plays that don’t have a reference to daggers in some form – most commonly deployed as symbolic of treachery. In the Scottish play, Macbeth has a vision of a dagger, pointing toward the King Duncan’s chamber and perhaps indicating that he should use it to follow through on his and Lady Macbeth’s plan of murdering the King.

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-oppressed brain?
I see thee yet, in form as palpable
As this which now I draw.