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?
William Shakespeare, Macbeth, Act II, scene i