Is this a dagger which I see before me, The handle toward my hand?

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 (1564-1616), Macbeth
-- Act ii, Sc. 1