Is this a dagger you see before you?

Macbeth.

"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-oppress'd 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."

Basically, Macbeth is so excited about killing King Duncan that he hallucinates a dagger hovering in the air before him.