A reference to Macbeth's speech in Act II, Scene i of Shakespeare's
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.
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.
Lady Macbeth mentions this dagger again in Act III, Scene iv:
O proper stuff!
This is the very painting of your fear:
This is the air-drawn dagger which, you said,
Led you to Duncan. O, these flaws and starts,
Impostors to true fear, would well become
A woman's story at a winter's fire,
Authoriz'd by her grandam. Shame itself!
Why do you make such faces? When all's done,
You look but on a stool.