"Is this a dagger...."
This passage has long been a personal favorite of mine. The rhythm is predominantly straightforward iambic pentameter, which makes it one of the easier speeches to illustrate the fundamentals of Shakespeare's versification. Add to it the pure psychological insight of a man standing on the precipice of regicide, alongside the vivid language and imagery, and it's not difficult to see why this speech is viewed as a paragon among the Bard's greatest soliloquies.
Macbeth has long been one of Shakespeare's most gripping tales, dispensing with the usual subplots and humorous digressions in favor of a singular and direct plot action. As one wag once put it, the premise may be reduced to "behind every great man is a wife fully prepared to goad him into murder if it enhances the couple's social standing." The psychology behind Macbeth is a bit more complex, however. The tale is a tragedy of ambition studied through the prism of temptation. As such, it stands as a starkly humanistic morality play, more observing of Macbeth's evil than editorializing upon it.
The unspoken conflict is between free will and predestination; the subtle part of this study is the contrast of Macbeth and Banquo. Macbeth finds himself driven by external forces that seemingly conspire to abet his darker ambition. Banquo, on the other hand, resists temptation through his own choice, and yet passively fulfills his destiny even as Macbeth actively fulfills his own. Macbeth at first tries to distance himself from the dishonorable implications ("If chance will have me king, why, chance may crown me/Without my stir.") and is reticent to commit the greatest treason. However, there are more than enough hints that the subject has been previously debated, either with his wife or his own conscience. Macbeth, tempted or not, becomes a man betrayed by his baser nature. What makes it tragic is Macbeth's knowing complicity in his own damnation.
With this speech, Shakespeare foreshadows the toll that Duncan's murder will exact upon the conspirators. For now, the appearance of a bloody dagger in the air unsettles Macbeth. Even he doesn't know whether the dagger is real or a figment of his guilty imagination. It is, however, certainly a harbinger of bloodier visions to come. Macbeth will suffer more frightening apparitions in the scenes that follow, and Lady Macbeth will go mad trying to scrub away blood on her hands that only she can see. As Macbeth fears, the murder of Duncan is not a deed that will be "done, when 'tis done."
The last vestiges of the honorable Macbeth die at the end of this speech. It is a fleeting match between Macbeth's ambition and revulsion. The bell ultimately tolls for Macbeth as it does for Duncan; the dagger of the mind is as potent a killer as the dagger Macbeth wields in murder.
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-oppressèd 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' the 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. Now o'er the one half world
Nature seems dead, and wicked dreams abuse
The curtain'd sleep; witchcraft celebrates
Pale Hecate's offerings, and wither'd Murder,
Alarum'd by his sentinel, the wolf,
Whose howl's his watch, thus with his stealthy pace,
With Tarquin's ravishing strides, towards his design
Moves like a ghost. Thou sure and firm-set earth,
Hear not my steps which way they walk, for fear
Thy very stones prate of my whereabout
And take the present horror from the time
Which now suits with it. Whiles I threat, he lives:
Words to the heat of deeds too cold breath gives.
Act II, sc. i