Is this a dagger which I see before me (2.1.33-61)  

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' 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 halfworld
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.
A bell rings
I go, and it is done; the bell invites me.
Hear it not, Duncan; for it is a knell
That summons thee to heaven or to hell. 

fatal ] sent by fate. 

sensible ] perceived by the senses (particularly touch and sight).

heat-oppress'd ] fevered. In the Renaissance, heat was considered a fluid that could literally press on the brain and cause fever and delirium. 

marshall'st me ] guide or usher me. 

Mine eyes...rest ] Either Macbeth's sight is being fooled by his other senses or else his sight alone is the only sense to be trusted. 

dudgeon ] wooden handle. 

gouts ] From the Latin gutta, meaning large drops or splashes, often gushing or bursting. 

informs ] takes shape. 

Now o'er...dead ] Because it is night, half of the world is in darkness and everything seems dead. 

curtain'd ] A reference to the curtains drawn around a four-post bed (the standard bed in Elizabethan England). 

Witchcraft...offerings ] Hecate, daughter of Perses and Asteria, was a magician who raised a temple to Diana in which she performed human sacrifice. Medea and Circe are her children. Note that her name is disyllabic in the play (you do not pronounce the final 'e'). Hecate's offerings are her ritual sacrifices. 

Alarum'd ] summoned to action. 

sentinel ] watchman; guard. 

Tarquin's ravishing strides ] The Roman king, Tarquin (Sextus Tarquinius), rapes Lucrece, the act upon which Shakespeare's long poem of the same name is based. Macbeth and Tarquin have many similarities. Compare Macbeth's soliloquy to the following two stanzas from The Rape of Lucrece: 

Now stole upon the time the dead of night,
When heavy sleep had closed up mortal eyes:
No comfortable star did lend his light,
No noise but owls' and wolves' death-boding cries;
Now serves the season that they may surprise
The silly lambs: pure thoughts are dead and still,
While lust and murder wake to stain and kill.

And now this lustful lord leap'd from his bed,
Throwing his mantle rudely o'er his arm;
Is madly toss'd between desire and dread;
Th' one sweetly flatters, th' other feareth harm;
But honest fear, bewitch'd with lust's foul charm,
Doth too too oft betake him to retire,
Beaten away by brain-sick rude desire.

sure ] solid. 

stones prate ] the stones speak. Macbeth knows that, although those around him are unaware of his crimes, the earth and the heavens know all. Notice also the connection to Habakkuk 2.10,11: "Thou hast consulted shame to thine own house, by destroying many people, and hast sinned against thine own soule. For the stone shall cry out of the wall and the beam out of the timber shall answer it, woe unto him that buildeth a town with blood". For more biblical imagery in this passage see my article: Biblical Imagery in Macbeth.

take...it ] The noise of the stones reveal Macbeth's movement toward Duncan, and thus his evil intentions. The sound cuts through the "present horror", i.e. the dreadful silence, from the time that suits it best -- the dead of night. 

words...gives ] Talking about the murder is wearing away his courage to follow through. 

COMMENTARY

Macbeth, after discussing the crime with Lady Macbeth, has decided to go through with the "terrible feat" (1.7.75). Now he sits alone, waiting for the bell which will summon him to murder Duncan, pondering his decision one final time. The focus of the soliloquy, the invisible dagger, is our first glimpse of Macbeth's powerful imagination – imagination that is largely responsible for his mental torment throughout the drama. Although Macbeth knows that the dagger is an optical illusion, and suspects that it could be brought about by his potentially "heat-oppressed brain" (39), he nonetheless allows the phantom dagger, soon stained with imaginary "gouts of blood" (46), to affect him greatly. Enhancing the ominous and eerie atmosphere of the speech is the use of successive allusions to people and practices which conjure up images of satanic and earthly evil. Hecate, the goddess of witchcraft and a strong presence overall in Macbeth, is preparing her sacrificial victims, and murder himself, summoned by his trusted watchman, the wolf, moves with the power and speed of evil king Tarquin towards his prey. (To read a transcript of an actual ritual where Hecate is invoked, click here). 

Just as talk of the murder is about to stifle his courage, Macbeth's intense illusion is shattered by the bell, a signal from Lady Macbeth that Duncan's chamberlains are asleep, and Macbeth races away to commit the heinous crime. One can only wonder if a few more moments of deliberation would have changed Macbeth's mind.