The first scene opens with a conference among the witches, and Shakespeare uses imagery to convey the point that evil forces will be at work throughout the play. With lightning flashing and references to "filthy" air, the witches agree to meet with Macbeth, departing with a warning that "what's fair is foul/what's foul is fair." When Macbeth actually meets the witches, and discovers that part of their prophecy has come true, he references the evil nature of his own ambitions, which he describes as "black and deep desires." But the best example of evil at work early in the play can be seen in Lady Macbeth's chilling soliloquy after reading her husband's letter describing the witches' prophecy in Scene 5 of the first act. She asks the "spirits that attend mortal thoughts" to keep her conscience from standing in the way of what she knows she has to do to push her husband to the throne:
...unsex me here
And fill me, from the crown to the toe, top-full
Of direst cruelty! Make thick my blood,
Stop up the access and passage to remorse,(45)
That no compunctious visitings of nature
Shake my fell purpose nor keep peace between
The effect and it!
Thus three evil forces are introduced early in the play: the witches' malignant tampering; Lady Macbeth's ruthlessness; and Macbeth's ambition. They will go on to converge to destroy Duncan and many of the other characters in the play, including the Macbeths themselves.