How does "fair is foul and foul is fair" explain the passage of events in Macbeth—in other words, how do these words work thematically in the play?

In Shakespeare's Macbeth, one of the
witches' first utterances in the play sets the tone for the rest of the play. Once the
witches become involved, the theme of "fair is foul and foul is fair" is seen in every
corner of the drama. Shakespeare uses this theme to cast doubt on even the most noble of
characters, especially in that Macbeth, a seemingly noble and devoted subject (and
cousin and friend) to Duncan, is soon blamed for killing the King and of other terrible
acts. The witches seem to call up this dark image, perhaps like a spell, and "command"
that it hover through the air, where nothing is left
untouched.

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ALL:

Fair
is foul, and foul is fair.

Hover through the fog and filthy
air. (I.i.11-12)

When Macbeth
enters the scene from the battlefield, he echoes the same words, perhaps showing how
closely aligned he actually is to what the witches have planned for
him: perhaps he is more willing than even he knows, seen especially
in how quickly he accepts their prophecies.

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MACBETH:

So
foul and fair a day I have not seen.
(I.iii.39)

Macbeth himself
reflects this theme. He seems so "fair" in that he is such a hero,
supportive of Duncan, and loved by Duncan. No one would, at the place's start, question
Macbeth's loyalty, yet this quickly changes. He represents what
seems fair ("good") and what is really foul
("evil").

When Duncan enters the courtyard of Inverness
(Macbeth's castle), he comments upon how lovely it is. He has no way of knowing that
what seems so fair will actually be the last place he lives to see. No one would suspect
it for the castle and the air are so
fine.

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DUNCAN:

This
castle hath a pleasant seat; the air

Nimbly and sweetly
recommends itself

Unto our gentle senses.
(I.iv.1-3)

When Macbeth hires
the men to kill Banquo, he presents Banquo as "foul," blaming the murderers'
disappointments in life on Banquo. The truth is that Banquo is the good one and Macbeth
has caused their hardships, but the truth becomes cloudy, as if the air truly is "foggy
and filthy."

When Macduff has had enough of Macbeth's
treachery, he travels to England, where Duncan's son Malcolm, true heir to the throne,
has taken refuge in the court of Edward the Confessor. However, because Macduff has left
his family unguarded at home and comes from Macbeth's court, Malcolm distrusts Macduff,
who is actually a very good man. Because of Macbeth's behavior, Macduff seems "evil,"
while he is actually a "faithful" and dedicated citizen of
Scotland.

Malcolm, in order to test Macduff, presents
himself as an evil man who would destroy Scotland, being a worse
King even then Macbeth. In doing so, he pretends to be a threat to his homeland. When
Macduff hears Malcolm's words, he believes that Malcolm is not fit to
live, let alone to govern Scotland. As Macduff mourns the fate of
Scotland, Malcolm realizes that Macduff is a good man. Very soon after, news comes that
Macduff's family has been slaughtered at Macbeth's order, and Malcolm is further
convinced that the man before him is as just and noble as he
claims.

Of course, one would believe that being King and
Queen would be wonderful for Macbeth and Lady Macbeth, but we soon see that their lives
have deteriorated terribly, supporting this theme.

Once the
words "fair is foul and foul is fair" are uttered by the witches, this theme works its
way throughout the play, and it is not put to rest until the Macbeths are dead and
Malcolm returns to the throne, where "God" has "ordained" he should
be.