The core plot is about the corruption, by evil, of Macbeth, how there is an apparent metamorphosis from a good, noble, loyal servant of the king (Act 1, scene 2 is included to show just this), to a ghastly, treasonous, murderer. It is clear that the witches either caused, or catalysed, this corruption of Macbeth, originally through their prophecy and then through their apparitions, and they are therefore of paramount significance.
The first scene, in which only the witches appear, opens up the play with an eerie atmosphere of the supernatural and evil, which grabs the audience straight away. Shakespeare begins with this scene to illustrate to the audience that the witches are very important, if not pivotal, to the play. The audience immediately discover that the witches are strange and sinister by the fact that they meet only in foul weather and for them “fair is foul, and foul is fair”. This shows that they are unnatural (which is a parralel of evil) and is clearly a sign of things to come.
The witches are portrayed as highly unnatural. They meet only in foul weather, which, to them, is good weather. This being an example of their inverted values. They have the ability to foresee the future (as shown in Act 1 Scene 3), which is clearly very unnatural. They possess strange powers, such as the ability to sail in a sieve. The unnatural, if not now, was in the time of Shakespeare a parrallel of evil, and therefore fits perfectly into the moral structure of the play. (That though the witches are exceedingly evil and hold inverted values, human good nature cannot be surpressed by their evil). The witches have no morality and act maliciously for the sheer sake of it, except when one of the witches condemns a sailor to a horrible fate, not ever to sleep again, because his wife did not offer her any chestnuts.
The moral structure of the play, as reffered to above, plays a large role in making the play as interesting as it is. The underlying message is positive, as the evil of the witches, though it corrupts Macbeth and Lady Macbeth, fails to cause lasting damage and suffering to Scotland (as Macbeth is defeated at the end of the play), and the general goodness of human nature cannot be surpressed, in particular in the case of Lady Macbeth (who cannot rid herself of her human feelings) but also in the case of Macbeth (who displays an uncharacteristic reaction to the suggestion of murder in act I, scene III , "whose murder yet is but fantastical").
The witches constantly speak in paradox and with inverted values. Their apparitions, who are also unnatural and represent everything that the witches do, speak in a misleading fashion such as ‘for none of woman born shall harm Macbeth’. This statement leads Macbeth to believe that he is completely safe, as it basically implies that no human will harm him. This prophecy is of great significance as Macbeth continually refers to it and becomes arrogant because of it. It turns out, however, that Macduff was born by a suzarian ("Macduff was from his mother's womb untimely ripp'd). The witches say things like ‘fair is foul and foul is fair’ and ‘lesser than Macbeth, and greater’. The first of these shows that to the witches good is evil and evil is good. The second is a paradox, which typifies the witches’ speech. The use of language for the witches implies that they are evil and unnatural and is used as a way of displaying the loss of morality of other characters, particularly Macbeth and Lady Macbeth, as when they start to sound like the witches, we know that they have started to lose some of their morals and become slightly evil.
The apparitions of the witches in act IV, scene I are very significant in the future unfolding of the plot. They make Macbeth more dependent on the witches and from this point on, he is not his former, strong, powerful self. One apparition tells him to beware of Macduff and one leads him to believe that Macduff will not harm him, which turns out to be very significant as he decides, because of this, to attack Macduff's castle, even though the odds are against him. In the battle that ensues, Macbeth is defeated by Macduff who, it turns out, was not of woman born. These two prophecies appear to be conflicting to Macbeth, who as a result becomes more agitated. The third apparition makes Macbeth believe that he will not be defeated, as "Macbeth shall never vanquish'd be until Great Birnham Wood to high Dunsinane Hill shall come against him." This is also very significant because it makes Macbeth arrogant and plays a role in his decision to attack Macduff's castle. This scene proves to be the turning point in Macbeth's sanity as, after it, he begins to make rash decisions and sound agitated and unconcise.
In act 1 scene 5, Lady Macbeth begins to sound like the witches, she expresses a wish to be made evil, to become unsexed. It is an indication of how evil she wants to become that she has to call upon supernatural powers to do so. Once more, the unnatural is linked to evil. The term unsexed, which she uses, links back to when Banquo tells the witches that they should be women, but yet are really not (Lady Macbeth would like to be as evil as the witches are, even though she cannot, although she doesnt yet know this). Later on in the play, Lady Macbeth begins to mentally deteorate, partly due to her desire to be made evil, this can be seen both by rather nonsensical comments she makes; "out damned spot! out! One; two", (in this example she makes some sense as she is referring to the doctor, but still sounds agitated and, frankly, mad) and by the fact that she begins to sleepwalk and eventually takes her own life.
Macbeth’s speech (near the end of Act 3, scene 4) shows that he has become dependent on the witches. The appearance of the ghost of Banquo contributes greatly to this. He begins to doubt his ability to consolidate power and therefore decides that he will go to see the witches, whose apparitions lead him to believe that he is safe, although the audience realise that the statements are actually misleading. The appearance of the kings encourages Macbeth and he tells Lennox that he will attack the castle of Macduff, because he believes that he does not have to fear Macduff. After this meeting with the witches, Macbeth becomes very agitated and he makes rash decisions.
Whilst it could be argued that it is the witches who lead to the downfall of Macbeth, they are not, in fact, totally responsible for his actions. Macbeth had clearly thought of the idea of being king before he ever came across the witches, as he reacts to the witches’ prophecy in a way that makes this apparent. His wife Lady Macbeth spurs him on throughout the play. At several points in the play, when he calls upon the witches, he is not directly encouraged, but he becomes more evil and malicious as a result of both their influence and their actions.
