There is a great argument about the significance and relevancy of the witches in Macbeth. Some critics consider them a part of the reality of the play according to the Elizabethan audience. On the contrary, other critics consider them a mere imagination according to the modern approach of the twentieth century. Above all, it is believed that their prophecies are a part of Macbeth's inner conflict even if he is responsible for his actions.
Some critics say that the witches may be real through the play because they create the atmosphere of evil. In the Elizabethan Age, the audience can believe in the supernatural. The first scene of their appearance establishes the play's dramatic premise through their awakening of Macbeth's ambition. At the same time, they establish a dark mood that permeates the entire play. The stage directions indicate that the play begins with a storm, and malignant supernatural forces immediately appear in the form of the three witches. Shakespeare has the witches speak in language of contradiction. Such speech adds to the play's sense of moral confusion by implying that nothing is quite what it seems. Moreover, Shakespeare uses the scenes involving supernatural element to increase the audience's sense of foreboding and ill-omen.
Generally, the witches are important because they move the plot forward. That is, they meet Macbeth for two times. In each time, they give him a set of prophecies which has a strong effect on him. In the first time, they come to give him a set of prophecies while he goes to them in the second meeting as he seeks the knowledge of the unknown from the metaphysical world. Perhaps their prophecies are constructed to wreck havoc in the minds of the hearers, so that they become self-fulfilling. It is doubtful, for instance, that Macbeth would have killed Duncan without his meeting with the witches.
Furthermore, the witches may be real due to their relation with the idea of fatalism. Close to the end of the play, Hecate, the queen of the witches, becomes angry because the witches does not consult her about their conspiracy against Macbeth. This means that Macbeth's life is conspired by the witches. Moreover, he feels a false sense of security because they tell him that he will not be defeated till the wood of Birnam moves towards Dunsinane. However, his depend on the supernatural to direct the course of his life leads to his end.
Contrary to the previous opinions, some critics say that the witches are just imagination with no effect upon Macbeth's moral downfall. For instance, Banquo asks Macbeth about teh credibility of the witches and the basics of their reality because imagination usually overcomes our sense of reason. Macbeth and Banquo are very exhausted in a remote place and anticipate a reward for their victory. This makes a real possibility that they imagine the witches. Because Banquo is flexible, he tries to persuade himself that they are from their own imagination. On the other hand, Macbeth has the seeds of evil nature from the beginning, so he believes in the witches.
Furthermore, some critics see that the witches' prophecies have no actual effect on Macbeth's actions. For example, the first apparition tells him to "beware Macduff'. Macbeth has already a previous fear from Macduff because he does not attend the banquet. Thus, teh witches are a psychological reflection of his inner fear. Likewise, their prophecies may be a more reading of future. After all, when Birnam Wood comes to Dunsinane at the play's end, the soldiers bearing the branches have not heard of the prophecy.
In conclusion, whatever the nature of the witches' prophecies, their sheer inscrutability is as important as any reading of their motivations and natures. The witches stand outside the limits of human comprehension. They seem to represent the part of human beings in which ambition and sin originate; an incomprehensible and unconscious part of the human psyche. In this sense, they almost seem to belong to the moral aspect of the play.