The apparitions further illustrate and emphasize the theme of equivocation and paradox. The witches use supernatural forces to further their purpose of misleading and deceiving Macbeth into committing further atrocities. They have been successful thus far and are intent on convincing the tyrant that he is invincible so that he may continue his evil.
When Macbeth visits them for further advice, they summon apparitions to advise him. The first spirit tells him to fear Macduff. This piece of information indicates nothing new since Macbeth knows that Macduff has turned against him. The witches' purpose is to show Macbeth that they know of what they are speaking. In the process, they further endear the gullible tyrant to them.
The second apparition, a bloody child, tells Macbeth to mock man's power for "none of woman born" shall harm him. At this, Macbeth sneers at Macduff's threat because he believes that the prophecy states that no human will be able to harm him for all humans are born of women. He realizes how foolish he had been when, in his confrontation with Macduff in Act 5, scene 8, the latter informs him that he was from his mother's womb "untimely ripp'd." It is clear that Macduff has not been born naturally but was cut from his mother's womb in a procedure now known as a Caesarean section.
The third apparition, a child crowned, with a tree in his hand, informs Macbeth that:
Macbeth shall never vanquish'd be until
Great Birnam wood to high Dunsinane hill
Shall come against him.
This information further convinces the murderer that he is invincible. He interprets the prediction literally and believes that the trees will have to uproot themselves and march towards his castle physically. This is clearly impossible. Macbeth is shocked later when a messenger tells him in Act 5, scene 5, that the trees are marching towards his castle. Malcolm has instructed his troops to each cut down a branch from a tree and bear it in front of them as camouflage to hide their numbers. When Macbeth realizes this, he declares:
I pull in resolution, and begin
To doubt the equivocation of the fiend
That lies like truth:
He now grasps that the witches have deceived him with paradoxical statements.
The witches have achieved their purpose by fooling Macbeth into believing their prognostications and performing terrible acts of evil. Banquo warned Macbeth about their deceit and slyness, but he chose to ignore his advice deciding, instead, to assassinate his friend and confidante later. In the end, both he and his wife pay the ultimate price for their greed and gullibility. Sadly, though, many innocents also become victims of their malice.
The paradox of the apparitions occurs in how Macbeth views what they show him and how the audience views their predictions. Macbeth needs to quell his fears that doom is upon him, so he takes the visions at face value. To the audience, the apparitions are symbols that foreshadow how the prophecies will be fulfilled. The armored head suggests war or rebellion, while the bloody child obscurely refers to Macduff's cesarean birth. Macbeth takes the comment at face value, and therein lies the irony. The crowned child is Malcolm and refers to the tree branches his soldiers will carry from Birnam Wood. The procession of kings reveals the future line of kings, all descended from Banquo.
Macbeth believes only what he feels will benefit him. He can no longer make rational judgments, and Hecate knows this. The contradictory nature of the apparitions will go right over Macbeth's head, and he will take from the apparitions only what he wants.