At this point in the play, Macbeth has murdered both Duncan and Banquo, succumbing fully to his lust for power and losing his humanity in the process. In Act 3, scene 4, Macbeth says, "I am in blood / Stepped in so far that / should I wade no more, / Returning were as tedious as go o'er" (Folger 2013 edition, lines 168-170). In other words, reverting back to being the good man he was at the start of the play is as difficult a task as it is to keep moving forward on his murderous mission.

Because of this change in Macbeth's character, the images the witches use to describe Macbeth in the opening scene of act 4 underscore his animalistic violence as a way to elucidate that that which makes Macbeth a "man" is seeping away from him as he indulges these acts of violence and cruelty. As the witches concoct their potion in the cauldron, they add increasingly disgusting animal elements: "eye of newt and toe of frog, / wool of bat and tongue of dog" are the more "benign" images that start the recited list of ingredients, which then progresses to things like "witch's mummy" (mummy being "mummified human flesh," according to the notes in the Folger edition), "liver of blaspheming Jew," "finger of birth-strangled babe," and "a tiger's chaudron [entrails]." The images become increasingly more rotten, just as Macbeth himself is allowing his morality and humanity to rot away in his quest to protect the throne that is not rightfully his.

To understand the function of this part of the text we need to look at the scene in context. The witches are preparing for the arrival of Macbeth and the further acts of witchery and deception that they are to practice on him, thanks to Hecate's commands. Thus the purpose of this scene, where the witches prepare the spell in the cauldron, is to deliberately prepare the minds of the audience for the arrival of Macbeth. The list of strange ingredients, ranging from "liver of blaspheming Jew" to "finger of birth-strangled babe" clearly symbolises the evil of the witches, but note how the evil of the ingredients and the witches themselves finds a suitable parallel in the character of Macbeth and the deeds that he has committed and the way that he has given himself over to the forces of evil. Thus, in a sense, the evil symbolism of the ingredients foreshadows the arrival of Macbeth: a man who has relinquished any lingering sense of good to the all-consuming power of darkness within him.