Throughout the play, the witches—referred to as the "weird sisters" by many of the characters—lurk like dark thoughts and unconscious temptations to evil. In part, the mischief they cause stems from their supernatural powers, but mainly it is the result of their understanding of the weaknesses of their specific interlocutors—they play upon Macbeth's ambition like puppeteers.
The witches' beards, bizarre potions, and rhymed speech make them seem slightly ridiculous, like caricatures of the supernatural. Shakespeare has them speak in rhyming couplets throughout (their most famous line is probably "Double, double, toil and trouble, / Fire burn and cauldron bubble" in 4.1.10–11), which separates them from the other characters, who mostly speak in blank verse. The witches' words seem almost comical, like malevolent nursery rhymes. Despite the absurdity of their "eye of newt and toe of frog" recipes, however, they are clearly the most dangerous characters in the play, being both tremendously powerful and utterly wicked (4.1.14).
The audience is left to ask whether the witches are independent agents toying with human lives, or agents of fate, whose prophecies are only reports of the inevitable. The witches bear a striking and obviously intentional resemblance to the Fates, female characters in both Norse and Greek mythology who weave the fabric of human lives and then cut the threads to end them. Some of their prophecies seem self-fulfilling. For example, it is doubtful that Macbeth would have murdered his king without the push given by the witches' predictions. In other cases, though, their prophecies are just remarkably accurate readings of the future—it is hard to see Birnam Wood coming to Dunsinane as being self-fulfilling in any way. The play offers no easy answers. Instead, Shakespeare keeps the witches well outside the limits of human comprehension. They embody an unreasoning, instinctive evil.
