Plainly these witches are not causes; Banquo, who has heard that his sons will be kings, does not immediately go off to commit murder to fulfill the prophecy, but Macbeth does. In fact, like all omens and portents in Shakespeare, the witches exist to be interpreted. They are the essence of ambiguity, ambiguous not only in their speech but in their gender: “You should be women, / And yet your beards forbid me to interpret / That you are so”
