Shakespeare uses various language features throughout the MacBeth play, one of these features in particular is metaphors. During Macbeths soliloquy he said this: “Or art thou but a dagger of the mind, a false creation, proceeding from the heat-oppressed brain” Macbeths ultimate goal during this time is to kill King Duncan so he can become Thane Of Cawdor; Due to this it has caused him horrendous stress or a “heat-oppressed brain” Macbeth is concluding that because he is so stressed in the mind his eyes are playing tricks on him hence when he said “the dagger of the mind”
