With this soliloquy, Shakespeare foreshadows the toll that Duncan's murder will exact upon Macbeth and Lady Macbeth. For now, the appearance of a bloody dagger in the air unsettles Macbeth, even though he is unsure whether the dagger is real or a figment of his guilty imagination. It is, however, certainly a harbinger of bloodier visions to come. Macbeth will suffer more frightening apparitions in the scenes that follow, and Lady Macbeth will go mad trying to scrub away blood on her hands that only she can see. As Macbeth fears, the murder of Duncan is not a deed that will be "done, when 'tis done."