Shakespeare uses this as a way to underline the uncertainty that lies in Macbeth's future as a King. The paradox, "I have thee not, and yet I see there," (II, 2, 35), stresses the conflict Macbeth is having between distinguishing appearance and reality. This gives the audience the reassurance that Macbeth is not completely incompetent and that he himself, is questioning the validity of the phantom dagger. .
The second half of the speech has Macbeth break into, "reality," and begin to muster up the strength and courage to perform the murder by talking himself into getting it over with. Shakespeare uses the allusion of, "Pale Hecate's offerings," (II, 2, 52), to justify what he believes he must do, murder Duncan. Just as Hecate may perform a ritual and offer a sacrifice, so it seems Duncan will be the sacrifice for Macbeth in order for him to become king. Ultimately, Macbeth is alluding to the ancient practices of sacrifice in order to serve an apparent greater good. This brings to the surface one of the many themes of Macbeth- the influence of the supernatural on people and events; by following and entrusting in supernatural beings, Macbeth's fortune will deteriorate without his conscious knowledge. Shakespeare also compares Macbeth to the Roman King Tarquin as well as having, "Moves like a ghost," (II, 2, 55 & 56). This shows just how swiftly and nimble Macbeth must approach Duncan in order to perform the murder, leaving no room for error. Finally, another major device Shakespeare uses is the symbolism of the bell ringing (II, 2, 61), performed by Lady Macbeth. This alerts Macbeth that the guards are fast asleep and that the coast is clear to assassinate Duncan. Not only does this bell alert Macbeth but also ushers in a point-of-no-return for him. He can either go ahead with his plan and acquire the title of King or not go through with the murder and continue with his fairly decent life as it is.