In act three, scene four, the ghost of the murdered Banquo appears to Macbeth at a banquet. Macbeth has recently had Banquo murdered, and he tried unsuccessfully to have Banquo's young son murdered as well. In this scene, Banquo's ghost perhaps signifies, or symbolizes, Macbeth's guilty conscience. Indeed, the ghost only appears to Macbeth, and Lady Macbeth tells her husband that it is "the very painting of (his) fear."
The ghost also signifies the disorder or chaos of Macbeth's mind. Macbeth is, at this stage of the play, "in blood / Stepp'd in so far" that he feels it is impossible for him to stop. He feels that he must carry on down the same path of blood and murder that he has travelled so far down already. After seeing Banquo's ghost, Macbeth acknowledges that "Strange things I have in head," indicating that he is aware of the psychological disorder that he has brought upon himself.
Banquo's ghost also signifies a greater disorder and discord than that which exists inside of Macbeth's head. The inextricable links between the natural and the supernatural, which are evident from the very first scene of the play, suggest that the world is out of balance. This greater sense of disorder and discord is suggested in act three, scene four, when, for example, Macbeth seems to be hallucinating, speaking and looking "but on a stool." It is also suggested when Macbeth incredulously proclaims:
If charnel-houses and our graves must send
Those that we bury back, our monuments
Shall be the maws of kites.
Here, Macbeth acknowledges that there no longer seems to be a fixed boundary between the living and the dead, just as there no longer seems to be a fixed boundary between the natural and the supernatural. A little later in the scene, Macbeth says, referring to the dead, "now they rise again ... this is more strange." This blurring of boundaries between the earthly realm on the one hand and the realms of the dead on the other is the fundamental discord which Banquo's ghost reveals to Macbeth, and also to the audience, in act three, scene four.