The opacity in Macbeth is not produced by the same radical excision of motivation Shakespeare so strikingly employed in Hamlet, Othello, and King Lear. If the audience does not know exactly why Hamlet assumes his madness or Iago hates Othello or Lear puts the love test to his daughters, it most assuredly knows why Macbeth plots to assassinate King Duncan: spurred on by his wife, he wishes to seize the crown for himself. But in a tortured soliloquy, Macbeth reveals that he is deeply baffled by his own murderous fantasies:
My thought, whose murder yet is but fantastical,
Shakes so my single state of man that function
Is smothered in surmise, and nothing is
But what is not. (1.3.138-41)
At the center of the familiar and conventional motive there is a dark hole—“nothing is / But what is not.” And this hole that is inside Macbeth is linked to the dark presence, within his consciousness and within the play’s world, of the witches. Do they actually arouse the thought of murdering Duncan in Macbeth’s mind, or is that thought already present before he encounters them? Do they have some affinity with Lady Macbeth—who calls upon the spirits that attend on mortal thoughts to “unsex” her (1.5.38-39)—or is their evil completely independent of hers? Does the witches warning—“beware Macduff” (4.1.87)—actually induce Macbeth to kill Macduff’s family, or has he already waded too deep in bloodshed to turn back? Do their ambiguous prophecies lead him to a final, fatal over-confidence, or is his end the result of his loss of popular support and the superior power of Malcolm’s army? None of the questions are answered. At the end of the play the weird sisters are left unmentioned, their role unresolved. Shakespeare refuses to allow the play to localize and contain the threat in the bodies of witches.
Macbeth leaves the weird sister unpunished but manages to implicate them in a monstrous threat to the fabric of civilized life. The genius of the play is bound up with this power of implication, by means of which the audience can never quite be done with them, for they are most suggestively present when they cannot be seen, when they are absorbed in the ordinary relations of everyday life. If you are worried about losing your manhood and are afraid of the power of women, it is not enough to look to the bearded hags on the heath, look to your wife. If you are worried about temptation, fear your own dreams. If you are anxious about your future, scrutinize your best friends. And if you fear spiritual desolation, turn your eyes on the contents not of the hideous cauldron but of your skull: “O, full of scorpions is my mind, dear wife!” (3.2.37).
The witches—eerie, indefinable, impossible to locate securely or to understand—are the embodiment of the principle of opacity that Shakespeare embraced in his great tragedies. Shakespeare’s theater is the equivocal space where conventional explanations fall away, where one person can enter another person’s mind, and where the fantastic and the bodily touch. - Stephen Greenblatt