That the Weird Sisters possess ... perennial and astounding vitality is attested by the whole sweep of Shakespearean criticism. All hands seem to be convinced that they symbolize or represent evil in its most malignant form, though there is to be found little unanimity of opinion regarding the precise nature of that evil, whether it is subjective or objective or both, whether mental or metaphysical. (pp. 55-6)The single purpose of this study is to examine, as thoroughly as possible, the nature of that evil which the Weird Sisters are said to symbolize or represent, and to reproduce one aspect at least of the metaphysical groundwork of the drama. It presupposes that in Shakespeare's time evil was considered to be both subjective and, so far as the human mind is concerned, a non-subjective reality; that is to say, evil manifested itself subjectively in the spirits of men and objectively in a metaphysical world whose existence depended in no degree upon the activities of the human mind. This objective realm of evil was not governed by mere vague and irrational forces; it was peopled and controlled by the malignant wills of intelligences--evil spirits, devils, demons, Satan--who had the ability to project their power into the workings of nature and to influence the human spirit. Such a system of evil was raised to the dignity of a science and a theology. (p. 58)Since ... this belief was so universal at the time, we may reasonably suppose that Shakespeare's Weird Sisters are intended to symbolize or represent the metaphysical world of evil spirits. Whether one considers them as human witches in league with the powers of darkness, or as actual demons in the form of witches, or as merely inanimate symbols, the power which they wield or represent or symbolize is ultimately demonic. Let us, therefore, exercise wisdom in the contemplation of the nature, power, and illusions of unclean spirits.In the meantime, we may conveniently assume that in essence the Weird Sisters are demons or devils in the form of witches. At least their control over the primary elements of nature ... would seem to indicate as much. Why, then, should Shakespeare have chosen to present upon his stage these witch-likenesses rather than devils in devil-forms? Two equally valid reasons may be suggested. In the first place, the rather sublime devil and his angels of the earlier drama, opponents of God in the cosmic order and destroyers of men, had degenerated in the hands of later dramatists into mere comic figures; by Shakespeare's time folk conception had apparently so dominated dramatic practice and tradition that cloven hoof, horns, and tail became associated in the popular imagination only with the ludicrous.... In the second place, witches had acquired no such comic associations. They were essentially tragic beings who, for the sake of certain abnormal powers, had sold themselves to the devil. As we have seen, everybody believed in them as channels through which the malignity of evil spirits might be visited upon human beings. Here, then, were terrifying figures, created by a contemporary public at the most intense moment of witchcraft delusion, which Shakespeare found ready to his hand. Accordingly he appropriately employed witch-figures as dramatic symbols, but the Weird Sisters are in reality demons, actual representatives of the world of darkness opposed to good. (pp. 59-61)[The] Weird Sisters take on a dignity, a dark grandeur, and a terror-inspiring aspect which is in no way native to the witch-symbol as such. In the first place, they are clairvoyant in the sense that whatever happens outwardly among men is immediately known to them. In the thunder and lightning of a desert place they look upon the distant battle, in which Macbeth overcomes the King's enemies, and conjecture that it will be lost and won before the day ends. They do not travel to the camp near Forres where Duncan receives news of the battle, but when Macbeth is created Thane of Cawdor they seem to know it instantly. They must be aware that it is Macbeth who murders Duncan, because Hecate berates them for having trafficked with him in affairs of death without her help. All the events of the drama--the murder of Banquo and the escape of Fleance, the striking down of Lady Macduff and her children, Macbeth's accumulating sins and tragic death--must, as they unfold in time, be immediately perceived by these creatures in whom the species of these things are connatural. Moreover, by virtue of their spiritual substance they are acquainted with the causes of things, and, through the application of wisdom gained by long experience, are able to prognosticate future events in relation to Macbeth and Banquo: Macbeth shall be king, none of woman born shall harm him, he shall never be overcome until Birnam wood shall come against him to Dunsinane; Banquo shall be no king, but he shall beget kings. The external causes upon which these predictions are based may to a certain extent be manipulated by these demonic forces: but the internal causes, i.e., the forces which move the will of Macbeth to action, are imperfectly known and only indirectly subject to their influence. They cannot read his inmost thoughts--only God can do that--but from observation of facial expression and other bodily manifestations, they surmise with comparative accuracy what passions drive him and what dark desires of his await their fostering. Realizing that he desires the kingdom, they prophesy that he shall be king, thus arousing his passions and inflaming his imagination to the extent that nothing is but what is not. This influence gained over him is later augmented when they cause to appear before him evil spirits, who condense the air about them into the shapes of an armed Head, a bloody Child, and a crowned Child. These demonic presences materialize to the sound of thunder and seem to speak to him with human voices, suggesting evil and urging him toward destruction with the pronouncement of half-truths. These are illusions created by demonic powers, objective appearances with a sensible content sufficient to arouse his ocular and auditory senses.Indeed, the Weird Sisters are always illusions when they appear as such upon the stage; that is to say, their forms clothe the demonic powers which inform them. This is suggested by the facility with which they materialize to human sight and disappear. King James suspects that the Devil is able to render witches invisible when he pleases, but these Weird Sisters seem of their own motion to melt into thin air and vanish like a dream. Instead of disappearing with the swift movement which characterizes demonic transportation of bodies, they simply fade into nothingness. This suggests that their movements from place to place are not continuous necessarily. Though one of them plans to sail to Aleppo in a sieve, we feel that for the most part they appear in one place at one instant and at another place the next instant, or at whatever time pleases them, without being subject to the laws of time and place. I would not, however, force this point. At any rate, all their really important actions in the drama suggest that they are demons in the guise of witches.But the witch-appearances constitute only a comparatively small part of the demonic manifestations in Macbeth. Many of the natural occurrences and all of the supernatural phenomena may be attributed to the activities of the metaphysical world of evil spirits. Whether visible or invisible these malignant substances insinuate themselves into the essence of the natural world and hover about the souls of men and women; they influence and in a measure direct human thought and action by means of illusions, hallucinations, and inward persuasion. For example, since they are able to manipulate nature's germens and control the winds, we may reasonably suppose that the storm which rages over Macbeth's castle and environs in Act II is no ordinary tempest caused by the regular movements of the heavenly bodies, but rather a manifestation of demonic power over the elements of nature. Indeed, natural forces seem to be partly in abeyance; o'er the one half-world nature seems dead. A strange, mephitic atmosphere hangs over and pervades the castle and adjacent countryside; an unnatural darkness, for ages the milieu of evil forces, blots out the stars and in the morning strangles the rising sun. Where Lennox lies--evidently not far distant--the night is so unruly that chimneys are blown down, lamentings and strange screams of death are heard in the air; and the firm-set earth is so sensitized by the all-pervading demonic energy that it is feverous and shakes. Macbeth senses this magnetization, and fears that the very stones will prate of his whereabouts. As the drunken Porter feels, Macbeth's castle is literally the mouth of hell through which evil spirits emerge in this darkness to cause upheavals in nature. Within the span of his seventy years the Old Man has experienced many strange and dreadful things, but they are as trifles in comparison with the occurrences of this rough night. Demonic powers are rampant in nature. (pp. 77-81)Macbeth's vision of a dagger is an hallucination caused immediately, indeed, by disturbed bodily humours and spirits but ultimately by demonic powers, who have so controlled and manipulated these bodily forces as to produce the effect they desire. And a like explanation may be offered of the mysterious voice which Macbeth seems to hear after the murder, crying exultantly to all the house, "Sleep no more! Macbeth does murder sleep" [II. ii. 32-3]. (p. 84)Banquo's ghost is an infernal illusion, created out of air by demonic forces and presented to Macbeth's sight at the banquet in order that the murderer may be confused and utterly confounded. The second appearance of Banquo's ghost, together with the show of eight kings [IV. i. 112], is undoubtedly the result of demonic machinations. Having persuaded and otherwise incited Macbeth to sin and crime, the Devil and his angels now employ illusions which lead to his betrayal and final destruction.And finally, certain aspects of Lady Macbeth's experience indicate that she is possessed of demons. At least, in preparation for the coming of Duncan under her battlements, she calls upon precisely those metaphysical forces which have seemed to crown Macbeth. The murdering ministers whom she invokes for aid are described as being sightless substances, i.e., not evil thoughts and "grim imaginings" but objective substantial forms, invisible bad angels, to whose activities may be attributed all the unnatural occurrences of nature. Whatever in the phenomenal world becomes beautiful in the exercise of its normal function is to them foul, and vice versa; they wait upon nature's mischief. She recognizes that they infest the filthy atmosphere of this world and the blackness of the lower regions; therefore she welcomes a night palled in the dunnest smoke of hell, so dense that not even heaven may pierce the blanket of the dark and behold her projected deed. Her prayer is apparently answered; with the coming of night her castle is, as we have seen, shrouded in just such a blackness as she desires. (pp. 85-6)What happens to Lady Macbeth in the course of Act IV is not immediately clear. Apparently there is a steady deterioration of her demon-possessed body until, at the beginning of Act V, the organs of her spirit are impaired to the point of imminent dissolution. Such a great perturbation of nature has seized upon her that she walks night after night in slumbery agitation, with eyes wide open but with the senses shut. There appears a definite cleavage in her personality. Her will, which in conscious moments guards against any revelation of her guilty experiences, is submerged; and her infected mind is forced to discharge its secrets in the presence of alien ears. Her symptoms in these circumstances resemble those of the ordinary somnambulist, but the violence of her reactions indicates that her state is what may be called "somnambuliform possession" or "demoniacal somnambulism." ... The most outstanding characteristic of this demoniacal somnambulism, which in the course of history has been more common than any other form of possession, is that the normal individuality disappears and seems to be replaced by a second personality, which speaks through the patient's mouth. This strange individuality always confesses wrong-doing, and sometimes relates a sort of life-history consisting frequently of the patient's reminiscences or memories. Now the physician to Lady Macbeth recognizes these symptoms in his patient. Sometimes, to be sure, he has known those who have walked in their sleep who have died holily in their beds. But this disease is beyond his practice; this heart sorely charged with perilous stuff needs the divine more than the physician. The demonic substances she welcomed into her body now employ her bodily functions to disclose her criminal experiences. (pp. 89-90)Shakespeare's age would undoubtedly have pronounced Lady Macbeth's sleep-walking an instance of demoniacal somnambulism. Practically everybody, so far as may be determined, accepted demonic possession as an established fact. The New Testament affirmed it; the Church Fathers had elaborated and illustrated it; the Catholic Church made of it a firm article of faith and proceeded to exorcise demons by means of recognized rituals involving holy water and cross, bell, book, and candle; and Protestants could not consistently deny it, or if some of them did, peremptory experience forced them to take a doubtful refuge in the conception of obsession, which produced the effects of possession.... Fortunately Shakespeare has spared us, in the case of Lady Macbeth, a representation of the more disgusting physical symptoms of the diabolically possessed, such as astounding contortions of the body and fantastic creations of the delirious mind. He merely suggests these horrors in the report of the Doctor that the Lady is troubled with thick-coming fancies and in the expressed opinion of some that she took her own life by self and violent hands. He is interested primarily in presenting not so much the physical as the spiritual disintegration of this soul-weary creature possessed of devils.In this manner, it seems to me, Shakespeare has informed Macbeth with the Christian conception of a metaphysical world of objective evil. The whole drama is saturated with the malignant presences of demonic forces; they animate nature and ensnare human souls by means of diabolical persuasion, by hallucination, infernal illusion, and possession. They are, in the strictest sense, one element in that Fate which God in his providence has ordained to rule over the bodies and, it is possible, over the spirits of men. And the essence of this whole metaphysical world of evil intelligences is distilled by Shakespeare's imagination and concentrated in those marvellous dramatic symbols, the Weird Sisters. (pp. 91-3)Walter Clyde Curry, "The Demonic Metaphysics of Macbeth," in his Shakespeare's Philosophical Patterns, Louisiana State University Press, 1937, pp. 53-93.