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...