In act 1, scene 5 of William Shakespeare's Macbeth, Lady Macbeth receives a letter from Macbeth, in which he tells her the story of the three witches and the prophecy—"Hail, King that shalt be!" This arouses Lady Macbeth's ambitions for Macbeth and for herself, and she starts thinking about what she can do to convince Macbeth to usurp the Scottish throne.
A messenger appears to tell Lady Macbeth—"The King comes here tonight"—and Lady Macbeth immediately decides to take advantage of the opportunity to murder King Duncan while he's under Macbeth's protection in Macbeth's castle.
Lady Macbeth calls on the "spirits / That tend on mortal thoughts" and the "murdering ministers" to put her in the proper murderous frame of mind to achieve her goal. To modern audiences, Lady Macbeth's conjuring of evil spirits is simply the raving of a madwoman and the opportunity for the actor playing Lady Macbeth to overact. To Shakespeare's audience, however, Lady Macbeth was committing witchcraft, and she was putting her life, and Macbeth's life, in serious jeopardy.
Witchcraft laws were first enacted in England in 1542, during the reign of King Henry VIII. These laws were expanded in 1563, during the reign of Queen Elizabeth I, in response to an alleged plot to kill Elizabeth and restore Catholicism to England by means of sorcery.
The Witchcraft Act of 1604, enacted early in the reign of King James I—an ardent anti-witch advocate who wrote a book on witchcraft and demonology, officiated at witch trials, and even attended executions of witches—further broadened earlier laws.
If any person or persons ... shall use, practise, or exercise any invocation or conjuration of any evil and wicked spirit: ... or shall use, practise, or exercise, any Witchcraft, Incantment, Charme or Sorcery, whereby any person shall be Killed ... that then every such Offender, or Offenders, their Ayders, Abettors, and Counsellors, being of the said offences duly and lawfully Convicted and Attainted, shall suffer paines of death as a Felon or Felons, and shall lose the priviledge and benefit of Clergy and Sanctuary. (An Act against Conjuration, Witchcraft and dealing with evil and wicked spirits, 1 Ja. I c. 12, 1604)
A person convicted of a minor offence of witchcraft was usually sentenced to a year in prison. A person who committed a second act of witchcraft, or who was convicted of witchcraft which caused or was intended to cause death or harm to another person, was hanged. However, anyone whose witchcraft involved bodily harm or death and treason—like Lady Macbeth's conjuring of spirits to help her murder King Duncan—was burned at the stake.
Even though Macbeth is set in Scotland, not England, and in a period long before Shakespeare's time, these are the laws that were in effect in England at the time Macbeth was first performed in about 1606. This is the context in which the play was performed, and many people in Shakespeare's audience knew the risk that Lady Macbeth was taking in conjuring evil spirits. They might also have decided that Lady Macbeth herself was a witch, which is one common interpretation of her character.
Lady Macbeth's appeal to the "spirits / That tend on mortal thoughts" goes far beyond the conventional interpretation, based on stereotypical male and female attributes, that Lady Macbeth is asking the spirits to make her less of a woman and more like a man. Lady Macbeth isn't asking the spirits to make her more like a man. She's asking them to make her wholly inhuman, neither male or female.
Lady Macbeth asks the spirits to remove any semblance of humanity from her being, to fill her with the "direst cruelty," and to remove from her any sense of remorse or pangs of conscience so that her "keen knife see not the wound it makes," and she can kill Duncan without any feelings of guilt or regret.
However, when the time comes to kill Duncan, Lady Macbeth finds that she hasn't lost all human sensibility, like she hoped she would, and her humanity momentarily interferes with her ambitions.
LADY MACBETH. Had he not resembled
My father as he slept, I had done't.
(act 2, scene 2, lines 15–16)