In act four, scene one, Macbeth visits the Three Witches for a second time in order to learn more about his future and whether or not he will be able to cement his legacy as King of Scotland. The Three Witches proceed to conjure several apparitions, which purposely mislead Macbeth and influence him to be overconfident. The first apparition warns Macbeth to beware of Macduff without giving him any further context. The second apparition encourages Macbeth to be "bloody, bold, and resolute" because no man born of a woman can harm him. Macbeth accepts this prophecy literally and reasons that even Macduff cannot harm him since every man is born of a woman. The third apparition also misleads Macbeth into becoming overconfident by telling him that he shall not be defeated until Birnam wood travels to Dunsinane hill, which is something Macbeth believes is impossible.
Despite the seemingly favorable prophecies, the fourth vision disturbs Macbeth as he witnesses a parade of eight kings and recognizes them as Banquo's descendants, which confirms Banquo's initial prophecy. After the witches disappear, Lennox arrives and informs Macbeth that Macduff has fled to England. Macbeth responds by saying,
The flighty purpose never is o'ertook
Unless the deed go with it. From this moment
The very firstlings of my heart shall be
The firstlings of my hand (Shakespeare, 4.1.161-164).
Macbeth is saying that unless a person does something the second he thinks of it, he will never get a chance to do it. Therefore, he will begin acting on his impulses by turning his thoughts into actions the second he thinks of them. Macbeth then instructs his agents to slaughter Macduff's entire family. Macbeth's quote emphasizes his change in his character as he becomes a completely impetuous, bloodthirsty tyrant who acts upon his impulses and is willing to kill all of his political enemies.