"If good, why do I yield to that suggestion(145)
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs,
Against the use of nature?" (I.iii.145-148)
In Act One, scene three, Banquo and Macbeth wonder about the witches' predictions, especially after Angus reveals that Macbeth has been appointed Thane of Cawdor. The first prediction of the witches has just come true. Banquo suggests that the witches' prophecies may be "instruments of darkness" meant as temptation (I.iii.134).
Even as Macbeth contemplates whether the witches' predictions are good or evil, he finds himself entertaining "a horrid image;" his own traitorous thoughts have already imagined possibly murdering King Duncan to obtain the throne. The realization and horror of the notion of regicide leaves Macbeth feeling completely unsettled and rattled, from the roots of his hair to the wild beating of his heart.