In act one, scene four, Malcolm describes to King Duncan how the former Thane of Cawdor acted before his execution, and the king responds by saying that there is no way to read a man's mind by looking at his face. King Duncan then tells his son,
"He [former Thane of Cawdor] was a gentleman on whom I built / An absolute trust" (Shakespeare, 1.4.13–14).
King Duncan's comment regarding the former Thane of Cawdor's capacity for dissembling echoes the motif "Fair is foul, and foul is fair," which is a theme that runs throughout the play and means that appearances can be deceiving. Similar to how the former Thane of Cawdor acted benevolent and loyal to Duncan while secretly colluding with the Norwegian King, Macbeth also secretly plots the king's demise while appearing to be Duncan's loyal subject. While Macbeth and his wife are planning the king's murder, Duncan is unaware of their wicked intentions, and Macbeth ends up assassinating him.