After Malcolm describes the Thane of Cawdor's behavior before he is executed for treason, Duncan replies:
There's no art
To find the mind's construction in the face:
He was a gentleman on whom I built
An absolute trust.
Duncan seems to express some regret and sadness as he hears about the traitor's execution. Cawdor has faced his death sentence with admirable courage. The King acknowledges now that in his dealings with Cawdor in the past, he obviously had no "art" or skill to discern Cawdor's true character. Cawdor's "mind's construction" or true motives and plans could not be read in his facial expressions or in his surface behavior towards King Duncan. Duncan remarks thoughtfully that he absolutely trusted Cawdor, who ultimately proved unworthy of trust and was adept at hiding his true feelings and schemes as he plotted against the King. Here Duncan also reveals his weakness in reading character, a weakness that proves fatal for Duncan.
The fact that Cawdor deceives the King contributes one thread to one of the themes of the play: appearances are deceiving. "Fair is foul, and foul is fair."
Later, in Act 1 scene 7, Macbeth remarks that "False face must hide what the false heart doth know" as he resolves to go forward with the plan to murder Duncan.