Final answer:
The lines from Shakespeare's 'Macbeth' show King Duncan's thoughts on the difficulty of judging character based on appearance, and his misplacement of trust in Macbeth, reflecting the play's theme of appearance versus reality.
Step-by-step explanation:
The lines you've quoted are from William Shakespeare's tragedy Macbeth. These lines reflect King Duncan's regret and surprise about the betrayal of the original Thane of Cawdor, and his misplaced trust in appearance.
Duncan believes one cannot judge a person's character simply from their outward appearance. Duncan's trust in Macbeth, who has been given the Thane of Cawdor's title, shows his inability to see the true nature of those around him, which is a central theme of the play.
Statements like "There's no art to find the mind's construction in the face" and "Look like the innocent flower, But be the serpent under't" point to the appearances versus reality motif that Shakespeare explores in Macbeth, highlighting the disparity between what characters seem to be and what they truly are.