Macbeth:
> There's no art
> To find the mind's construction in the face:
> He was a gentleman on whom I built
> An absolute trust. O, worthiest cousin!
> The sin of my ingratitude even now
> Was heavy on me. Thou art so far before
> That swiftest wing of recompense is slow
> To overtake thee.
Macbeth is speaking to Duncan, and the passage is a little tricky to interpret because it's actually meant to be a bit *deceitful* on Macbeth's part.
Here's the breakdown:
* "There's no art / To find the mind's construction in the face" - Macbeth is suggesting that appearances can be deceiving, that you can't judge a person's character just by looking at them. This is ironic, as he is about to murder Duncan.
* "He was a gentleman on whom I built / An absolute trust" - Macbeth is praising Duncan, calling him a "gentleman" and saying he had complete trust in him. This is intended to make Duncan feel secure.
* "O, worthiest cousin! / The sin of my ingratitude even now / Was heavy on me" - Macbeth is feigning guilt and remorse, implying that he feels awful for not being able to repay Duncan's kindness and generosity.
* "Thou art so far before / That swiftest wing of recompense is slow / To overtake thee" - Macbeth is praising Duncan's goodness, saying that even the fastest reward couldn't possibly be enough to repay him. This is meant to flatter Duncan and reinforce the idea of his own supposed "ingratitude."
In summary, while the passage appears to be about the duties of a king and his subjects, it's actually a masterful display of Macbeth's manipulation and foreshadowing of his upcoming evil deeds. He uses flattery and insincere remorse to disguise his true intentions.