These lines in Macbeth are spoken by King Duncan to his son Malcolm after Malcolm comments on the noble nature of Cawdor's death and just before Macbeth arrives on the scene. Duncan is saying that there is no way to tell what a man is thinking by looking at his face. If you could tell what someone is thinking then he would have known Cawdor was going to betray him. Duncan is also suggesting that Malcolm cannot know what Cawdor was thinking just because he died in a noble manner. The entire quote follows:
...There's no art
To find the mind's construction in the face.
He was a gentleman on whom I built
An absolute trust. (12-15)
The irony in the lines points backward and forward. Backward to Cawdor, because the king trusted him and he turned out to be a traitor, and forward because Macbeth is about to enter the scene and to be given the now vacant title of Cawdor by Duncan. Macbeth, too, of course, will become a traitor. Duncan will not see this betrayal coming, either.
The juxtaposition of Cawdor's noble death, Duncan's admittance that he cannot tell what is on his men's minds, and Macbeth's entrance into the scene create interest and anticipation in the audience.
This type of irony is called dramatic irony: the audience or the reader knows something important that a character in a play does not. We know what Duncan doesn't; we know that Macbeth has had treasonous thoughts, thus the iron.
In this line from Act I, of Macbeth, King Duncan is explaining to his son, Malcolm that there is no skill to determine what a man is thinking by just looking at his face. These lines are referring to the Thane of Cawdor, whom King Duncan placed "an absolute trust," but the Thane of Cawdor ended up becoming a traitor, which proves Duncan's statement true.
But now Duncan has rewarded Macbeth with this title for being the hero of the war, so this line is ironic because King Duncan does not know that Macbeth has been thinking about murdering the King after the witches' prophecy, and who should arrive shortly after Duncan states this, Macbeth.
In these lines, Duncan is saying that he knew the previous (before Macbeth) Thane of Cawdor was betraying him because he (Duncan) could see it in his face. The line you cite means "there's no difficulty in looking at someone's face and knowing what he's thinking."
This is ironic because he's about to name Macbeth the next Thane of Cawdor. We know that Macbeth is going to kill him. So he's saying he can tell what people are thinking from their faces and now he's going to go and give a high title to a man who's going to kill him.
thank you every one your answers have been grate help and i can now answere wat i was stuck on thank you x
Basically Duncan is saying here that "you can't tell what's going on inside someones head just by looking at their facial expressions". This is ironic because he is looking right into the face of Macbeth and Lady Macbeth and has no idea that inside their heads is the plot to kill him. Their facial expressions are not giving away the horrible things that they are thinking. It would interest the audience because they would possibly be looking for signs that Duncan may pick up on the plot against him.