Explain this line from Macbeth:"There's no art/ to find the mind's construction in the face."
This is a line that is spoken in Act I, Scene 4 by King Duncan. He is talking to Malcolm after Malcolm has reported to him that the Thane of Cawdor has been executed. What Duncan means by this is that there is no way to tell what a man is like -- what his character is -- by looking at his face.
When Duncan says "there is no art" he means "there is no way." When he refers to a "man's construction" he means the man's character.
What he is saying is that Cawdor basically fooled him -- he thought Cawdor was loyal but he turned out to be a traitor.
To understand the meaning of this line, it is important to consider its context. This line comes from Act I, Scene IV, and is spoken by King Duncan. Here, Duncan is speaking with his noblemen, including Macbeth, about the Thane of Cawdor and his treason. Specifically, Duncan is saying that he had no idea that Cawdor would turn out to be a traitor. After all, it is impossible ("there's no art") to know what a person is thinking ("the mind's construction") simply by looking at the face, because a person can hide his true intentions and feelings.
Looking deeper, this line is just as applicable to Macbeth as it is to Cawdor. In the previous scene, for example, Macbeth listened intently to the witches, who prophesied that he would become the next king of Scotland. This prompted to Macbeth to suddenly start thinking about murdering King Duncan. But, just like Cawdor, Macbeth hides these murderous thoughts. In this scene, Macbeth acts completely loyal and trustworthy to Duncan.
So, for the second time, Duncan is unable to read the "construction" of a person's mind. This time, it is Macbeth's mind, and Duncan is completely clueless about the impending murder.
King Duncan makes this observation in act 1 sc. 4 when Malcolm reports that the Thane of Cawdor who betrayed the king has been put to death. King Duncan means to say that what is hidden in the mind of a man can never be understood by looking at his face. That is to say, the face is never a true index of the mind. The king believed the Thane of Cawdor to be very trustworthy, but he secretly helped the Norwegian king against Duncan.
However this piece of comment also becomes an example of irony. As Duncan says this, Macbeth enters with Banquo and others. Having gone through the previous scene--act 1 sc. 3--we know that Macbeth, Duncan's most admired and trusted general, is tempted, by his own ambition and the prophecies of the witches, to kill the good old king and usurp the throne. Duncan has declared Macbeth as the new Thane of Cawdor, but it is an irony that this time it is going to be a worse kind of betrayal. Truly the 'mind's construction' can never be known from the face.