The question needs to be backed up with evidence supporting the response I believe the answer to be that not everything is at one percieves it to be in ones own mind. I thought this to be true because in his mind Duncan believed that he could trust thane of cawdor but in reality he couldn't.
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I think the lines you mean are:
There’s no art To find the mind’s construction in the face He was a gentleman on whom I built An absolute trust.
Here Duncan is commenting on how difficult it is to see what people are really thinking and feeling. People are very good at hiding their real emotions and motivations.