Foul is Fair and Fair is Foul

Foul is fair and fair is foul means that nothing is as it seems. Things that appear foul might be fair and vice versa. It means that you should not take everything you see, hear, taste etc. at face value. This is especially important when someone puts forward an argument. It may seem ok, but when you ask for evidence it falls flat on its face. Look a bit deeper into things before you accept them blindly.

This quote is from the play Macbeth by William Shakespeare and since it means to look deeper into things before accepting it blindly, it applies to the story when Macbeth lusts for power as soon as the witches told him about the prophecy that they saw in him.