What is the hidden truth in Macbeth in the paradox, "so foul and fair a day I have not seen"?
Macbeth says this to Banquo early in the play. On the surface the day is "fair" or good because Macbeth, Banquo and their armies have won a victory against their enemies, who are traitors to the king. It is "foul" because many lives have been lost on the battlefield, and more directly, because the witches have brought thunder and high winds with them to greet Macbeth and Banquo.
On a deeper level, this particular day will prove to be both fair and foul to Macbeth because it is the day on which the witches prophesy that he will become Thane of Cawdor and King of Scotland. He will become Thane of Cawdor, a piece of good or fair news for an ambitious and victorious soldier, but the prophecy that he will become king, will prove to be a tragic or foul one for Macbeth. Macbeth's offhand statement is full of meaning but Macbeth will not know until later the true import of his words on this fateful day.