What is the hidden truth in Macbeth in the paradox, "so foul and fair a day I have not seen"?
There are paradoxes aplenty in Macbeth and this is simply one of them. The day on which the play begins is both good and bad. Macbeth has vanquished the king's enemies on the field of battle and been generously rewarded by Duncan for his extraordinary feats of valor. In that sense it's a good day. The newly ennobled Thane of Cawdor is a man on the make, rising rapidly through the upper echelons of the Scottish court.
Yet as Macbeth and Banquo arrive to see the three witches, they are treated to a stormy scene—a howling gale and a blackened sky whipped up by the wizened crones' dark, demonic powers. This is what Macbeth means by "foul." Unbeknown to Macbeth, his very first words in the play eerily echo those of the witches in act 1, scene 1:
Fair is foul, and foul is fair.
These words, like Macbeth's own, embody a paradox. Their similarity allows us to see straight away that Macbeth is inextricably linked with the forces of darkness and evil.
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.