Final answer:
Macbeth's line 'so foul and fair a day I have not seen' reflects the day's paradoxical nature—bad weather coupled with battlefield victory. Shakespeare uses this to foreshadow the play's intertwining of fortune and dark deeds, and to highlight the theme of appearances versus reality.
Step-by-step explanation:
When Macbeth says, "so foul and fair a day I have not seen," he is commenting on the paradoxical nature of his experiences. The phrase signifies the dual aspects of the day he has encountered: on the one hand, the weather is foul, possibly with poor conditions such as storms or fog; on the other hand, the day is fair because he has just won a victory on the battlefield. Shakespeare paints the setting with words by using this contrast to foreshadow the blend of good fortune and dark deeds that will characterize the rest of the play. This line embodies the play's theme of appearances versus reality, as what seems fair (victory, kingship) is intertwined with what is foul (treachery, murder).