Joining, in a short phrase, two words of opposite meaning, such as "sweet bitterness."
In Don Quixote
not all the doctors and good scribes in the world will take him out of the blot of his madness: he is a mixed madman, full of lucid intervals.
"Madman" and "lucid intervals" join reason and its absence in one phrase, and Don Lorenzo's formula sums up what the book shows of the gentleman: a madness that does not impair his judgment.
Part Two: Mirrors, Lions, and Camacho, Don Lorenzo, to his father, after talking with the guest