Are they fickle? Are their affections mutable? Their virtue flexible? In Cosi fan tutte, Mozartand Da Ponte answer with resounding affirmatives.In the nearly 200 years since it was first performed at the Burgtheater in Vienna, Cosi fan tutte has waxedand waned and waxed again in popularity. The music, with its richness of melody, harmony, and texture, and its perfection of form, has never been considered less than brilliant. The libretto, however, has not been as well received as evidenced by attempts in the 19th century to "improve" it, making it more possible to the overly decorous sensibilities of the Victorians.