While at work on The Abduction from the Seraglio, Mozart posed for himself the great aesthetic conundrum of opera: how does drama become music? Reflecting, in a letter to his father, on the angry outburst of his operatic villain Osmin, he wrote, "Just as a man in such a towering rage oversteps all the bounds of order, moderation and propriety and completely forgets himself, so must the music too forget itself." And yet, as Mozart went on to say, unpleasant...