At the end of the 18th century, the musical stars aligned themselves in a way that may never be repeated. A gruff, odd-looking prodigy named Ludwig van Beethoven came from the provincial Bonn to worldly Vienna to seek out the celebrated Mozart and play for him. Though he recognized a genius in the rough, Mozart declined him as a student. Beethoven went instead to Haydn, the hero of Mozart, and the alchemy of art which then came forth would forge a new direction in the history of music. A touchstone of that great chemistry is the first set of Beethoven's lifelong work with the string quartet format, the Opus 18. That set consists of six works, and this Sunday, the splendid Mir Quartet will play them all. There may not be a better example in music, or any discipline, of a great artist with feet planted so firmly in two different worlds, looking both forward and backward with equal respect and technical fluency.