Sixtus, afterwards Sixtus III., Bishop of Rome, the immediate successor of Caelestine, to whom the next letter is addressed. His name is the forty-third in the list of Popes, and he was in office from 432 to 440 A.D. The 194th letter of Augustin was addressed to the same Sixtus, and is a very elaborate dissertation on Pelagianism. It is omitted from this selection as being rather a theological treatise than a letter. ↩