His third book, "On the Incarnation of the Lord", was a defense of orthodox doctrine against the views of Nestorius, and was written at the request of the Archdeacon of Rome, later Pope Leo I. In this book Cassian points out a link between Nestorianism which stresses the humanity of Jesus and Pelagianism which stresses human effort. Later theologians, however, labeled Cassian as "Semipelagian" because he stressed the role of the human will, as opposed to Augustine's stress on the totality of grace, in moving towards salvation.