Irenaeus wrote a number of books, but the most important that survives is the "Against Heresies" (or, in its Latin title, "Adversus haereses"). In Book I, Irenaeus talks about the Valentinian Gnostics and their predecessors, who he says go as far back as the magician Simon Magus. In Book II he attempts to provide proof that Valentinianism contains no merit in terms of its doctrines. In Book III Irenaeus purports to show that these doctrines are false, by providing counter-evidence gleaned from the Gospels. Book IV consists of Jesus' sayings, and here Irenaeus also stresses the unity of the Old Testament and the Gospel. In the final volume, Book V, Irenaeus focuses on more sayings of Jesus plus the letters of Paul the Apostle.