Doctrinal Works.

        In the doctrinal field Tertullian was not markedly creative, for he owed much to Irenaeus and Melito. He was also much influenced by Stoic philosophy and by what he had been taught by the church at Rome, where he was converted. Yet his work Against Praxeas is a notable defense of the doctrine of the Trinity, particularly against the followers of the Roman Sabellius, who flourished late in the second and early in the third century and held Monarchian and modalistic views. Praxeas in his solicitude for the divine unity identified Father, Son, and Holy Spirit, so that it was the Father himself who was born of a virgin and suffered on the cross. Tertullian wrote also On the Flesh of Christ, On the Resurrection ofthe Flesh, and On the Soul-a work which Harnack calls the first book on Christian psychology.