Pseudo-Dionysius was long believed to have been St. Paul's Athenian convert, Dionysius the Areopagite, mentioned in Acts 17:43. However, the presence, in the writings attributed to him, of concepts and categories derived from the 5th century Neoplatonic philosopher Proclus gradually led to a re-evaluation of this mysterious writer's identity, and so he became known as Pseudo-Dionysius. We can say nothing certain about his life or identity, for we only know his works. These are: The Mystical Theology, The Divine Names, The Celestial Hierarchy, The Ecclesiastical Hierarchy, and ten epistles. In these treatises Pseudo-Dionysius develops his apophatic or negative theology, which was to have an immense influence on Christian mysticism in the East and West.