Gaius Marius Victorinus (also known as Victorinus Afer; fl. 4th century) was a Roman grammarian, rhetorician and Neoplatonic philosopher. Victorinus was African by birth and experienced the height of his career during the reign of Constantius II. He is also known for translating two of Aristotle's books from ancient Greek into Latin: the Categories and On Interpretation (De Interpretatione). Victorinus had a religious conversion, from being a pagan to a Christian, "at an advanced old age" (c. 355).
Victorinus, at some unknown point, left Africa for Rome (hence some modern scholars have dubbed him Afer), probably for a teaching position, and had great success in his career, eventually being promoted to the lowest level of the senatorial order. That promotion probably came at the time when he received an honorific statue in the Forum of Trajan in 354 (Jerome supplied biographical information but was not his student). Victorinus' religious conversion from Platonism to Christianity (c. 355), "at an advanced old age" according to Jerome, made a great impression on Augustine of Hippo, as recounted in Book 8 of the latter's Confessions. His conversion is historically important in foreshadowing the conversion of more and more of the traditionally pagan intellectual class, from the gods who in pagan belief had made Rome great.
Brought up a Christian, Emperor Julian had converted to a philosophical and mystical form of paganism; and once in power upon the providential death of Constantius II, then Julian attempted to reorganize the highly decentralized pagan cults, on lines analogous to the Christian Church. The emperor, wanting to purge the schools of Christian teachers, published an edict in June 362 mandating that all state appointed professors receive approval from municipal councils (the emperor's accompanying brief indicated his express disapproval of Christians lecturing on the poems of Homer or Virgil with their religion being incongruous with the religion of Homer and Virgil). Victorinus resigned his position as official rhetor of the city of Rome, professor of rhetoric, not an orator. The sprightly old professor kept writing treatises on the Trinity to defend the adequacy of the Nicene Creed's definition of Christ the Son being "of the same substance" (homoousios in Greek) with the Father. After finishing this series of works (begun probably in late 357), he turned his hand to writing commentaries on the Pauline Epistles, the first in Latin. Although it seems from internal references that he wrote commentaries on Romans and the Corinthians letters as well, all that remains are works, with some lacunae, on Galatians, Ephesians, and Philippians (the comments from the first 16 verses of this latter are missing).
We are fairly well informed on his previous works, mostly texts for his teaching areas of grammar and rhetoric. His most important works from the standpoint of the history of philosophy were translations of Platonist authors (Plotinus and Porphyry at least), which are unfortunately lost. They greatly moved Augustine and set him on a road of creating a careful synthesis of Christianity and Neoplatonism that was tremendously influential. Victorinus wrote a brief treatise De Definitionibus (On Definition) that lists and discusses various types of definitions used by rhetoricians and philosophers; he recommends the substantial definitions preferred by the latter (prior to the late 19th century this work was ascribed to Boethius). Victorinus' manual of prosody, in four books, taken almost literally from the work of Aelius Aphthonius, still exists. It is doubtful that he is the author of certain other treatises attributed to him on metrical and grammatical subjects. His commentary on Cicero's De Inventione is very diffuse.
He retained his Neoplatonic philosophy after becoming Christian, and in Liber de generatione divini Verbi, he states that God is above being, and thus it can even be said that He is not. Victorinus noted, "Since God is the cause of being, it can be said in a certain sense, that God truly is (vere ), but this expression merely means that being is in God as an effect is in an eminent cause, which contains it though being superior to it."
Mary T. Clark has identified the following works of Marius Victorinus
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