Proclus

Proclus headed the Platonic Academy at Athens in the fifth century, in the closing period when a pagan philosophical school was operating inside a Christian empire, and his instinct was to codify everything before it was lost.

The Elements of Theology is the extreme case: two hundred and eleven propositions, each demonstrated from those before it, laying out the procession of all things from the One and their return to it in the form of a geometry. Between the One and the intelligible world he inserted the henads, unities that are the gods of traditional cult given a metaphysical office, a move that let him keep the entire pagan pantheon inside a rigorous monistic system rather than choosing between them.

Two consequences reach the later tradition. The first is structural: his ranked orders of gods, angels, daemons and heroes are the model for the elaborate hierarchies of medieval angelology and of Renaissance and modern occult cosmology. The second is a transmission accident of the first importance. The Christian writer known as Dionysius the Areopagite adapted Proclus's scheme into a Christian celestial hierarchy of nine angelic orders, and because he was believed until the Renaissance to be the Athenian converted by Paul, that hierarchy carried apostolic authority through the whole Middle Ages. A great deal of Christian angelology is Proclus, unattributed.

The Elements of Theology is Proclus at his most austere: two hundred and eleven propositions, each proved from the ones before, a geometry of the divine written by the last great head of the Athenian school. Its afterlife is one of the strangest transmission stories in the history of ideas, because the most influential readers of this militantly pagan book were Christians and Muslims who did not know they were reading it. Around the turn of the sixth century an unknown Christian writing as Dionysius the Areopagite, Paul's Athenian convert, rebuilt Proclus's hierarchies into the celestial and ecclesiastical hierarchies of Christian mysticism; the apostolic name carried the system into the heart of the church, which took from it much of its angelology and its negative theology alike.

Separately, an Arabic compiler distilled the Elements into a short work that reached Latin readers as the Liber de Causis and circulated for a century as Aristotle's own theology. Aquinas, comparing it against William of Moerbeke's fresh translation of Proclus in 1268, recognised the true source and said so. Proclus himself, hymn writer, commentator on Euclid and the Timaeus, and practitioner of the theurgy he defended, was on the losing side of history in his lifetime: the school he served was closed by Justinian in 529, and his system survived best inside the religion that closed it.