Photius

Photius was twice Patriarch of Constantinople, one of the great scholars of the Byzantine world, and the compiler of the *Bibliotheca*, a reading log of some 280 books that preserves several works otherwise lost. The *Mystagogy of the Holy Spirit* is his case against the Latin addition to the creed, and it remains the argumentative basis of the Eastern position.

Sections 37 and 38 are the most on-point patristic text about a fourth divine person that this Library has located. If the Spirit proceeds from the Father and the Son, Photius argues, then a fourth hypostasis should proceed from the Spirit, and a fifth from that, and the doctrine ends in polytheism; and if you refuse the fourth procession while granting the second, you are playing favourites among equals.

Read for what it establishes rather than for what it attacks, the passage is instructive in an unexpected way. The fourth person is the absurdity the argument ends at — "there is no fourth" is Photius's premise, agreed on both sides without discussion, not his conclusion. In the one place one would most expect the point to be defended, it is assumed.