"In one particularly successful exercise in Patriarch-baiting, Photius had even gone so far as to propound a new and deeply heretical theory that he had just thought up, according to which man possessed two separate souls, one liable to error, the other infallible. His own dazzling reputation as an intellectual ensured that he was taken seriously by many – including Ignatius – who should have known better; and after his doctrine had its desired effect, by making the Patriarch look thoroughly silly, he cheerfully withdrew it. It was perhaps the only completely satisfactory practical joke in the history of theology, and for that alone Photius deserves our gratitude."

from The Popes by John Julius Norwich