Crick* gives the following waggish translation from Gerald of Wales* (and discusses Gerald's somewhat hypocritical attitude)...

"A Welshman from the neighbourhood of Caerleon was endowed with occult and prophetic gifts. Most notable among them was his ability to detect lies, whether written, spoken, or merely thought, a process facilitated by devils who indicated to him the offending person or passage (the man himself was illiterate). When he was harrassed beyond endurance by these unclean spirits, Saint John's Gospel was placed on his lap, and then they all vanished immediately, flying away like so many birds. If the Gospels were afterwards removed and the "History of the Kings of Britain" by Geoffrey of Monmouth put there in its place, just to see what would happen, the demons would alight all over his body, and on the book, too, staying there longer than usual and being even more demanding."