This is not the book I set out to write. My original aim had been to write a study of Gregory the Great (pope 590–604) in his world. Gregory was neither a faithful disciple of Augustine, Ambrose and Cassian: the fathers he liked, above all, to consider his forebears; nor an original, creative thinker such as they had been. His greatness and his creativity lay in another dimension. His own judgement on his work – ‘a despicable little trickle’ compared with the ‘deep torrents and the clear flow of Ambrose and Augustine’ – does full justice to it on one level, and is a crude caricature on another. This is the paradox I had wanted to explore. What was it about Gregory's world that so changed the framework of thought, the assumptions about the world, God and man, the shape of discourse, that his questions came to differ so profoundly from those of Augustine and his contemporaries, and that the traditions he inherited from them now furnished material for answers to questions so different? What was the alchemy that so changed the whole configuration of the intelligible world over two centuries?

These were the questions I wanted to consider. It took me longer than it should to realise that this would involve considering the slow change of a whole world of discourse over the two centuries.