The Laurentian library in Florence holds many medieval copies of writings of the ancient Greek historian Herodotus, most of them still dangling the long chains which once bound them to the wooden desks of Michaelangelo’s library in San Lorenzo. One manuscript, copied in 1318, looks at first glance like just another late-Byzantine copy of Herodotus’s classic work. But a century or so after its creation an eccentric philosopher, George Gemistos, better known as “Pletho”, went through the manuscript carefully editing, deleting, or rewriting selected passages. His alterations bring Herodotus into line with a new history of philosophy which Gemistos wrote as part of his clandestine revival of ancient Greek Paganism. Like so many of Gemistos’s “ancient” novelties, this looks like one side of a competitive dialogue with the monotheistic religions of his day. The Florentine manuscript illustrates how the dynamics of co-production, visible within Judaism, Christianity and Islam, could also spill over their borders into the more esoteric movements of the late medieval world.