John of Salisbury's Polycraticus is one of the major works of twelfth-century literature. Belonging to the traditional genre of the ‘advice to princes’, it draws on a remarkable range of classical and patristic learning. Among the works paraphrased, but in this case not directly quoted, is the Institutio Traiani supposedly written by Plutarch to the emperor Trajan. These excerpts are the only evidence for the existence of such a work and have been much discussed. While Plutarch and Trajan were certainly contemporaries, there is no other reference to any connection between them; moreover, if such a work did exist it must originally have been written in Greek and later circulated in a Latin translation. It has been suggested that it was a late-classical fake: another theory would spread its composition over several centuries. Janet Martin has argued that there never was such a work and that John invented it in order to give authority to a number of his own morally improving stories. The evidence for this lies in John's treatment of his other classical sources, particularly in one case when we know the very manuscript on which he drew and where we see him interpolating material of his own which is then sourced to the classical author.

Welcome to the busy arena of literary forgery! John may have assumed that the cognoscenti among his audience would have picked up the fraud and been amused by it – or by their own superior acumen in spotting it.