The movements of individuals and small groups supply much fresh information on communications between Carolingian Europe and the Mediterranean worlds of Byzantium and the Caliphate. Generally speaking, that information is of high quality. Before we begin to exploit it, however, a word on the strengths and weaknesses of my evidentiary base is in order.

The investigation has approached exhaustiveness for sources in Latin, Greek, and Old Church Slavonic. They have been scrutinized for travelers from continental western Europe who connected with Byzantium and the Arab world from 700 to 900. Only those travelers for whom the information appeared to me reliable have been counted – though I have tried to use in other ways the valuable evidence that comes from people who never really traveled merely because they never existed (Ch. 9.3). Some reliably documented travelers attested in these languages must have been missed. But within the limits of this inquiry, I do not think there are many.

Experienced practitioners of early medieval sources know how fluid and still relatively uncharted their languages are. Constant philological attention is essential if the evidence is to be shorn of the inaccurate understandings that have sometimes enveloped our witnesses' words. The need for searching linguistic scrutiny simply to grasp precisely what the sources were saying in the languages with which I am more or less comfortable made me reluctant to venture beyond them.