Theodore of Tarsus and the Study of Computus at the Canterbury School

The Irish Computus Einsidlensis (CE) of c. 700 contains a reference to a certain Theodore. This article makes the case that this Theodore should be identified with Theodore of Tarsus, archbishop of Canterbury from 668/9 to his death, 690, on the basis of comparison not only with other contemporary Latin versions of the same argumentum, but also Byzantine computi. The passage under discussion represents the only known computistical tract that can with confidence be ascribed to the famous Canterbury school under Theodore and Hadrian. From the evidence provided, it appears that Theodore learnt this algorithm while studying in the Byzantine Empire and introduced it through his teaching to his Canterbury audience; his Irish students brought it to Ireland, from where it got popularised on the Continent through Willibrord.