More than thirty years ago the artist-philosopher Jacques Derrida was working in the Old Bodleian Library in Oxford when he strayed into the shop. On offer were postcards  richly and decoratively redolent of the Western heritage. The shop has since moved to an enlarged space on the far side of the Schools quad and won a prize for its innovative approach to museum marketing. But we didn’t care about commercial opportunities in our universities then. When Derrida visited, in 1977 and 1979, the postcard stand was just a draughty space, clustered at one end of the seventeenth-century Proscholium.

Derrida chose a reproduction of a medieval illustration showing Plato standing behind Socrates. It showed Plato looking over Socrates’s shoulder as if he, Plato, were the teacher and Socrates the scribe. Seeing these two philosophers pictured in the wrong chronological order, Derrida instantly got an idea. What did it mean if the…