Isidore of Seville’s encyclopaedia played a key role in the transmission of antique knowledge in the Middle Ages. Its first part (books I-X) discusses the artes liberales, law, medicine and the organization of human society at large. Historical and philological evidence suggests that it circulated separately in a number of versions. This paper argues that Isidore wrote this first part for king Sisebut, who was intent on acquiring Latin erudition. A close reading shows that Isidore had assembled basic information which could be of help for more advanced studies and that he considered some fields of knowledge to have become obsolete. He selected his material according to his understanding of the language as an epistemological tool, whose origin faithfully reflected the structure of the world.