Of the 60 orations in his name available in Roman times, 21 remained in transmission by the end of the medieval period. The earliest manuscripts dated from the ninth or tenth century, until fourth century copies of Isocrates' first three orations were found in a single codex during a 1990's excavation at Kellis, a site in the Dakhla Oasis of Egypt. We have nine letters in his name, but the authenticity of four of those has been questioned. He is said to have compiled a treatise, the "Art of Rhetoric," but there is no known copy. Other surviving works include his autobiographical "Antidosis", and educational texts such as "Against the Sophists".