This is the first new critical edition of this text since 1908, and the first to appear in the Oxford Classical Texts series. The edition is informed by a comprehensive analysis of the entire tradition of and , and by a thorough rethinking of the text documented in the accompanying commentary volume. are a key body of evidence for the development of Academic scepticism, one of the two varieties of scepticism in antiquity. The texts also shed light on the re-emergence of dogmatic Platonic philosophy in the first century BC.