  At the gates of the city of Megara in 369 BC, Eucleides and Terpsion hear a slave read out Eucleides’ memoir of a philosophical discussion that took place in 399 BC, shortly before Socrates’ trial and execution (142a–143c). In this, the young Theaetetus is introduced to Socrates by his mathematics tutor, Theodorus. Socrates questions Theaetetus about the nature of expertise, and this leads him to pose the key question of the dialogue: “What is knowledge?” (143d–145e). Theaetetus’ first response (D0) is to give examples of knowledge such as geometry, astronomy, harmony, arithmetic (146a–c). Socrates objects that, for any x, examples of x are neither necessary nor sufficient for a definition of x (146d–147e). Theaetetus admits this, and contrasts the ease with which he and his classmates define mathematical terms with his inability to define knowledge (147c–148e). Socrates offers to explain Theaetetus’ bewilderment about the question “What is knowledge?” by comparing himself with a midwife: Theaetetus, he suggests, is in discomfort because he is in intellectual labour (148e–151d).