Summary of the Dialogue
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 of 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). Thus prompted, Theaetetus states his first acceptable definition, which is the proposal (D1) that "Knowledge is perception" (151d-e). Socrates does not respond to this directly. Instead he claims that D1 entails two other theories (Protagoras' and Heracleitus'), which he expounds (151e-160e) and then criticises (160e-183c). Socrates eventually presents no fewer than eleven arguments, not all of which seem seriously intended, against the Protagorean and Heracleitean views. If any of these arguments hit its target, then by modus tollens D1 is also false. A more direct argument against D1 is eventually given at 184-7. In 187a10-e4, Theaetetus proposes a second definition of knowledge: (D2) "Knowledge is true belief." D2 provokes Socrates to ask: how can there be any such thing as false belief? There follows a five-phase discussion which attempts to come up with an...
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