Set immediately prior to the trial and execution of Socrates in 399 BC, Theaetetus shows the great philosopher considering the nature of knowledge itself, in a debate with the geometrician Theodorus and his young follower Theaetetus. Their dialogue covers many questions, such as: is knowledge purely subjective, composed of the ever-changing flow of impressions we receive from the outside world? Is it better thought of as "true belief"? Or is it, as...