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 many modern philosophers argue, 'justified true belief', in which the belief is supported by argument or evidence? With skill and eloquence, Socrates guides the debate, drawing out the implications of these theories and subjecting them to merciless and mesmerising criticism. One of the founding works of epistemology, this profound discussion of the problem of knowledge continues to intrigue and inspire.