What is knowledge? A systematic treatment of this question takes the form of a dialogue between the great philosopher Socrates and his student, Theaetetus. They explore a variety of answers, including knowledge as perception; knowledge as true belief; knowledge as true belief plus an account (i.e., a justified true belief); as well as variations on each of these possibilities. Like most Socratic dialogues, it ends without a definitive answer, leaving the subject open for the reader's further consideration. One of Plato's later masterpieces, the Theaetetus poses eternal questions that keep the dialogue relevant not only for students of philosophy but also for every serious reader and thinker. This edition, translated by the noted classical scholar Francis M. Cornford, features extensive commentaries by the interpreter that provide helpful background information and valuable insights.