This chapter examines Plato’s Theaetetus, the dialogue most explicitly devoted to the question “What is knowledge?” Despite this central focus, the dialogue ends aporetically without a definitive answer. The analysis centers on key arguments that shape the role of δόξα in it: The Contradiction Argument (161d–162a, 168c–171c) and The Reasoning Argument (184d–187a), both targeting Theaetetus’ first definition of knowledge as perception; The Puzzles of False Belief (188a–200a), which reveal how differing premises affect outcomes; The Judges Example (200a–201c), refuting the second definition of knowledge as true belief; and The Dream Theory (201e–202c), testing but ultimately rejecting the third definition of knowledge as true belief with λόγος. The discussion highlights how these refutations shape the distinction between belief and knowledge and concludes by assessing the philosophical value of belief within the dialogue.