This chapter sketches the reasons for taking Plato’s term episteme and related ‘knowledge’ terms to refer to an irreducible kind of knowledge that is not standardly included in lists of key types of knowledge, and for identifying corresponding irreducible senses of ‘being’ and ‘truth’. Second, it reviews existing ways of understanding Plato, exploring the difference between developmental and unitarian readings, and situating this work among them. Third, it considers the reasons why one might include a search for a definition as part of a philosophical enquiry, and examines when, if ever, such a search could be helpful, and why a successful definition might not be Plato’s main goal or a goal at all. Fourth, it examines the relation between knowledge and propositional utterances expressing knowledge. Finally, it sketches the plan for the rest of the book.