Plato thinks the serious person has need of much playfulness, and his dialogues describe many things as play that we might deem to be serious matters, for example, education, religious rites, philosophical inquiry, and books. The Athenian Stranger in the Laws even says that “we should live playing.” Despite his judgment that play has great worth, Plato did not publish an inquiry into its essence. This lecture attempts to explain Plato’s thought on what play is, its value, and how it can serve or be part of seriousness.