We think of a myth as a fictional story, and Plato was the first to use the term "muthos" in that sense. But Plato also used "muthos" to describe the practice of making and telling myths, the oral transmission of all that a community keeps in its collective memory. In the first part of this text, Luc Brisson reconstructs Plato's multifaceted and not uncritical description of "muthos" in light of the latter's famous Atlantis story. The second part of the book contrasts this sense of myth, as Plato does, with another form of speech which he believed was far superior: the "logos" of philosophy. Brisson's work is part lexical, part philosophical, and part ethnological, and Gerard Naddaf's substantial introduction shows the originality and importance both of Brisson's method and of Plato's analysis in the context of contemporary debates over the origin and evolution of the oral tradition.