In this work, Umberto Eco demonstrates how myths and lunacies can produce historical developments of no small significance. In Eco's words, even errors can produce interesting side effects. This book shows how: believers in a flat earth helped Columbus accidentally discover America; how the medieval myth of Prester John, the Christian king in Asia, assisted the European drive eastward; and how the myth of the Rosicrucians affected the Masons, leading in turn to the widespread belief in a Jewish masonic plot to dominate the world and other forms of paranoid anti-Semitism in the 19th and 20th centuries.