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.