1.  The philosophical theory that concepts of what are and are not real must be agreed upon (not by a test or statement, but by “consensus,” a general agreement) by a group or culture. That is, if enough people believe some concept is “real,” whether it is or not, the cultural view is that it is real. An example of this is the concept that at one time it was believed by most Western Europeans that the Earth was flat. Maps and expeditions were made based on this belief. In reality, the Earth is not flat. In fact, the belief that most people during the Middle Ages and before believed that the Earth was flat may, in itself, be a form of consensus reality. There is now a belief that educated and influential people knew the Earth was spherical, and the wide spread popularity of the flat Earth myth actually developed only in the 19th century due to the popularity of Washington Irving’s book, The Life and Voyages of Christopher Columbus, a novel that claimed most thought the world was flat until Columbus.