There are many competing theories about the origins and nature of UFOs. For example, researchers Brad Steiger and Sherry Hansen Steiger have a list of seventeen theories at their website. In UFO: The Complete Sightings (1995) by Peter Brookesmith, there is a list of nine theories. Further, these theories are often categorized.In The Mammoth Encyclopedia of Extraterrestrial Encounters (2001) edited by Ronald D. Story, the subject is divided into “conventional (unintelligent)” and “unconventional (intelligent)” encounters.