In ufology, a close encounter is an event in which a person witnesses an unidentified flying object (UFO) at relatively close range, where the possibility of misidentification is presumably greatly reduced. This terminology and the system of classification behind it were first suggested in the American astronomer and UFO researcher J. Allen Hynek's book The UFO Experience: A Scientific Inquiry (1972). Categories beyond Hynek's original three have been added by others but have not gained universal acceptance, mainly because they lack the scientific rigor that Hynek aimed to bring to ufology.