Over the history of psychiatry, the term psychosis has had a number of different definitions. Currently, the most restrictive definition of this term implies the presence of delusions or prominent hallucinations where there is a lack of insight into the pathological nature. A delusion is described as a permanent and false belief. This belief cannot be changed by any logical argument, and the person does not recognize the belief as untrue. A hallucination is when a person believes he or she is experiencing a sensory input when there is none present. This can include the sense of sight, hearing, smell, or touch. Most commonly in psychotic illnesses the hallucinations are auditory (hearing) or visual (sight). In other words people who have psychotic symptoms have a difficult time differentiating reality from fantasy.