Hallucinations are the apparent perception of an external object when no such object is present. They are a fixed report of a perception without an appropriate stimulus. Frequently the stimulus appears to be located in the world outside the hallucinating person, although it can be demonstrated that it is within the person. Hallucinations have the characteristics of one or several sensory systems; they may be visual, auditory, tactile, olfactory, or gustatory. Organic disturbances may cause hallucinations, although in most instances they can be explained by psychological and cultural circumstances.
In adults in the Western world, hallucinations should raise the question of mental pathology. In primitive individuals and in primitive cultures in which myth and reality are not clearly differentiated, hallucinations occur more frequently.