These words refer to mental experiences that appear realistic or believable but have, despite their vividness, no objective reality. Delusion refers to the most extreme and inclusive form of this mental phenomenon, since it may combine vivid sensory imagery with complex notions or conceits; in the psychotic person, these delusions are totally mistaken for reality and are not voluntarily called up: a delusion that he heard voices urging him to kill; a delusion that he was Winston Churchill; the final stages of alcoholism in which delusions are commonplace. The phrase delusions of grandeur is actually a clinical term but is often used commonly for anyone with inflated self-regard.

Fantasy and illusion are considerably less extreme than delusion , referring to mental activity that everyone, not just a psychotic person, has commonly experienced. Fantasy applies mostly to an imaginary scene, such as that in a day dream, whether called up voluntarily or not, which is acted out mentally with vivid sensory imagery, but which is not, except in the mentally ill, mistaken for reality. The word may emphasize a general tendency of imagination towards the fanciful, whimsical, surrealistic or grotesque: a fantasy in which he watched his parents mourning over his coffin; a delightful element of fantasy that makes his ghost stories chilling and also convincing. Illusion can refer to an ideational cluster of notions that everyone experiences, voluntarily or not, but which do not correspond to any objective view of things: under the illusion that he could accomplish the whole job without the help of his co-workers. Two related, but milder, uses of this word exist; one refers to the bundle of ideals or necessary lies each person maintains to buoy up his ego: an illusion that the society in which he lived was very near perfect; an illusion that he was well liked by his fellow workers. Another use of illusion refers to confused optical phenomena that trick the eye into seeing a situation as other than it is: Heavy fog had created an optical illusion that made the opposite shore seem closer than it was. Mirage is specifically restricted to this last sense of illusion : a mirage that made the highway ahead seem to be flooded with water. When a person is under extreme stress, however, his inward mental disturbance may co-operate with deceptive optical clues to create something more similar to a full-blown delusion : gasping through lips cracked by the desert sun, he claimed that he saw a silver palace just over the next dune, unaware that the whole vision was nothing but a mirage .

Hallucination is mainly restricted to vivid sensory experience, like fantasy , but in its intensity a hallucination approaches the believability of a delusion . This experience would tend to occur involuntarily to any person under certain extreme circumstances, as during a long-term fever, after heavy dosages of painkilling or other drugs, during delirium tremens, or in connection with certain physical illnesses such as brain tumours. In this sense, a hallucination might be most typically fleeting, like a waking dream. In other uses, however, the word may point to nervous malfunction expressive of a deep-seated mental imbalance, in which case it indicates one possible constituent of a psychotic delusion .