Hallucination

There is apparently uninstigated perception of what is not actually there, the state occurring in response to drugs, physical or mental illness (for example, high fever, paranoia) or pain. The perception is most usually taken to be visual, although it may in fact refer to any of the five senses. The individual may or may not be aware of the hallucinatory nature of what is perceived. Although hallucination may be pleasant, it may also be unpleasant or frightening. The state is similar to a waking dream and may reveal useful information on the subconscious. Hallucination is difficult to distinguish from illusion. The former is generally said to cover seeing an internal image as physically present whereas the latter as a misrepresentation of an object actually there.

Common instances of hallucination are: hypnotic trance, when the individual perceives and responds to suggestions of the hypnotist, whether seeing something that is not there or not seeing something that is; dreams and hypnagogic hallucinations when half awake. In addition to conditions mentioned above, hallucination may be triggered by fatigue or by prolonged fasting.