Definition of hallucinations from the British Encyclopedia of 1933

HALLUCINATIONS, according to Esquirol, are morbid conditions of mind in which the patient is conscious of a perception without any impression having been made on the external organs of sense. Hallucinations are to be distinguished from illusions, for in these there are real sensations, though they are erroneously interpreted.

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Hallucinations are not confined to those whose mental faculties have been alienated, but occasionally assail and torment even the sane. Occasionally hallucinations supervene where the system is healthy, and the individual fully conscious of the unreality of the objects that address his senses. Sometimes this disorder is associated with much ability and wisdom in the conduct of life.

Amongst well-known and authenticated hallucinations are that of the second Earl Grey, who was haunted by a gory head, which, however, he could dismiss at will, and that of Bernadotte, King of Sweden, who was beset in his rides by a woman in a red cloak, although perfectly conscious of the hallucination under which he laboured.