A hallucination is in general, any belief or notion for which there is no foundation in fact. The word is used frequently as synonymous with illusion and delusion, but in its technical use there is a distinction to be made. An illusion is a false mental conception of something which really exists. If the stirring of a curtain by the wind makes one believe for a moment in a human presence, we describe his belief by the word illusion. If a person has a settled conviction that bis friends are trying to poison him, he is said to suffer from delusion, a word implying that the false conception is fixed and not susceptible to correction by the subject himself. An hallucination, specifically, is an apparent perception by the senses of some external object which does not exist as such. If one seems to hear bells ring when there are no bells, sees faces when there are no persons present, we call it hallucination. In other words, the sensation is real, but what psychologists call the "objective stimulus" is lacking.
Hallucinations were regarded formerly as the result of a disordered mind. Lord Brougham at one time seriously endeavored to establish a law which should make hallucination a proof of insanity. At present it is believed probable that all minds experience hallucinations in some form. It is only when the judgment is unable to discriminate and the will unable to keep hallucinations in the background that the subject is in any sense unbalanced or insane.
Hallucinations which seem of the nature of visions as, for instance, the apparition to one person of another who is dead or is at a great distance, the hearing of voices which convey messages when one knows that in reality no sound waves are set in motion, form a class of phenomena which have received the attention of the Society for Psychical Research. Some have become convinced that such occurrences are supernormal. Others regard them as perfectly normal but the result purely of misjudgment, and that experience, the cultivation of the reasoning faculties, and the training of the powers of observation, will lead to the condition where hallucinations will always be interpreted correctly.
