An hallucination differs from an illusion in the fact that it is entirely subjective. In a strict sense it is a disorder of perception in which the external sense organs do not participate and it leads to a belief on the part of the patient that he perceives some external object, whereas in fact there is no such object corresponding to the perception. In other words, it is a disorder of perception giving rise to a false belief in a sensation. For instance, a person believes that he hears the voice of an acquaintance speaking to him, when there is no voice either of the ac quaintance or of anyone else; or, again, a per son thinks he has a vision of a deceased friend or of some other spiritual being, whereas there is no external object or person whatever giving rise to this or any other sensation. The whole process, therefore, is subjective or within the patient's own mind; it is without any objective equivalent. It is thus readily seen that an hal lucination is much more deeply seated than an illusion, and is a much more direct evidence of a disorder of the mind. Some authorities doubt indeed whether a true hallucination ever occurs in a normal person; what appears to be so in any given case would probably be found on strict inquiry to be an illusion. Although this may be an extreme position, the fact remains that hallucinations are very rare in the sane and very common in some forms of insanity; and that, as in the case of illusions the sane tend to correct them, while the insane adhere to them with unswerving belief. The most common hallucinations of the insane are the visual and the auditory, and they are sometimes very grave symptoms. Auditory hallucinations are espe cially dangerous, for they may lead the patient to commit acts of violence in response to their suggestions.
A delusion differs from both an illusion and an hallucination in the fact that it is not a sen sory disorder — but a derangement entirely with in the intellectual sphere. It is an erroneous belief, but one which is due entirely to mental disorder. The latter part of this definition is essential, because a delusion is essentially some thing more than a mere error of belief or judg ment. It is an erroneous belief that is due to insanity, or to a disordered mind. Mere errors of belief may and do occur in persons who are sane: no person, in fact, is exempt. Such er rors may be due to ignorance, prejudice or faults of education, but they are not evidence of insanity; otherwise the whole world would be insane. In the case, however, of an insane pauper who believes that he is a multi-million aire, or that he is emperor of the United States, there is obviously something more than a mere erroneous belief in the sense of an error of judgment or opinion. Such a patient has de veloped an idea which no sane man in similar circumstances could have developed; in other words, he has a delusion, and he is not insane because he has the delusion, but he has the de lusion because he is insane. The insanity or mental unsoundness is the fundamental fact and is shown usually by other symptoms besides the tendency to form delusions. As in the cases of illusions and hallucinations, the insane cling to their delusions with great tenacity; no argu ment, no logic moves them. The delusions of the insane are usually distinguished as systema tized and unsystematized. In the former the delusion has a certain consistence, coherence and endurance; in the latter the ideas are more or less incoherent and changing. Delusions again may be expansive or depressive, accord ing to the emotional tinge. See also IMAGINA