A _delusion_ may be defined as a perversion of the judgment, a chimerical thought; an _illusion_, an incorrect impression of the senses, counterfeit appearances; hence we speak of a delusion of the mind, an illusion of the senses. Lawyers lay great stress on the presence of delusions as indicative of insanity. An _hallucination_ is a sensation which is supposed by the patient to be produced by external impressions, although no material object acts upon his senses at the time. =Partial Intellectual Mania=, or =Monomania=, also called =Melancholia=, is a form of the disease in which the patient becomes possessed of some single notion, contradictory alike to common-sense and his own experience. =General Moral Mania.=--This is a morbid perversion of the natural feelings, affections, inclinations, temper, habits, moral dispositions, and natural impulses, without any remarkable disorder or defect of the intellect, or knowing and reasoning faculties, and particularly without any insane illusion or hallucination. It is often difficult to distinguish this form of mania from the moral depravity which we associate with the criminal classes. =Partial Moral Mania--Paranoia--Delusional Insanity.=--In this form one or two only of the moral powers are perverted. Delusions are always present, and very frequently are those of persecution. The patient's conduct is dominated by his delusion; thus murder and suicide may be committed. There are several forms: _Kleptomania_, a propensity to theft; common in women in easy circumstances. _Dipsomania_, or _Oinomania_, an insatiable desire for drink. _Morphinomania_, a craving for morphine or its preparations. _Erotomania_, or amorous madness. When occurring in women this is also called _Nymphomania_, and in men _Satyriasis_. It consists in an uncontrollable desire for sexual intercourse. _Pyromania_, an insane impulse to set fire to everything. _Homicidal mania_, a propensity to murder. _Suicidal mania_, a propensity to self-destruction. Some consider suicide as always a manifestation of insanity. =Insanity of Pregnancy.=--This may show itself after the third month of pregnancy in the form of melancholia. It is not recovered from until after delivery. =Puerperal Mania.=--This form of mania attacks women soon after childbirth. There is in many cases a strong homicidal tendency against the child. =Insanity of Lactation= comes on four to eight months after parturition, either as mania or melancholia. The mother may repeatedly attempt suicide.