What is Insanity?

Legal Definition
Insanity, craziness, or madness is a spectrum of behaviors characterized by certain abnormal mental or behavioral patterns. Insanity may manifest as violations of societal norms, including a person becoming a danger to themselves or others, though not all such acts are considered insanity; likewise, not all acts showing indifference toward societal norms are acts of insanity. In modern usage, insanity is most commonly encountered as an informal unscientific term denoting mental instability, or in the narrow legal context of the insanity defense. In the medical profession the term is now avoided in favor of diagnoses of specific psychiatric diseases; the presence of delusions or hallucinations is broadly referred to as psychosis. When discussing psychiatric illness in general terms, "psychopathology" is considered a preferred descriptor.

In English, the word "sane" derives from the Latin adjective sanus meaning "healthy". Juvenal's phrase mens sana in corpore sano is often translated to mean a "healthy mind in a healthy body". From this perspective, insanity can be considered as poor health of the mind, not necessarily of the brain as an organ (although that can affect mental health), but rather refers to defective function of mental processes such as reasoning. Another Latin phrase related to our current concept of sanity is "compos mentis" (lit. "sound of mind"), and a euphemistic term for insanity is "non compos mentis". In law, mens rea means having had criminal intent, or a guilty mind, when the act (actus reus) was committed.

A more informal use of the term insanity is to denote something considered highly unique, passionate or extreme, including in a positive sense. The term may also be used as an attempt to discredit or criticise particular ideas, beliefs, principles, desires, personal feelings, attitudes, or their proponents, such as in politics and religion.
-- Wikipedia
Legal Definition
Med. jur. A continued impetuositv of thought, which, for the time being, totally unfitsga man for judging and acting in relation to the matter in question, with the composure requisite for the maintenance of the social relations of life. Various other definitions of this state have been given, but perhaps the subject is not susceptible of any satisfactory definition, which shall, with, precision, include all cases of insanity, and exclude all others. Ray, Med. Jur. 24, p. 50.

2. It may be considered in a threefold point of view: 1. A chronic disease, manifested by deviations from the healthy and natural state of the mind, such deviations consisting in a morbid perversion of the feelings, affections and habits. 2. Disturbances of the intellectual faculties, under the influence of which the understanding becomes susceptible of hallucinations or erroneous. impressions of a particular kind. 3. A state of mental incoherence or constant hurry and confusion of thought. Cyclo. Practical Medicine, h. t.; Brewster's Encyclopaedia, h. t.; Observations on the Deranged Manifestations of the Mind, or Insanity, 71, 72; Merl. R¬ępert. mots Demenoe, Folie, Imbecilite; 6 Watts & Serg. 451.

3. The diseases included under the name of insanity have been arranged under two divisions, founded on two very different conditions of the brain. Ray, Med. Jur. ch. 1, 33.

4. - 1. The want of, or a defective development of the faculties. 1st. Idiocy, resulting from, 1. Congenital defect. 2. An obstacle to the development of the faculties, supervening in infancy. 2d. Imbecility, resulting from, 1. Congenital defects. 2. An obstacle to the development of the faculties, supervening in infancy.

5. - 2. The lesion of the faculties subsequent to their development. In this division may be classed, 1st. Mania, which is, 1. Intellectual, and is general or partial. 2. Affective and is general or, partial. 2d. Dementia, which is, 1. Consecutive to mania, or injuries of the brain. 2. Senile, or peculiar to old age.

6. - There is also a disease which has acquired the name of Moral insanity. (q. v.)

7. Insanity is an excuse for the commission of acts which in others would be crimes, because the insane man has no intention; it deprives a man also from entering into any valid contract. Vide Lunacy; Non compos mentis, and Stock on the Law of Non Compotes Mentis; 1 Hagg. Cons. R. 417; 3 Addams, R. 90, 91, 180, 181; 3 Hagg. Eccl. R. 545, 598, 600; 2 Greenl. Ev. 369, 374; Bouv. Inst. Index, h. t.
-- Bouviers Law Dictionary
Legal Definition
Unsoundness of mind; madness; mental alienation or derangement; a morbid psychic condition resulting from disorder of the brain, whether arising from malformation or defective organization or morbid processes affecting the brain primarily or diseased states of the general system implicating it secondarily, which involves the m-teliect, the emotions, the will, and the moral sense or some of these-faculties, and which is characterized especially by their non-development, derangement or perversion, and is manifested, in most forms, by delusions, incapacity to reason or to judge or by uncontrollable impulses. In law, such a want of reason, memory, and intelligence as prevents a man from comprehending the nature and consequences of his acts or from distinguishing between right and wrong conduct. From both the pathologic and the legal definitions are to be excluded temporary mental aberrations caused by or accompanying alcoholic or other intoxication and the delirium of fever. See Crosswell v. People, 13 Mich. 427, 87 Am. Dec. 774; Johnson v. Insurance Co., 83 Me. 182, 22 Atl. 107; McNeil v. Relief Ass'n, 40 App. Div. 581, 58 N. Y. Supp. 122 ; Haile v. Railroad Co., 60 Fed. 560, 9 C. C. A. 134, 23 L. R. A. 774 ; Meyers v. Com., 83 Pa. 136; Somers v. Pumphrey, 24 Ind. 245; Frazer v. Frazer, 2 Del. Ch. 263. Other definitions. Insanity is a manifestation of disease of the brain, characterized by a general or partial derangement of one or more faculties of the mind, and in which, while consciousness is not abolished, mental freedom is pel-verted, weakened, or destroyed. Hammond, Nervous System, 332. The prolonged departure, without auy adequate cause, from the states of feeling and modes of thinking usual to the individual in health. Bouvier. By insanity is not meant (in law) a total deprivation of reason, but only an inability, from defect of perception, memory, and judgment, to do the act in question, [with an Intelligent apprehension of its nature and consequences.] So, by a lucid interval is not meant a perfect restoration to reason, but a restoration so far as to be. able, beyond doubt, to comprehend and to do the act with such reason, memory, and j'udgment as to make it a legal act. Frazer v. Frazer, 2 Del. Chi 263. Synonyms.
See also
-- Black's Law Dictionary
Legal Definition
Unsoundness of mind; mental derangement. See 34 Wis. 117.
-- Ballentine's Law Dictionary