Med. Jur. A defect or imbecility of the understanding, general or partial, but confined to individual faculties of the mind, particularly those concerned in associating and comparing ideas, whence proceeds great confusion and incapacity in arranging the thoughts.
Demency is attended with a general enfeeblement of the moral and intellectual faculties which were originally well developed and sound. It is characterised by forgetfulness of the past, indifference to the present and future, and a childish disposition. It differs from idiocy and imbecility. In the latter, the powers of the mind were never possessed, while in demency they have been lost.
Demency may also be distinguished from mania with which it is sometimes confounded. In the former, the mind has lost its strength and thereby the reasoning faculty is impaired; while in the latter, the madness arises from an exaltation of vital power or from a morbid excess of activity.
Demency is divided into acute and chronic. The former is a consequence of temporary errors of regimen, fevers, hemorrhages, etc., and is susceptible of cure. Chronic demency may succeed mania, apoplexy, epilepsy, masturbation, and drunkenness, but is generally that incurable decay of the mind which occurs in old age.
When demency has been fully established in its last stages, the acts of the individual of a civil nature will be void because the party had no consenting mind. Having no legal will or intention, he cannot knowingly commit a crime.