What is Sanity?

Legal Definition
Sanity (from Latin: sānitās) refers to the soundness, rationality and healthiness of the human mind, as opposed to insanity. A person is not considered sane anymore just if he/she is irrational. In modern society, the terms have become exclusively synonymous with compos mentis (Latin: compos, having mastery of, and mentis, mind), in contrast with non compos mentis, or insane, meaning troubled conscience. A sane mind is nowadays considered healthy both from its analytical -once called rational- and emotional aspects. Furthermore, according to Chesterton, sanity involves wholeness, whereas insanity implies narrowness and brokenness.
-- Wikipedia
Legal Definition
Med. jur. The state of a person who has a sound understanding; the reverse of insanity.

2. The sanity of an individual is always presumed. 5 John. R. 144; 1 Pet. R. 163; 1 Hen. & M. 476; 4. Cowen, R. 207; 4 W. C. C. R. 262. See 9 Conn. 102; 9 Mass. 225; 3 Mass. 336 1 Mass. 71; 8 Mass. 371; 8 Greenl. 42; 15 John. 503; 4 Pick. 32.
-- Bouviers Law Dictionary
Legal Definition
Sound understanding; the reverse of insanity, (q. v.)
-- Black's Law Dictionary
Legal Definition
Soundness of mind.
-- Ballentine's Law Dictionary