Sanity refers to the soundness, rationality

Rationality

In philosophy, rationality is the exercise of reason. It is the manner in which people derive conclusions when considering things deliberately. It also refers to the conformity of one's beliefs with one's reasons for belief, or with one's actions with one's reasons for action...

 and health

Health

Health is the level of functional or metabolic efficiency of a living being. In humans, it is the general condition of a person's mind, body and spirit, usually meaning to be free from illness, injury or pain...

iness of the human

Human

Humans are the only living species in the Homo genus...

 mind

Mind

The concept of mind is understood in many different ways by many different traditions, ranging from panpsychism and animism to traditional and organized religious views, as well as secular and materialist philosophies. Most agree that minds are constituted by conscious experience and intelligent...

, as opposed to insanity

Insanity

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 becoming a danger to themselves and others, though not all such acts are considered insanity...

. A person is sane if they are rational

Rationality

In philosophy, rationality is the exercise of reason. It is the manner in which people derive conclusions when considering things deliberately. It also refers to the conformity of one's beliefs with one's reasons for belief, or with one's actions with one's reasons for action...

. In modern society, the terms have become exclusively synonym

Synonym

Synonyms are different words with almost identical or similar meanings. Words that are synonyms are said to be synonymous, and the state of being a synonym is called synonymy. The word comes from Ancient Greek syn and onoma . The words car and automobile are synonyms...

ous with compos mentis , in contrast with non compos mentis

Non compos mentis

Non compos mentis is a term meaning 'not of sound mind'. Non compos mentis derives from the Latin non meaning "not", compos meaning "having ", and mentis , meaning "mind"...

, or insane, meaning troubled conscience.
A theory of sanity was proposed by Alfred Korzybski

Alfred Korzybski

Alfred Habdank Skarbek Korzybski was a Polish-American philosopher and scientist. He is remembered for developing the theory of general semantics...

 in his general semantics

General Semantics

General semantics is a program begun in the 1920's that seeks to regulate the evaluative operations performed in the human brain. After partial program launches under the trial names "human engineering" and "humanology," Polish-American originator Alfred Korzybski fully launched the program as...

. He believed that sanity was tied to the structural fit or lack of it between our reactions to the world and what is actually going on in the world.