The first fact to realize about insanity is that it is a culturally defined disease, in the sense that the culture and/or society decide what "normal" is. If something is sufficiently unusual and uncommon, and not explicable by reason or understood as having a well-defined cause, then society will often label the action as evidence of an "unsound mind". For instance, until the American Psychiatric Society voted (and not by 100% to zero) that homosexuality was not a mental disease, homosexuals were often "treated" by psychiatrists. Similarly, in the 1950's and 1960's, if a Russian citizen claimed that communism was not the best political system, he was often classified as schizophrenic, confined to a mental hospital, and treated with Thorazine and other anti-psychotics. In the Southern USA before 1860, if a black slave wanted to run away, he was often labeled as "crazy because of 'escapitis'", because why would he try to run away if he were not insane.
You also cannot "prove" that you are sane. It generally takes the testimony of two psychiatrists who have examined you to certify that you are insane or not insane. The default assumption is that you are sane, unless a judge requires you to prove you are sane before he will let you stand trial. Of course, the psychiatrists are themselves culturally bound, so that an American who attempts suicide is considered to have a mental problem, while a Japanese who attempts hari-kari is not, and probably everyone who sees Shakespeare's "Julius Caesar" understands Brutus' suicide. If you saw and spoke with the image of your dead grandmother, you might be considered insane, unless you were a teenage girl from a Latin American culture, where such such events can occur on or near her fifteenth birthday.
Then we come to the insanity defense. I am not commenting pro or con on its use, but how can you decide today that someone was insane two years, ago, if he/she were not examined at that time. However, most doctors will label a person as having a drinking problem, if the patient has drinks more on a daily basis than does the doctor. Similarly, as I tell my residents time and time again, all prescribing is a negotiating process, so if a hospitalized patient refuses to take a prescribed medicine, this is not grounds for a psychiatric consult.
One problem that occurs frequently is with people who hear voices or see images and claim that such sensations came from God. Clearly one cannot "prove" by any logical process that they were not, any more than they can prove that they were. There is a condition called the "Jerusalem Syndrome", wherein pious Christians and Jews are so overcome with the holiness of Jerusalem when they land at Lod airport that they become instantly convinced that they are the Jewish or Christian Messiah. There is a ward of Jerusalem Hospital that has permanently set aside beds for members of this group. Bernadette of Lourdes and Joan of Arc had difficulty convincing the authorities that they were divinely inspired while Abraham had no difficulty believing that the Lord commanded him to sacrifice his son, nor did Joseph Smith have difficulty believing in the Angel Moroni. Since religion is part of every culture, ideations that might otherwise be considered manifestations of paranoia are acceptable when they occur in a religious context.
In Salem, Massachusetts, women were judicially killed for being witches. Such behavior is not condoned now in the United States. However, there is a mental condition that occurs in males from Asia (especially Japan) as well as males in West Africa: They become convinced that their penis is shrinking and being pulled up and vanishing inside their bodies. Some men even go so far as to tie weights with ropes to their penises to arrest this process. However, in Africa, this belief is considered as evidence that a witch has placed a curse on you, and the immediate cure is to kill the witch. This has occurred, and the males involved are never prosecuted. I might observe in all societies it seems that only female witches are executed, and never male warlocks.
I would like to close this blog with a comment on free will. Most of us believe that we have free will, but most psychiatrists do not so believe. They believe that we are bound up in childhood problems, and try to solve them in the adult world in which we live. We try to resolve the unsolved problems of childhood and therefore (they believe) we marry the person who is most similar to the parent with whom we had the most problems.
This raises the question/problem of how much free will any of us have if we are still trying to solve childhood problems as an adult, since we never recognize the displacement 1nvolved in the battle(s). Since the justice system can only function logically and properly and fairly if we all have free will, that is precisely the default belief of all justice systems. Therefore you are responsible for all of actions, unless you can prove that you are not. To have any justice system function, it must be assumed, incorrectly or not, that you are legally responsible for 100% of your actions; this is the same assumption we make on Voting Day, as well as when we choose members of a jury.