A more thorough explanation:

Criminal insanity is a mental illness or disease that makes it impossible for a person to understand that their actions are wrong or that they were committing a crime. If a defendant is found to be criminally insane, they can use an insanity defense in court. This means that they admit to committing the crime but argue that they are not responsible for it because of their mental illness.

The tests used to determine if a defendant is criminally insane vary from state to state. Some states follow the Model Penal Code (MPC) rule, which looks at whether the defendant lacked the capacity to appreciate the wrongness of their actions and to conform their actions to the law. Other states follow common law tests, such as the M'Naghten Rule or the irresistible impulse test.

For example, under the M'Naghten Rule, a defendant is considered criminally insane if they lacked the capacity to know they were committing a crime due to a mental defect. Under the irresistible impulse test, a defendant is considered criminally insane if they lacked total capacity to conform with the law.

The first use of the insanity defense in the United States was in People v. William Freeman (1847) in Cayuga County, New York.

It's important to note that states have the freedom to decide how they want to handle the insanity defense. In Kahler v. Kansas, the court ruled that states are not required to acquit a defendant who believed their actions were morally just.

For example, if a person with a mental illness commits a crime but did not understand that their actions were wrong, they may be able to use an insanity defense in court. However, if a person commits a crime because they believe it is morally just, even if they have a mental illness, they may not be able to use an insanity defense in some states.