A more thorough explanation:

Definition: "Not guilty by reason of insanity" is a plea or verdict in a criminal trial where the defendant claims that they were mentally disturbed or incapacitated at the time of the offense and did not have the required intention to commit the crime, and are therefore not guilty.

For example, if someone commits murder but was suffering from a severe mental illness at the time, they may plead not guilty by reason of insanity. This means that they admit to committing the act but argue that they were not responsible for their actions due to their mental state.

The defense of not guilty by reason of insanity is based on the concept of mens rea, which refers to the mental state of the defendant when they committed the crime. If the defendant can prove that they were insane at the time of the offense, their culpability in the crime may be negated.

Legal insanity is usually proven through expert testimony on the defendant's psychological evaluation. Different jurisdictions use different tests to judge such pleas, such as cognitive tests, irresistible impulse tests, and substantial capacity tests.

It's important to note that a verdict of not guilty by reason of insanity may require the defendant to be admitted into a mental institution for treatment.

Example: John is charged with arson for setting fire to a building. He pleads not guilty by reason of insanity, claiming that he was suffering from a severe mental illness at the time and did not have the required intention to commit the crime. The court orders a psychological evaluation, and the expert testifies that John was indeed suffering from a mental illness at the time of the offense. The jury may then find John not guilty by reason of insanity, and he may be admitted to a mental institution for treatment.