A more thorough explanation:

The M’naghten rule is a legal test used to determine criminal insanity. It was first used in England in 1843 during the trial of Daniel M’Naghten, who shot and killed the secretary to the Prime Minister, Edward Drummond, believing he was the Prime Minister. M’Naghten claimed he needed to murder the Prime Minister because “the tories” were conspiring against him and wished to murder him. His defense argued that he was insane at the time of the crime.

The M’Naghten rule states that a defendant is presumed to be sane unless they can prove that, at the time of committing the criminal act, their state of mind caused them to either not know what they were doing or not know that it was wrong. For example, if a person is acting on orders from “God,” they may not know that their actions are wrong.

The M’Naghten rule was the standard test for insanity in both the United States and the United Kingdom. However, some states in the US have implemented different tests, such as the irresistible impulse test, the Durham Test, or the Model Penal Code test.

An example of the M’Naghten rule in action is the case of People v. William Freeman in 1847. Freeman killed four people with an axe, claiming that he was doing so under orders from God. The court found him not guilty by reason of insanity under the M’Naghten rule.