For centuries, courts have struggled to protect the mentally ill while also trying to distinguish between sanity and insanity. In the 1700s, the British courts relied on the “wild beast” test as their barometer for the latter: if the defendant’s understanding of his crime was no better than that of a infant or beast, he couldn’t be found guilty. From there, the insanity defense began its tortuous evolution.

In 1843, a Scottish woodcutter named Daniel M’Naghten attempted to assassinate British Prime Minister Robert Peel, shooting and killing his secretary instead. M’Naghten believed that Peel and the British government had singled him out for persecution and were responsible for all his personal and financial woes. He was found not guilty by reason of insanity and acquitted, leading to public outrage over the verdict.

In response, the House of Lords and a panel of the Queen’s judges put together the M’Naghten…