Principle: Nothing is an offence if done by a person who, at the time of doing it, by reason of unsoundness of mind is incapable of knowing the nature of the act or what he is doing is wrong or contrary to law.
Factual Situation: A, an author of macabre stories, was undergoing treatment for somnambulism. One night, he was given a high dose of sedative as he had become violent. There was a 'Do not disturb' board out side his room. Early morning, while A was still asleep, a nurse went to his room to check his condition. She started to note his pulse and thereafter to measure his BP. While she was fixing the equipment to measure the BP, A saw the nurse, but in a delirium he thought the nurse to be a vampire going to drink his blood. Suddenly in order to exorcise the ghost, A grabbed the BP apparatus and hit the 'ghost' on the head several times. The nurse collapsed and died subsequently.