Section 84 Act of a person of unsound mind.
Nothing is an offence which is 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 that he is doing what is either wrong or contrary to law.
Insanity needs to be proved
The accused was charged and committed under section 302, I.P.C. for having caused the death of his wife and a female child with a chopper. Rejecting the plea of insanity the Supreme Court observed that the law presumes every person of the age of discretion to be sane unless the contrary is proved. It would be most dangerous to admit the defence of insanity upon arguments derived merely from the character of the crime. The mere fact that no motive was proved as to why the accused murdered his wife and child nor the fact that he made no attempt to run away when the door was broken open, could not indicate that he was insane or that he did not have the necessary mens rea for the commission of the offence; Seralli Wali Mohammed v. State of Maharashtra, AIR 1972 SC 2443.