Fact of the case :

In this case, McNaughten was charged with the murder by shooting Edward Drummond, who was the Private Secretary of the then Prime Minister of England Sir Robert Peel. The accused McNaughten produced medical evidence to prove that, he was not, at the time of committing the act, in a sound state of mind. 

He claimed that he was suffering from an insane delusion that the Prime Minister was the only reason for all his problems. He also claimed that as a result of the insane delusion, he mistook Drummond as the Prime Minister and committed his murder by shooting 

The statutory recognition to the defence of insanity as developed by the Common Law of England was rendered in the case of R. Vs. Daniel McNaughten, 1843 RR 59.:

Every person is presumed to be sane and to possess a sufficient degree of reasonableness to be responsible for his crimes, until the contrary is proved.

  In order to be exempt from the criminal liability of a conduct, it must be proved that at the time of committing the act, the person due to a defect of mind was not aware of nature and quality of the act he was doing, or 

if he did know it, he did not know that what he was doing was wrong.  If a person was conscious at the time of doing the act, to the fact that his act was contrary to the law of the land, he is to be held criminally liable.

  When a person commits certain acts suffering from delusional insanity and thus not knowing the true nature of his acts, he will be held liable.

Held :  The plea of insanity was accepted and McNaughten was found not guilty on the ground of insanity.

Note: The Indian law of unsoundness of mind is based on the opinion of McNaughten case