Residuary Powers (Art. 248)

Section 84 - Nothing is an offence which is done by a person who, at the time of doing it, by the reason of unsoundness of mind, is incapable of knowing the nature of the actor that he is doing what is either wrong or contrary to law.

Thus, a person claiming immunity under this section must prove the existence of the following conditions -

1.     He was of unsound mind - Unsound Mind is not defined in IPC. As per Stephen, it is equivalent to insanity, which is a state of mind where the functions of feeling, knowing, emotion, and willing are performed in an abnormal manner. The term Unsoundness of mind is quite wide and includes all varieties of want of capacity whether temporary or permanent or because of illness or birth defect. However, mere unsoundness of mind is not a sufficient ground. It must be accompanied by the rest of the conditions.

2.     Such incapacity must exist at the time of the act - A person may become temporarily out of mind or insane for example due to a bout of epilepsy or some other disease. However, such condition must exist at the time of the act. In S K Nair vs State of Punjab 1997, the accused was charged for the murder of one and grievous assault on other two. He pleaded insanity. However, it was held that the words were spoken by the accused at the time of the act clearly show that he understood what he was doing and that it was wrong.  Thus, he was held guilty.

3.     Due to incapacity, he was incapable of knowing - 

1.     either the nature of the act.

2.     or that the act is wrong. 

3.     or that the act is contrary to law.

The accused is not protected if he knows that what he was doing was wrong even if he did not know that what he was doing was contrary to law. In Chhagan vs State 1976, it was held that mere queerness on the part of the accused or the crime does not establish that he was insane. It must be proved that the cognitive faculties of the person are such that he does not know what he has done or what will follow his act.