Section 84 deals with Acts done by 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.
According to section 84 acts done by an unsound person is not an offence. But the main question behind this section is who is a sound person and who is unsound? If we see medical scope even a person under the influence of drugs is an unsound person, but legal scope is much narrower as the general principle of law is to protect the society. If we take medical perspective into our account then any person under influence of drugs commits a crime and take a plea of sec 84 that he had an unsound mind when he committed that offence. This would create chaos in the society.
Basically there are three faculties that law recognizes in human mind:
Indian law only recognizes Cognitive faculty which means, he is incapable of knowing the nature of the act, or that he is doing what is either wrong or contrary to law. Incapacity is different from potentiality and incapacity of mind to differentiate what is wrong/Right is Unsoundness.
The Emotional and will components are also form partial defence such as in cases of Grave and sudden provocation, offences done in heat of argument but are not considered as unsoundness or in capacity.
The medical evidence also plays a dominant role and behaviour of the accused also taken into consideration, if all the points related to inacapcity, behaviour, medical evidence are satisfied then only the case fits into the four corners of section 84.