Latin term meaning of Non Compos Mentis is “Not of a sound mind, memory, or understanding.” Defendants could argue in their defence that they were not responsible for the crime, owing to the fact that they are not sound in mind and therefore they cannot be held liable for their act. This type of defence can be taken only if the defendant totally lack in reasoning and understanding, and defendant should not be able to differentiate between right and wrong. If the defendant comes under the above said conditions then the juries deliver the verdict of “Non Compos Mentis.” Section.84 of the Indian Penal Code deals with act of the person of unsound mind.
There are four kinds of persons who may be said to be Non Compos Mentis
- An Idiot;
- One made Non Compos by illness
- A lunatic or a mad man
- One who is drunk
Person who can take up the defense of Non Compos Mentis are who cannot count twenty, or tell the days of the week, or who do not know their father or mother. Section 84 of IPC itself provide that the benefit is available only after proving that at the time of committing the act, he must be affected by the disease of mind and he must not be knowing that the act he was doing is lawful or not. In Siddhapal Kamala Yadav v state of Maharashtra the appellant murdered a person and pleads before the Supreme Court of India that he was in an unsound mind while performing the act, so he can’t be held under section 302 of IPC, but the doctors who attended the appellant gave a report that the appellant was normal and he was not in unsound mind when the act was done, taking the medical records into account Supreme Court declared that section 84 of IPC does not have any application to the facts of the case.
In Paramjeet Singh v state, a man was arrested for murdering his own daughter and causing injuries to the wife and the other two children, he claimed that he had unsound mind at the commencement of the act and he held that there was not motive in committing such a act, The Delhi high court held that motive cannot be a ground to access 84 of IPC, further on looking into the facts the neighbor came to the spot and said the appellant to through his knife away and so did the appellant which proves that he was not in a state of unsound mind he knows what was happening. So the Delhi High court dismissed the case on the grounds that article84 of IPC cannot be applicable.
 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.
 See Archbold’s Criminal Pleadings, Evidence and Practice, 35th Edn. pp.31-32; Russell on Crimes and Misdemeanors, 12th Edn. Vol., p.105; 1 Hala’s Pleas of the Grown