Section 84 of the Indian Penal Code says that any act done by a person who is of unsound mind at the time of doing act and the person is incapable of knowing the nature of the act and the person does not know that the act which he is doing is wrong or contrary to Law.

Origin of Law of Insanity: McNaughten Case

There were various tests used to declare a person legally insane such as Wild Beast Test , this test was the first to check the insanity laid down in the case of Arnold , in this case it was held that if any person do not have the capacity to determined what is right or wrong then he would get insanity defense . Then came the Insane Delusion Test and test of capacity to distinguish between right or wrong , these three tests laid the foundation of McNaughten rule.

In 1843 the accused McNaughten was suffering from persecution mania and because of this disease he thought that whatever the difficulties he is facing is because of British Prime Minister Robert Pel and McNaughten shot mr drummond believing under a mistake that he was killing Prime Minister who was the private secretary of PM and this happened because McNaughten was sick.

So he pleaded insanity and the House of Lords acquitted him . This generated a lot of public sentiment and lot of pressure was put on the House of Lords and they constituted a special committee of Judges and laid down the rules for plea of insanity and this is known as McNaughten's Rules.

These rules are as follows:

1. All are presumed to be sane , until contrary be proved for the satisfaction of the Court
2. To claim the defense of the Insanity , one should clearly show that at the time of the act the accused was suffering from the defect or mental illness
3. At the time of doing act he did not know the nature of the act
4. At the time of doing the act the accused do not know the that what he was doing was wrong

English Law considers insanity as a valid defense . The definition of insanity is based on Mc Naughten rules.

Indian Law on the concept of Defense of Insanity

In India the law on this subject is mentioned under section 84 of IPC , the provisions are same as mentioned in McNaughten case held by House Of Lords . By the way Section 84 use a more comprehensive term unsoundness of mind rather than the word Insanity.

Essential ingredients of section 84 of Indian Penal Code (IPC)

1. Act must be done by a person of unsound mind
2. Such person was unsound at the time of committing the act
3. Such incapacity should be of unsoundness of mind of the accused
4. Such person was not capable to know the nature of the act or the act he was doing was either wrong or contrary to law

Distinction between Legal Insanity and Medical Insanity

It is important to note that Insanity plays a vital role in criminal law and it is legal sense is to be understood rather than medical sense in order to claim defense under IPC . Legal insanity is narrower than medical insanity because some illness which are termed as insanity according to medical science but it cannot fulfil the criteria of Legal Insanity in order to get the defense under IPC.

Medical Insanity:

According to medical science , unsoundness of mind or insanity is a type of disease which messes with mental faculty of a person . this disease in brain or nervous system leads to working of one or two functions of brain in an abnormal way or these functions of brain do not function at all because of the disease in brain . There are four kinds of person which are categorised into unsound mind and these are as follows:

1. Idiot: A person who is non-sane since his/her birth without any lucid intervals
    
2. Lunatic : Lunatic is a type of person who is triggered of mental disorder only for certain period . Lunacy is acquired insanity but idiocy is a natural insanity
    
3. Non Compos Mentis (not of sound mind) by illness: This type of person do not know what he/she is doing and prolonged disease cause fits of delirium and under such fits the person is not able to identify what is right or wrong
    
4. One who is under intoxication

Legal Insanity:

Legal conception of the insanity is different from the Medical conception of the insanity , not every kind of insanity is made excusable in the eyes of Law . Section 84 of IPC cannot be invoked unless it is established that the accused was suffering from legal insanity . To invoke the benefit of section 84 of the IPC it must be proved that unsoundness of mind was of such a degree which should be capable to fulfil any one of the test laid down under this section and they are:

1. The accused was incapable of knowing the nature of the act
2. The accused was not capable of understanding that what he was doing was either wrong or contrary to Law

Incapability to know the nature of the Act:

To get the defense of this one should prove that Insanity should have affected the cognitive faculty which guide our actions .Insanity not only affects ours cognitive faculty but also affects our emotions which prompt our action , but our Indian Law like the England Law only gives exceptions to those insanity cases which affects only cognitive faculty and in cases like insanity affects the emotions are not considered in the exception because if insanity affects our cognitive faculty person is not capable to control his actions and also do not know the effects of the action.

Incapacity to know right or wrong:

In order to use the defense of Insanity , it is not mandatory that both the requirements mentioned in the section have to be fulfilled , a person if able to know the nature of the act but not able to know what was wrong or contrary to law can still get insanity defense . This ground of exemption is most important in cases where mental disease has caused partial insanity . Situations like delusions eg : a person under delusions but otherwise sane cannot be acquitted on the ground of insanity unless those delusions caused the person to think those things which if existed would have excused his act.

Burden Of Proof in the cases of Insanity

The burden of proving the offence is always on the prosecution, the prosecution has to prove the offence beyond reasonable doubt . But onus of proving the elements mentioned in section 84 of the IPC are on the accused ( section 105 of the Evidence act ).

To claim the defense of insanity defense have to prove that at the time of the occurrence of the incident accused was unsound mind and the rules for burden of proof in cases of insanity are as follows:

1. Prosecution have to be prove beyond the reasonable doubt that the offence was committed by accused with mens rea
2. Insanity is a rebuttable presumption
   
3. The accused can bring oral , circumstantial or documentary evidence to rebut the presumption of sanity at an time and claim defense of section 84 of IPC and accused do not have to prove elements of section 84 IPC beyond reasonable doubt
   
4. Even if accused is not able to establish the ingredients of section 84 as to the acts committed by him but still creates a doubt in the minds of the Court . Then the Court would be entitled to acquit the accused on the ground that the general burden of proof resting on the prosecution was not discharged

Material circumstances which help to draw interference regarding mental condition.

The question whether accused was unsound at the time of occurrence of crime varies from case to case this thing has to be decided by the facts of the case . The circumstances which help to draw the interference regarding mental condition of the accused at the time of the offence are:

1. Motive
2. Preparation
3. Desire for concealment
4. Making statements which are false
5. Conduct before , at the time and after the commission of the offence
6. Conduct after the commission of the offence , showing guilt and trying to avoid detention