Insanity is a legal term denoting that the individual is so confused and deranged as a result of mental illness that he is not legally result of mental illness that he is not legally responsible for his actions. Mental health professionals used the term “abnormality” instead of insanity, and they used it in a much broader sense then it is used it in a much broader sense than it is used by lawyers and refer to almost anything from simple anxiety to dementia (which is interpreted as synonymous with gross mental deterioration). The medical science relies on the illness perspective, in which the idea is to classify all known mental diseases in a logical and consistent manner. The law is concerned with the question of responsibility rather than the type of distress. The prevailing test of law is not “Is this a paranoid or mania?” but, “Did the accused know from wrong?” Another test is whether a person who knows that the act is wrong, but does it anyway compulsively, on an “irresistible impulse.” 
I. Development of Law relating to Insanity
The earliest case on law of insanity is of R v Arnold  Edward Arnold was tried for wounding and making an attempt on the life of Lord Oslow. There was enough evidence of the mental derangement of the accused. Tracy J in directing the jury made the following observation:
If he was under the visitation of God and could not distinguish between good and evil and did not know what he did, though he committed the greatest offence, yet he could not be guilty of any offence against any law whatsoever. On the other side we must have cautious it is not every kind of frantic humor, or something unaccountable in a man’s action, that points him out to be such a mad man as is exempted from punishment; it must be a man that is totally deprived of his understanding and memory, and doth not know what he is doing, no more than an infant, than a brute or wild beast. Such a one is never the object of punishment.
1. The M’Naughten Rule
The trial of M’Naghten and his acquittal caused considerable sensation and was made the subject of debate in the House of Lords and as result the House of Lords called on the fifteen judges to lay down a law on the subject of criminal responsibility in cases of alleged lunacy in answers to questions propounded by them. This course appears to have taken with a view to some legislation then contemplated on which actions seems to have been taken. Fourteen of the judges united in their answers. Maule J. returned separate answers which, however did not materially differ from his colleagues. The opinion of the majority was delivered by Tindal C.J. These questions and answers are known as the M’Naghten Rules which form the basis of the modern law on insanity.
1. That every man is presumed to be sane and to possess a sufficient degree of reason to be responsible for his crimes, until the contrary be proved to the satisfaction of the jury or the court.
2. To establish defence of ground of insanity it must be clearly shown that at the time of committing the act, the accused was labouring under such a defect of reason from disease of mind, as not to know the nature and quality of the act he was doing, or he did not know it, that he did not know that what he was doing was wrong.
3. If the accused was conscious that the act was one which he ought not to do and if that act was at the same time contrary to the law of the land, he is punishable.
4. A medical witness who has not seen the accused before trial should not be asked whether on evidence he thinks that the accused was insane.
5. Where the Criminal Act is committed by a man under some insane delusion as to the surrounding facts, which conceals from him the true nature of the act he is doing, he will be under the same degree of responsibility as he would have been on the fact as he imagined them to be.
II. Law of Insanity in India
The defence of Insanity in criminal cases is to be found in section 84 of the Indian Penal Code, 1860, which is reproduced below:
“Nothing is an offence which is done by a person who at the time of doing it, by reason of unsoundness of mind, is in capable of knowing the nature of the act, or that he is doing what is either wrong or contrary to law.”
This section embodies a fundamental maxim of criminal jurisprudence, viz. that an act does not constitute a crime unless it is done with a guilty intention. In order to constitute a crime, the intent and the act must concur. The section fastens no culpability on insane persons because they can have no rational thinking or the necessary guilty intent  .
1. Semantic Selection
Section 84 uses the expression “unsoundness of mind” to include “insanity”, “madness” or “lunacy” for the definition of each of these may differ in degree and kind. These terms are often used synonymously, but to precise definitions are used to come by. Whether the “unsoundness of mind” is temporary or permanent, natural or supervening, whether it arises from disease, or exists from the time of birth, it is included in this expression. Thus an idiot, a person non compos mentis by sickness, a lunatic who had lucid intervals of reason, a person naturally mad and/ or delirious and one whose reason is clouded by alcohol, all are persons of “unsound mind”, provided that their unsoundness makes them oblivious to the nature and criminality of an act: their unsoundness must reach that degree which the latter part of this section requires.
2. Test of Insanity
Section 84 embodies two mental conditions which exempt a man from responsibility for his wrongful act, namely,
1) That his unsoundness of mind was such that he was “incapable of knowing the nature of the act,” or
2) That it had precluded him from understanding that the act he was doing was wrongful  .
Of these the first seem to refer to the offender’s consciousness of the bearing of his act on those who are affected by it, the second to his consciousness of its relation to himself. These two elements need not be simultaneously present in each case, nor indeed, are they invariably so present. The absence of both or either relieves the offender from liability to punishment. Situations like automatism, mistake and simple ignorance such as can occur only in gross confused state are covered by the first category , whereas the second category embraces cases where mental disease has only partially extinguished reason  .
3. Nature of Unsoundness of Mind
What should be the nature of unsoundness of mind in order to attract exemption? Every form of mental abnormality or derangement is not immune from criminal responsibility. So is every mental aberration or deviation from normal conduct. In order to get an exemption from liability the insanity must be out of a particular and appropriate kind. There are a few deciding factor.
4. Degrees of Insanity
It may be said that between the normal and the abnormal there is only difference of degrees but not of kind. The mind may be unsound, affected by disease, disorderly or disturbed or abnormal. These factors must be of such degree, which renders the accused capable of knowing the nature of his act or that what he is doing is either wrong or contrary to law. It should be obliterate the perceptional or volitional capacity  .
In Hazara Singh v. The State  the Punjab High Court said:
“In order to earn immunity from criminal liability the disease, disorder or disturbances of mind must of degree, which should obliterate perceptual or volitional capacity. A person may be a fit subject for confinement in a mental hospital, but that fact alone will not permit him to enjoy exemption from punishment. Crotchetiness of cranks, feeble mindedness, any mental irresponsibility , mere frenzy , emotional imbalance , heat of passion , uncontrollable anger or jealously , fits of insensate hatred , or revenge , moral depravity , dethroning , reason , incurable perversions , hypersensitive excitability , ungovernable fits of temper, stupidity , obtuseness, lack of self-control, gross eccentricity and idiosyncrasy and other similar manifestations, evidencing derangement of mental functions, by themselves, do not offer relief from criminal responsibility”.
5. Insanity Medical and Legal Insanity
Every mentally diseased person is not ipso facto exempted from criminal liability. This is so because according to the courts, the legal definition of insanity differs considerably from medical definition. Medical insanity and Legal insanity differ in degree and standards. From the medical points of view, it is probably correct to say that every man at the time of the committing the criminal act is insane. He is insane in the sense that he is not in a sound, healthy and normal condition and therefore a needs treatment .But from the legal point of view, so long as he is able to distinguish between right and wrong and to know that the act done by him is wrong or contrary to law, a man a must be held to be sane. There are following factors for making the distinction  .
A. Difference in Degree
In medical parlance, ‘unsoundness of mind’ would admit of a variety of conditions of varying degree of severity. It is said that these conditions manifest far too many characteristics to justify any precise definition of applicable to all cases. For the sake of precision and certainty law exempts from criminal responsibility only that ‘unsoundness of mind’ which materially affects the cognitive faculties of mind. Persons whom medical science would pronounce as insane do not necessarily takes leave of their, emotions and feeling, like fear, frustration, ambition and revenge. Fear and threat may have a deterrent influence on them. Though insane, in one sense such persons would refrain from committing any acts of violence or mischief if more powerful men are present at the scene. As been pointed out early, mad men may not have yielded to their insanity if a policeman had been their elbow. One is insane in legal sense only if one could have still yielded to his not insanity in such circumstances as one is not aware of one’s act and its consequences. The degree of unsoundness of mind for legal insanity is their higher than of medical insanity .
B. Time Factor
In the case of legal insanity, the mental condition referred to it in the Code must be established to have existed at the time when the act was committed. If a man is found to be insane before or after the commission of the offence it raises no presumption that he was of sound mind at the time of the commission of offence. All the same the state of mind of the accused before or after the crucial time may become relevant in as much as to the fact whether the accused was in such a state of minds as to be entitled to exemption can only be established from circumstances which preceded, attended and followed the crime. It means that for the purposes of legal insanity. Unsoundness of mind of an accused at the time other than at the commission is only relevant to prove the state of the mind of the accused at the time of the commission of the crime whereas , to medical insanity , that may be determinative in the sense that a man may be medically insane at any time  .
C. Proof of legal Insanity
For the purpose of legal insanity the degree of proof required is also greater than that required for proving medical insanity. A court will look for some clear and distinct proof of mental delusion or intellectual aberration existing immediately before, or at the time of, or immediately after the perpetration of the offence. Medical men recognize that there may be delusion or aberration, springing up in the mind suddenly, and not revealed by the previous conduct or conversation of the accused. Thus the criteria deployed by the medical men to detect insanity are different from those employed by the courts.
Medical men in far insanity also from the absence of certain factors as motive, attempt at concealment or escape and accomplice. The fact that the person was conscious of the criminality of the act is immaterial for establishing medical insanity. The legal criteria for the existence of insanity are the act of the person and his consciousness of its criminality. To a lawyer insanity is ‘conduct of a certain character’ whereas as physician it means ‘a certain disease one of the effects of which is to produce such a conduct’. To men of medicine and psychiatry as to men of law motive is not decisive in determining insanity  .
In Kalicharanv. Emperor  the appellant, who had some ill-feeling towards his wife struck four persons to death including, his wife, a boy and a two months old kid. He also injured two other persons in an atrocious manner. One of his victims he inflicted not less than 13 injuries. He used three weapons altogether to commit these multiple murders. In the trial court he said he remembered striking only his first two victims his wife and his brother- in- law’s son. He did not take the plea of insanity in the trial court. No family history was disclosed there. Such a plea was taken in the High court for the first time. The High Court observed that a crime is not excused for his own atrocity. The court has to look outside the act itself for evidences as to how much the accused knew about it. Since these factors were found against the accused he was held guilty of the crime charged. Distinguishing between ‘legal’ and ‘medical insanity’ the court pointed out that exemption is applicable only to the former cases where the cognitive faculties of the accused are completely impaired making the offender incapable of knowing the nature of the act or that he is doing what is wrong or contrary to law. It could not be extended to cases where the accused acts without any motive and under sudden and overpowering impulse.
“According to medical sciences insanity is another name for mental abnormality due to various causes and existing in various degrees. Even an uncontrollable impulse driving to kill or wound comes within scope. But a man whom the mental science would pronounce as insane does not necessarily take leave of his emotions and feelings. Hope, ambition, revenge, etc., may still govern his mind. Fear may have exercised its influence over him, and threats may have a deterrent effect.
-  Subash Chandra Singh “Insanity Defence: the Psychiatry Neglected”99 Criminal Law Journal 65 (1993).
-  (1724) 16 St. Tr. 695.
-  G Sadasivan Nair “Defence of Insanity: Need for Reform” 12 Cochin University Law Review 146 (1988).
-  Pancha v Emperor AIR 1932 All. 233.
-  K.M Sharma “Defence of Insanity in Indian Criminal Law” 337The Journal of the Indian Law Institute (1965).
-  Supra, 337.
-  AIR 1958 Punj 104.
-  G Sadasivan Nair “Defence of Insanity: Need for Reform” 12 Cochin University Law Review 147 (1988).
-  Ibid.
-  G Sadasivan Nair “Defence of Insanity: Need for Reform” 12 Cochin University Law Review 148 (1988).
-  Supra, 149.
-  AIR 1948 Nag 20.