R v. M’ Naghten

Read this analysis of the landmark judgement which led to the establishment of the M'Naghten rules for the plea of insanity as defence which have been embodied in the Indian Penal Code as well.
CITATION(1843) 8 ER 718
COURTHouse of Lords


Actus reus and mens rea are the two main constituents of a crime. A criminal act committed with a criminal intent will result in the occurrence of a crime. Criminal act is the actus reus and the criminal intent is the mens rea. It is based on the legal maxim ‘actus non facitreum nisi mens sit rea’, which means that no person can be held guilty without a guilty intention. Thus, a person cannot be held liable for a crime that he has committed without anticipating or understanding the consequences of it. A person who is not able to foresee the after effects of his act is said to be an insane person. An insane person cannot be punished because he does not possess the requisite mens rea or the guilty mind. Insanity is the conventional defense used in the trail for criminal cases and it has come into existence through this particular case.


The facts of the case are as follows: In the present matter, the defendant M’Nagthen was accused for the murder of Edward Drummond who was the Secretary to the Prime Minister. The defendant has misunderstood the Secretary as the Prime Minister and had shot him down. He used the defense of insanity to escape from the punishment during the trail. During the trail many witnesses were brought before the Court as evidence to prove that M’Nagthen was suffering from a mental condition called morbid delusions. As per this medical condition, the person committing any act would be unaware of what is right and wrong and would be out of his senses. During the trail it was proved that M’Nagthen was suffering from acute insanity and that he was obsessed with the fear of being killed by the Prime Minister, Robert Peel.


The main issue in the case were:

  1. How does the Court analyze the state of mind of a person as to whether he was insane or not while committing the crime;
  2. What are the criteria to be followed by the jury when insanity is used as a defense;
  3. Whether a person can be excused for the commission of a crime if he was under the influence of mental delusions;
  4. Whether there is any suitable standard that is set up by the jury to determine the legally pardonable insanity.
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Summary of court decision and judgment

In the present matter, the counsel for the defendant brought witnesses to Court who testified regarding the obsession and delusion of the defendant. The Court was able to reach a conclusion that the defendant was suffering from acute mental insanity as a result of which he could not apprehend the cognizance of the act that he has committed. The jury recognized the defense of insanity as a ground of acquittal of the defendant. 

Further, Lord Chief Justice Tindal stated that “the question to be determined is, whether at the time the act in question was committed, the prisoner had or had not the use of his understanding, so as to know that he was doing a wrong or wicked act. If the jurors should be of opinion that the prisoner was not sensible, at the time he committed it, that he was violating the laws both of God and man, then he would be entitled to a verdict in his favor: but if, on the contrary, they were of opinion that when he committed the act he was in a sound state of mind, then their verdict must be against him.”

After the trial, there were public outrages against the judgment and a result of this a meeting was held at the House of Lords which was attended by fifteen eminent judges who were asked to define the standards of insanity. It was decided that M’Nagthen rules would be set up as a measure to evaluate the canons of insanity that led somebody to the commission of a crime so as to prevent any sort of chaos or confusion in the future.


The Court observed that a person can be punished for the commission of an offence only if he is capable of knowing that he had committed an act against law and that he would be punished for the same. It stated that every person possesses some kind of sanity which makes him realize the consequences of his act. Unless and until a person knowingly commits an act with a criminal intent, he cannot be proved guilty before the Court of law. In order to claim insanity as a defense, it must be shown to the Court that he lacks mental stability that is needed to evaluate what is right and wrong. The same has to be substantiated through evidences as well. The Court should be satisfied that the person while committing the crime was under the influence of some mental disease that restricted his scope of thinking and that he was merely carried away by delusions of mind. If the Court has any doubt regarding the medical condition of the person, then it can use expert medical opinion to understand his state of mind. 

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The M’Nagthen Rules: The M’Nagthen Rule says that if an accused has to get an exemption from criminal liability on the ground of insanity, then it must be proved that due to the nature of his mind, he was neither able to understand the nature and quality of his act nor knew what he was doing is wrong. But it is difficult for the Court to analyze the terms and kinds of mental delusions. If any person who is a threat to public safety and he commits a crime, can he be set free on the defense of mental insanity? If a person has some kind of temporary illness, it is difficult for the Court to study his behavioural pattern to see if he was really suffering from delusions or not. There are chances that people may misuse this defense to commit crimes and to get away with it.  This can sometimes help a person with a minor delusion or insanity to commit a big offence without him being punished. It is not clear as to how to deal with those people who commit an act knowing that it is against law but is uncontrollable due to their mental nature.

Indian Penal Code and M’Nagthen Rules: The law relating to insanity has been codified under section 84 of the Indian Penal Code. Section 84 states, ‘Acts 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’

This section has been formulated on the basis of M’Nagthen rules of England. Instead of the term insanity, Indian law uses a more coherent term ‘unsoundness of mind’. The essential features of section 84 are[1]:

  • An act must be done by an insane person.
  • The act must have resulted in some kind of harm.
  • The person doing committing the act must be suffering from insanity, mental abnormality or unsoundness of mind at that time.
  • The accused person should not be capable of understanding the consequences of his act because of insanity, mental abnormality or unsoundness of mind.
Also Read  Hari Ram & Anr. v. State of Haryana & Ors.

If the above conditions are satisfied, then the defendant can claim for the defense of insanity.


It took time for the world to understand and accept mental insanity as a ground for defense in a criminal trial. But now M’Nagthen rules is a recognized exception in all the common law countries. It sets a common standard against which criminal liability can be judged. A person who is not aware of right or wrong cannot be made punishable as he lacks the criminal intend to do something against the law and hence cannot be proved guilty before the law.

[1] Ratanlal and Dhirajlal, Commentary on Indian Penal Code, 33rd Ed, Lexis Nexis, Gurgaon (2016).