## Facts

- Daniel M’Naghten, experiencing paranoid delusions, fatally shot Edward Drummond, private secretary to Prime Minister Sir Robert Peel, believing political enemies pursued him.
- A jury returned a verdict of not guilty by reason of insanity.
- Widespread public concern led the House of Lords to seek guidance from the common law judges, submitting five questions on the scope of the insanity defence.

## Issues

1. What legal test should govern criminal responsibility when insanity is pleaded?
2. How should “disease of the mind” and the resulting “defect of reason” be defined?
3. Must the mental disorder render the accused unaware of the act’s nature and quality, or of its wrongfulness, to warrant acquittal?

## Decision

- The judges’ responses crystallised into the M’Naghten Rules.
- An accused is excused only if, at the time of the act, he was labouring under a defect of reason caused by a disease of the mind.
- Because of that defect he must either (a) not know the nature and quality of the act, or (b) know the act but not that it was wrong.
- The Rules focus on cognitive impairment; an irresistible impulse alone does not suffice.

## Legal Principles

- “Disease of the mind” is a legal concept encompassing disorders that impair reasoning, later applied to conditions such as arteriosclerosis (R v Kemp) and epilepsy (R v Sullivan).
- “Defect of reason” demands more than transient confusion; brief absent-mindedness, as in R v Clarke, is insufficient.
- Ignorance of “nature and quality” concerns physical awareness of the act (e.g., mistaking strangulation for squeezing a lemon); ignorance of wrongfulness relates to legal knowledge, illustrated by R v Windle.
- The Rules remain the common-law standard, applied in cases like DPP v Beard, though criticised for their exclusive cognitive focus and limited concordance with modern psychiatry.

## Conclusion

The House of Lords established the two-limb M’Naghten test, excusing only those whose disease-based defect of reason left them unaware of the act’s nature, quality, or legal wrongfulness; this framework endures as the core common-law standard for insanity despite sustained criticism.


R v M'Naghten (1843) 10 Cl & Fin 200 (HL)