Insanity is a defence which has three elements: M’Naghten  UKHL J16
- At the time of the commission of the offence, the defendant was suffering from a ‘defect of reason’;
- This defect of reason was due to a disease of the mind; and
- The defect rendered the defendant unaware of act’s nature and quality or made him unaware the act was ‘wrong’.
A ‘Disease of the Mind’
Diseases of the mind do not necessarily line up with what medical professionals would call mental illness. It covers any internal medical condition which produces defects of reason, including from physical illnesses: R v Sullivan  AC 156; R v Kemp  1 QB 399. External causes, by contrast, are excluded and may only be subject to the defence of automatism.
Diabetes: Note that hypoglycemia (too little sugar from an insulin overdose) is an external cause. Hyperglycemia (too much sugar from taking insufficient insulin) is an internal cause. Contrast R v Hennessy  1 WLR 287 with R v Quick  3 WLR 26.
Nature and Quality of the Act
This limb of the third requirement of the insanity defence applies where the defect of reason leads the defendant to not realise what he is physically doing: R v Sullivan  AC 156. For example, if the defendant hits the victim thinking he is knocking a vase over, this limb would apply.
This limb of the third requirement of the insanity defence applies only where the defendant believed he was acting legally: R v Johnson  EWCA Crim 1978. This includes cases where the defendant falsely believed he had a defence (e.g. self-defence) because of a delusion: M’Naghten  UKHL J16. If the defendant knew his acts were illegal, however, his appreciation of morality is irrelevant. This is true even if he was acting in accordance with an ‘irresistible impulse’: R v Kopsch (1925) 19 Cr App R 50.
Insanity, Negligence and Strict Liability
The defence of insanity is available even if the offence is one of negligence or strict liability: Loake v CPS  EWHC 2855.
The Effect of Establishing Insanity
If the defence of insanity is established, the defendant is found not guilty by reason of insanity. While they are considered not guilty, they may still be subjected to incarceration under a hospital order: R v Birch  11 Cr App R (S) 202.