Dr Memon rightly observes the typographical error in relation to the inception of the M'Naghten rules.

The M'Naghten rules are a unique source of English common law, the relevant section in this discussion being:

The courts have been consistent towards the two ‘knowledge’ requirements, namely ‘nature and quality’ and ‘wrong’. Memon's criticism of Mackay's statement in our article seems to miss the point. Reference MackayMackay (1995) stresses that the courts adopt an extremely narrow cognitive and legal approach to the interpretation of the M'Naghten rules. The principal impact of the judgment in R v Cordere(1916) is that the Court of Appeal refused to draw any distinction between ‘nature’ and ‘quality’, holding that the phrase referred to the physical character of the act and not its moral quality. Furthermore, in R v Windle [1952] the same court held that the meaning of the word ‘wrong’ was restricted to legal wrong rather than the broader concept of moral wrong. Mackay considers this narrow cognitive approach to signify an attempt by the courts to ensure that the rules are applied only in extreme cases of mental disturbance.

Of greater importance is how psychiatrists interpret the M'Naghten rules, and this is perhaps best exemplified by considering the ‘wrongness’ limb of the rules. Previous research of psychiatric reports indicates that psychiatrists interpret the wrongness issue in a liberal fashion by often considering elements of moral justification rather than applying the strict legal criteria mentioned above (Mackay & Reference Mackay and KearnsKearns, 1999). This approach has not been entirely rejected by the courts and we should perhaps welcome psychiatry's common-sense approach to the M'Naghten rules.

Finally, we are unsure whether Memon requires us to classify mental disorders according to whether they fit the M'Naghten rules. This approach has limitations. First, it assumes that the legal concepts of ‘disease of the mind’ and ‘defect of reason’ are congruent with medical classification. They are clearly not. Two well-known House of Lords’ decisions, both of which concern offenders with epilepsy ( Bratty v Attorney-General for Northern Ireland [1963], R v Sullivan [1983]), make it clear that the concept of ‘disease of the mind’ is a broad one, capable of encompassing all forms of mental impairment that give rise to a defect of reason. Second, it risks a preconceived functionalist division of mental disorders into those that receive a status that deserves exculpation from criminal responsibility and those that do not. Although it is true that the law has largely considered personality disorders to fall outside the meaning of ‘disease of the mind’, the courts are less rigid about this. In fact, successful insanity defences have been held based on a diagnosis of personality disorder in both England and the USA (Reference BuchananBuchanan, 2000: pp. 84–107). Furthermore, the American courts have rejected an approach that avails the insanity defence to one of a number of diagnostic categories.

In summary, the English judiciary has a broad approach towards the scope of the term ‘disease of the mind’, but this is counterbalanced by the narrow interpretation given to the knowledge elements mentioned above. It is therefore not just the ‘psychopath’ who struggles to establish a successful insanity defence.