The McNaughton rules for determining whether a person can be successfully defended on the grounds of mental incompetence were determined by a committee of the House of Lords in 1843. They arose as a consequence of the trial of Daniel McNaughton for the killing of Prime Minister Sir Robert Peel’s secretary. In retrospect it is clear that McNaughton suffered from schizophrenia. The successful defence of McNaughton on the grounds of mental incompetence by his advocate Sir Alexander Cockburn involved a profound (...) shift in the criteria for such a defence, and was largely based on the then recently published scientific thesis of the great US psychiatrist Isaac Ray, entitled A Treatise on the Medical Jurisprudence of Insanity. Subsequent discussion of this defence in the House of Lords led to the McNaughton rules, still the basis of the defence of mental incompetence in the courts of much of the English-speaking world. This essay considers one of these rules in the light of the discoveries of cognitive neuroscience made during the 160 years since Ray’s treatise. A major consideration is the relationship between the power of self-control and irresistible impulse as conceived by Cockburn on the one hand, and by cognitive neuroscience on the other. The essay concludes with an analysis of the notion of free will and of the extent to which a subject can exert restraint in the absence of particular synaptic connections in the brain. (shrink)