There is a rule of law still extant that can be indirectly attributed to Robert Peel. In 1843 he was enjoying a second term as Prime Minister. On Friday 20 January a young Glaswegian woodturner called Daniel M'Naghten (variously spelt McNaughton or McNaghten) shot Edward Drummond, the Prime Minister's secretary, at point-blank range in the back of the head. The wound was mortal, ensuring Drummond a lingering death. Had it not been for the intervention of one of Peel's new police constables – James Silver of A Division – M'Naghten would have fired again and finished off his victim on the spot. A clearer case of premeditated murder could hardly be found, the intended victim was no less than the Prime Minister. Yet the jury, with the active encouragement of the judge, acquitted M'Naghten on the basis that he was insane and so not guilty of the offence.

The issue of insanity as a defence to felony had always been left to the discretion of the trial judge and to the good sense of the jury. No statute had been enacted laying down the law; no considered judgment of senior members of the judiciary sitting in banc had determined it; no binding precedent had been established. The formula usually applied was that of the trial judge in Edward Arnold's Case of 1724. 2 ‘Crazy Ned’ had shot and wounded Lord Onlsow. The judge directed the jury on the test for insanity in the following terms: ‘whether the accused is totally deprived of his understanding and memory and knew that he was doing no more than a wild beast or a brute, or an infant’.