The remarkable case of Cogden offers a good example of the exculpatory force of sane delusions. Mrs Cogden killed her daughter under the influence of a nightmare that she was being attacked by North Korean soldiers. She was tried before the Supreme Court of Victoria and acquitted. Cogden is usually treated as an early example of sane automatism by sleep-walking. It was not a case of insanity and there was no reason for imposing a special verdict. A case such as Cogden may also be analysed in terms of the defence of mistake of fact. A mistaken belief may negate some fault element. It may also activate a defence (such as self-defence) not otherwise available on the objective facts. Delusions will be exculpatory in the second situation to the extent that the subjective beliefs of the accused are relevant to the particular defence which is raised. This note canvasses the law on the borderline of sanity, where sane delusions shade into insane delusions. Is there a difference between sane and insane delusions? Is there a distinction between the latter and fult blown insanity? Recent cases yield some interesting answers.