Fast-paced developments in psychiatry, neuroscience and emerging neuroimaging technologies place continual pressure on the legal recognition of mental illness and disease across jurisdictional boundaries. Nevertheless, the Canadian legal definition of exculpatory mental disease in the context of criminal liability has remained largely static, sheltered from the immediate influence of medical theory and advancements. In order to effectively reflect on the intersection of mental health and criminal justice systems in this area, it is important to understand its historical development and the English common law origins of the current approach. Published in the International Journal of Law and Psychiatry, Volume 35, No. 2.