Charles Mercier (1852–1919), a contemporary of Dr Henry Maudsley, lectured on insanity at Westminster Hospital and London School of Medicine for Women and later held a post at Charing Cross Hospital in London. He authored several books, whose titles (Astrology in Medicine, Crime and Insanity, A New Logic) attest to the breadth of his interests and knowledge. He wrote for the Journal of Mental Science and was President of the Medico-Psychological Association of Great Britain and Ireland in 1908–1909. Mercier addressed some thorny issues in psychiatry that continue to be controversial to this day: the nature of insanity and the definition of disease.

Insanity, he argued, is a disorder of conduct. The statement does not seem that outlandish when one considers that we base our diagnosis on the patient's behaviour and what he or she says.

His two-part essay ‘What is a Disease?’ was published on both sides of the Atlantic, in England as well as in the USA, in 1917. There is the commonly held notion that diseases exist in nature and that clinicians ‘discover’ them in much the same way as Columbus discovered America; indeed, we sometimes honour the discoverer by naming the disease after the clinician. Disease, Mercier averred, is a ‘mental construct or concept, consisting of a symptom or a group of symptoms, correlated with or by a single intra-corporeal cause’.

As a forensic psychiatrist he was aware of the problem that our inability to define insanity presents in courts of law. With his characteristic sense of humour he suggested that, if counsel were to ask a psychiatrist to define mental disease in the courtroom, the psychiatrist should confound the lawyer by responding with a counterquestion to define law.

Mercier's views of insanity as a conduct disorder and of diseases as mental constructs were controversial then as they are likely to be today. Nevertheless, as the debate about how diseases are defined continues a century after these views were published, his proposed definition may be worth re-examining.

Sir William Osler, in his obituary of Mercier, described him as having a ‘rich vocabulary and a keen wit, he had no equal among us as a controversialist’.