Born on June 21st, 1852, the son of the Rev Lewis P and Anne Mercier, of French Huguenot descent. On the death of his father, the Mercier family were left in such straitened circumstances that after a few months' education at the Merchant Taylors' School in 1862, Mercier went as a cabin-boy on a voyage to Mogador; he then worked in a city woollen warehouse as a clerk. Rescue came; he entered the London Hospital, distinguished himself as a student, and qualified MRCS at 22, only one year above the minimum age. Whilst acting as a Medical Officer at the Buckinghamshire County Asylum, near Aylesbury, at the City of London Asylum, and at Flower House Private Asylum, Catford, by hard study and with great ability he gradually attained the highest qualifications - in 1904 the FRCP London, and in 1905 the MD of the University of London with the Gold Medal in Mental Science, thus becoming a remarkable combination of psychologist, physician, and logician. In this achievement he was influenced in particular by two teachers - at the London Hospital by Dr Hughlings Jackson; and by the writings of Herbert Spencer founded on Darwin and Evolution. Attention was drawn to Mercier by a continued series of publications beginning with a "Classification of Feelings" in *Mind* (1884, ix, 325, 509) and by his *Text-book of Insanity* (8vo, London, 1902; 3rd ed, 1921). He was appointed Lecturer on Insanity, first at Westminster Hospital, then at Charing Cross Hospital, where he was Physician for Mental Diseases from 1905-1913, and lectured on the subject, 1906-1913. At the London University he served as Examiner; at the Oxford Meeting of the British Medical Association in 1904 he was President of the Section of Psychiatry. As a member of the Departmental Committee on the Treatment of Inebriety he contributed largely to the Report. He represented the Royal College of Physicians of London before the Royal Commission on the Care and Control of the Feeble-minded. For long a member, he also held the office of President of the Medico-Psychological Association of Great Britain and Ireland; he was also a most valuable member of the London Medico-legal Society. It was on the interrelations of insanity and crime that Mercier's genius found its particular expression. His work *Psychology, Normal and Morbid* (8vo, London, 1901) made his name widely known in philosophical circles. He entered upon the discussion of the foundations upon which Criminal Law rests, and was twice awarded the Swiney Prize - in 1909 for his work on *Criminal Responsibility* (8vo, Oxford, 1905), and in 1919 for his *Crime and Criminals* (8vo, London, 1918), his last work. As a dialectician, wit, controversialist, he was ready to argue at Oxford on the uselessness of logic as taught, and to abuse Aristotle as having done irreparable damage to the human mind. In spite of his sheer cleverness in controversy and a caustic pen, he had a warm generous heart. In later years he was the victim of osteitis deformans showing the classical symptoms and signs. He suffered very great pain, sank four inches in height, had an enlarged head bent forwards and bowed legs. It in no way impaired his mentality; indeed, the disease came to a standstill. His general health enabled him to recover after an operation for gangrenous appendicitis. All this came upon him after a great loss - the death of his wife. Never complaining, he worked to the end, as he promised, "with all flags flying like Barère's 'Vengeur'." He died at Moorcroft, Parkstone, Dorsetshire, on September 2nd, 1919. Publications:- *Lunatic Asylums: their Organisation and Management*, 1894. *A New Logic*, 8vo, London, 1914. *Astrology and Medicine: Fitzpatrick Lectures*, 1913, 8vo, London, 1914. *Leper Houses and Mediaeval Hospitals: Fitzpatrick Lectures*, 1914, 8vo, London, 1915. *On Causation, with a Chapter on Belief*, 8vo, London, 1916. *Spiritualism and Sir Oliver Lodge*, 12mo. London, 1917. *The Principles of Rational Education*, 8vo, London, 1917. *Human Temperaments. - Studies in Character*, 12mo, London, 1916; 2nd ed, 1917. Editor, with Preface, of Lépine's *Mental Disorders of War*, 1919.