Medical doctor George Cheyne, little known today, was among the most quoted men in eighteenth-century Britain. A 450-pound behemoth renowned for his Falstaffian appetites, he nevertheless advocated moderation to his neurotic clientele. Cheyne was an early admirer of Isaac Newton and a writer on mathematics and natural philosophy, yet he also linked science and mysticism in his writings. This inventor of the all-lettuce diet was both an author of learned tomes and, to his patients, a fellow sufferer who struggled with obesity and depression.

Scientist and mystic, patient and healer,...