A sliding-doors moment shaped his origin, as his father declined a science post aboard the HMS Beagle and became a Canadian frontier clergyman. Born in Ontario in 1849, William Osler trained for the priesthood before the writings of Sir Thomas Browne convinced him that studying the body was itself a form of reverence. He earned his medical degree at McGill in 1872 and carried that redirected intensity into science.

As the first physician-in-chief at Johns Hopkins in 1889, he pulled students to the bedside with the clinical clerkship and built the first medical residency, while his 1892 textbook reorganized medicine around physiological systems. Yet the same man ran an elaborate prankster alter ego, joked about euthanizing the elderly, and held deeply exclusionary racial views. He died in 1919 of pneumonia, the disease he had called the friend of the aged.

• The Beagle invitation his father turned down and the faith that shaped him
• The clinical clerkship and the punishing origins of the residency
• His landmark textbook and the treatments still trapped in the past
• The Egerton Yorick Davis hoaxes and the fixed-period controversy
• His prejudices, his son’s death at Passchendaele, and a poetic end