The author of an acclaimed new book tells how Hitler used works by psychiatric patients in his culture war

On a winter’s day in 1898, a stocky young man with a handlebar moustache was hurrying along the banks of a canal in Hamburg, north Germany. Franz Karl Bühler was in a panic, fleeing a gang of mysterious agents who had been tormenting him for months. There was only one way to escape, he thought. He must swim for it. So he plunged into the dark water, close to freezing at this time of year, and struck out for the far side. When he was hauled on to the bank, soaked and shivering, it became clear to passersby that there was something odd about the man. There was no sign of his pursuers. He was confused, perhaps insane. So he was taken to the nearby Friedrichsberg “madhouse”, as it was known then, and taken inside. He would remain in the dubious care of the German psychiatric system for the next 42 years, one of hundreds of thousands of patients who lived near-invisible lives behind the asylum walls.

Bühler’s incarceration disturbed him, but it also marked the beginning of a remarkable story, one in which he played a leading role. It reveals the debt art owes to mental illness, and the way that connection was used to wage history’s most destructive culture war.