As with many nightmares, Mary Arnold-Forster was being chased. She seemed to be in London around the First World War, and she had somehow become embroiled in dangerous espionage.

“I had succeeded in tracing the existence of a complicated and dangerous plot against our country,” she noted in her diary. “The conspirators had turned upon me on discovering how much I knew.” Eventually she found shelter, but they were closing in. “The arch-conspirator, a white-faced man in a bowler hat, had tracked me down to the building where I was concealed, and which by this time was surrounded.”

At which point, many of us might have woken up in a cold sweat. But Arnold-Forster was made of steelier stuff. She had discovered a method of “dream control”, meaning that she was perfectly aware that she was asleep, and that everything around her – the pursuer, his bowler hat, the very ground she was standing on – was simply a figment of her mind.