Even the events leading up to William’s amnesia are highly puzzling. At the time of the surgery, the dentist didn’t realise that anything was wrong; it was only after they asked him to take off his protective dark glasses that they saw he was pale, and struggled to stand up. They called his wife. “He was lying on a couch,” Samantha recalls. (Both their names have been altered to preserve the family's privacy). “His eyes were staring; he looked surprised to see me – he had no clue what was going on.” By 5pm he was removed to hospital, where he stayed for three days. Even after some of the mental fog dissipated, he was still unable to recall anything for more than a few minutes.

The doctors initially suspected that he’d reacted badly to the anaesthetic, causing a brain haemorrhage – yet they failed to find evidence of injury. So he was discharged with the mystery still hanging over him, and the family moved back to England, where he was referred to the office of Gerald Burgess, a clinical psychologist in Leicester.

The brain’s printing press

The obvious explanation would have been that William had a form of “anterograde amnesia”, like Henry Molaison, whose experiences have informed much of what we know about memory. During brain surgery to treat his epilepsy, surgeons sliced out a large chunk of Molaison’s grey matter, including his hippocampi – seahorse-shaped regions at the centre of the brain. They act as our memory’s printing press, stamping the “episodic memories” of events into long-term storage – and without them, Molaison was unable to hold on to anything that occurred after the surgery.