‘In my second year of medical school,’ Gessen said, as he topped up their glasses of peppermint tea, ‘an interesting case, quite rare, was admitted to the Salpêtrière hospital. I didn’t meet the patient myself, but his malady was described in detail during one of our lectures. The boy, just shy of his eighteenth birthday, had been suffering from stomach pains, and presented with a distended abdomen and significant weight loss. A CAT scan showed a mass growing in the boy’s abdomen, just under the liver. His doctors thought it was a tumour, at first, but when they opened him up to remove it, they were shocked to discover that it was no ordinary tumour. The lump of tissue contained a spine and a rudimentary brain. Even vestigial fingers and eyes.’ He studied Vidor’s face for signs of squeamishness. But he didn’t look uncomfortable, merely bored, as if counting the minutes until he could return to his room.

‘What they found in the boy’s body is called a foetus in fetu,’ Gessen said, taking a sip of his tea. ‘A parasitic twin, in common parlance. It’s exceedingly rare. Worldwide, there have been fewer than two hundred recorded cases. The medical staff at the hospital were buzzing with excitement, as if an alien creature was growing inside the boy, though the reality was more prosaic.’ He waited to see if Vidor would jump in with an explanation of his own. ‘As it turned out, the boy’s mother had been pregnant with twins, one of whom failed to develop. As the surviving twin grew, it enveloped the vestigial twin, which couldn’t survive on its own without a functioning brain. But it continued to thrive, as a type of parasite, by attaching itself to the blood supply of the living twin.

‘The boy, who was otherwise perfectly fit, complained often of disturbed sleep due to regularly occurring dreams in which he and another boy were trying to solve a complex puzzle. The boy knew that his companion in the dream was his twin. He even asked his mother if she’d given birth to two boys, and if one of them had died, because he could sense the spirit of this ghost twin inside him. She reassured him there had only been one child. But the boy was haunted by the sensation, both mental and physical, that he wasn’t alone. That someone – or something – shared his thoughts. A psychiatrist diagnosed the boy as schizophrenic. But medication didn’t help, and he continued to feel dogged by this other presence.’

A log in the fireplace collapsed into coals. The room had grown overly warm and Gessen considered opening a window. Vidor absently studied the skin on his palm. A picture of boredom.