It was a quote Gessen vaguely remembered from their student days. Very apropos, and so like Bertrand to have a suggestion at the ready.

‘The thing is,’ he said, trying to put his thoughts into words, ‘I keep having the sense that I’ve met the patient before.’ His attention was distracted by the sound of laughter from the student table. ‘He grew up in Paris, and it isn’t farfetched to imagine I might have bumped into him somewhere, perhaps in one of the cafes by the Sorbonne – even this one – though he claims it’s unlikely. He says he left for Cambridge about the same time I arrived in the city.’

‘Someone you knew?’ Bertrand signalled the waiter for another round of espressos. ‘How strange the wheels of fate. Fontaine would have been pleased.’

‘That he grew up in Paris and left for England at eighteen appears to be the case.’ Gessen flicked a crumb from the table. ‘But he has this odd tic. Once or twice, while we were in session and I was pushing him to tell me about his father, he reached up absently and flicked his ear, like this.’ Gessen demonstrated. ‘While I was on the train, I was thinking about my first weeks at the Sorbonne and I remembered this student in one of my philosophy lectures who always sat at the back of the room. Once, I came in late and took a seat behind him. I noticed he wasn’t taking any notes, but when another student interrupted the lecturer and launched into a heated argument his shoulders tensed, and he flicked his ear. Exactly like my patient.’

‘The plot thickens.’ Bertrand’s face grew animated. ‘The Case of the Nervous Tic. Perhaps your patient has a doppelgänger. The mysterious double we’re all supposed to have, somewhere. Or, maybe it’s an Ankou.’

‘On cue?’ Once again, Bertrand had lost him.

‘In Breton mythology, the Ankou is the servant of Death. Or henchman, if you prefer, and is sometimes described as the personification of another’s soul. A skeletal or shadowy figure who assists in the collection of the dead.’ Bertrand paused to sip his coffee. ‘For the one who hears it, the cart’s squeaking wheel is thought to be a harbinger of death.’

Gessen felt a chill. ‘I’m fairly sure my patient is alive.’ He smiled at his friend.

Bertrand’s gaze grew filmy as if he were receding into the past, then snapped back as he cocked his head to eavesdrop on the two American tourists complaining about the rudeness of the French. ‘What makes you think that boy and your patient are the same person?’