‘Let’s examine the evidence, shall we?’ He retrieved Vidor’s file from his desk and flipped it open. ‘Periods of amnesia, and what you’ve referred to as sleepwalking, may not have been sleepwalking at all, but your other personality controlling your nocturnal habits. For example,’ Gessen consulted the yellow pad, though it wasn’t necessary. Such was the unusual nature of Vidor’s case that every detail was etched in his mind. ‘You’ve told me that there were a couple of occasions when, having gone to bed early and slept deeply, you entered the kitchen in the morning to find a half-drunk cup of cocoa on the counter, or an empty packet of biscuits. Yet, you’ve previously claimed not to have a sweet tooth and mentioned your dislike of cocoa. Perhaps, after considering the possibilities, you decided it was a simple matter of sleepwalking. A perfectly plausible explanation why you couldn’t remember these activities when you woke the next morning.’

He flipped to another page in the file. ‘A regression to a childlike state during times of stress could explain your hankering after sweets, but I believe my explanation is a better fit. In which case, you didn’t go to sleep at nine o’clock at all. At that time your other personality would emerge. This other personality, more nocturnal than you, as Vidor, would make hot cocoa, eat a packet of biscuits, watch television, and perhaps go for a late-night stroll through the neighbourhood, before going to bed at one or two in the morning.

‘It would explain why you might have felt tired the next day, even after an early bedtime, or discovered mud on your shoes when you remembered cleaning them before turning in. “Vidor” is a man of regular habits, who keeps a tidy home and eats dinner at seven and goes to bed at nine-thirty, while your alter is more slovenly. Perhaps, we might even call him angry or rebellious. He prowls the house at night or wanders the neighbourhood. Tracks mud into the front hall, leaves the television on and indulges in sweets, while Vidor has been cautioned by his doctor to watch his sugar intake.’ Gessen waited half a beat. The air in the room had grown stale. ‘How would you explain it?’

Vidor’s mouth flattened into a thin line. ‘What you’re saying is preposterous. I am not two people. I am one person. Professor Vidor Kiraly, chair of the Neurobiology department at Cambridge University. Who happens, I readily admit, to suffer from bouts of tiredness and mild anxiety from time to time. Who occasionally sleepwalks at night. It’s a known phenomenon, often exacerbated by stress, as I’m sure I don’t need to tell you.’