Sinclair closed the clasps on his briefcase with a click. ‘Well, it has been argued that patients with psychotic illness are simply at one end of a spectrum. Some practitioners say that psychosis is merely another way of constructing reality, and not necessarily a sign of illness at all. Only a small proportion of people who experience hallucinations are actually troubled by them.’

The psychiatrist smiled past Cooper, who turned to see Gavin Murfin hovering near the desk.

‘Well, I suppose police officers have to be feet-on-the-ground sort of people. No time for the imagination, eh? But most hallucinations are quite neutral, you know. They might simply take the form of a voice commenting on what you’re doing. People get used to that. In fact, for some individuals it’s rather comforting, as if they have a permanent, invisible companion, perhaps a loved one who’s died. Believe me, it’s not uncommon.’

‘I can imagine that.’

‘Well, now – instead of something neutral or reassuring, think of a voice that constantly makes negative comments about you and your actions. That’s the troubling sort of hallucination experienced by people in clinical groups, who need psychiatric help to deal with them. Treatment isn’t necessary unless the experience is disturbing.’

They walked down the corridor towards the stairs, passing the busy incident room on their way towards the reception area. By the front desk, the psychiatrist paused to shake hands. He held Cooper’s hand a little too long, giving him an appraising stare.

‘You know, many hallucinations occur in the phases of consciousness between waking and sleeping. You might hear your name being spoken, or get a feeling of someone being in the room, or of falling into an abyss. All of those are common. There’s a sort of paralysis that occurs in the hypnopompic stage just before waking – a sensation like a heavy weight on your chest that prevents you from moving or calling out for help.’

‘Oh, yes?’ said Cooper, wondering what he might be giving away by his expression.

Sinclair smiled reassuringly. ‘Generally, people write those experiences off as dreams, DC Cooper. They wake up. And then they get on with their lives.’

Cooper walked slowly back to the CID room, and found Murfin already at his desk, watching Fry heading towards him.

‘You know this voice that constantly makes negative comments about me and my actions,’ said Murfin.

Cooper looked up in surprise. ‘Yes, Gavin?’

‘It’s a relief to know it’s only a hallucination. I thought it was my DS.’