He cautioned me to stay inside the car and disappeared for five minutes. He came back holding a small box. Its screen showed a digital map of Greater London, a ragged omelette shape which he magnified progressively until we were watching a little red sphere making its way along a side road in the vicinity of Holborn.

‘So. That’s where the dog is buried,’ said Bykov, using a conventional Russian idiom that sounded oddly sinister in the circumstances. ‘Let’s go.’

He glanced at the boot and I climbed back in.

This time I was in there for no more than a few minutes when I felt the car pull up abruptly. I heard a hasty rap on the lid and Bykov’s face appeared, silhouetted by the – to me – dazzling daylight. ‘Quick,’ he said, ‘in front.’

He had stopped on a cul-de-sac near the Haymarket. I took my place in the passenger seat. The smell of air freshener was so strong that for a moment I wondered if I hadn’t perhaps been better off in the boot. Bykov passed me the device. ‘Where to?’ He wanted me to navigate.

I wrestled with my imperfect knowledge of Russian imperatives, aspectual tenses and verbs of motion in order to direct him through the streets in pursuit of the red sphere. At first glance, I had had an intuition about where Jack had gone, but it seemed so absurd that I dismissed it. And yet, as I watched, it became clear that I was right: Jack was making his way to Johnson’s old house in Gough Square.

At number 17, boxed in by unprepossessing offices, stands the eighteenth-century building that was once Johnson’s home. All creaky floors and odd angles, it’s an unlikely time traveller. It escaped the Blitz by a whisker (the roof was burned to ashes in an air raid) and it somehow survived the redevelopment of the City that followed. When you come upon it from Fleet Street, following a finger-post down an unpromising alleyway between a McDonald’s and a Starbucks to get there, the age and detail of it transport you to a moment when the city had a more human scale. I’ve spent much time there myself as a visitor and giving talks. On its top floor, beneath the restored twentieth-century ceiling, is the garret where Johnson and his assistants compiled the dictionary. A swinging wooden partition on the front wall can be pulled out to divide the room into two. There is something both ingenious and jury-rigged about it. This, as much as any of the more conventional memorabilia in the house, gives a flavour of the physical character of Johnson’s world.

‘He’s stopped,’ I said. ‘I think I know where he is.’

The traffic was heavy, and in spite of Bykov’s muscular driving, we got bogged down in the heaving streets around Covent Garden. When we’d finally extricated ourselves and were nearing Chancery Lane, the sphere, which had been motionless for five or so minutes, began to move again, and now very quickly. There was no hope of catching him now. All we could do was watch as he sped up Ludgate Hill, past St Paul’s, and shot north, finally coming to a stop about 150 metres south of London Wall.

There was no longer any need to hurry. We resumed our pursuit with a deepening misgiving. On Wood Street, just beyond the sad deconsecrated tower of bomb-ruined St Alban’s, we pulled up outside Jack’s new location: the City of London Police Headquarters.

Bykov parked the car on the far side of the church, from where we could just see the front door of the police station. ‘You’ll have to go in and sort it out,’ said Bykov.

‘Me?’

‘He’s your responsibility, Nikolai.’

One of the indices of Bykov’s growing contempt for me was that he’d started to call me by my first name and use the informal second person singular.

‘What about identification for him?’ I asked.

He looked uneasy. ‘Better without.’

As I opened the car door, there was a thunderclap. It was an instant downpour, pounding out of the gunmetal sky, spattering off the pavements and the exhausted stone of the vestigial church. Entering the police station, I was soaked. I dripped over to the front desk, where I showed my university ID to a shirt-sleeved receptionist with big forearms and a shaven head like a Regency prizefighter.

‘How can I help you, Dr Slopen?’ he asked, with the hearty confidence of one pillar of the establishment addressing another.

‘I believe you may have a patient of mine in custody,’ I said, as breezily as I could manage.

The policeman inclined his head slightly towards me. ‘What’s his name?’

‘His name is Jack Telauga.’

Without glancing down at the admissions record in front of him, the policeman said pregnantly: ‘I can tell you now that there’s no one of that name in the log book.’

‘He was in the vicinity of Gough Square. He might have called himself …’

The man met my hesitation with a blue, unblinking gaze.

‘He might have called himself …’ I found myself unable to say it.

‘Called himself what, sir?’ There was the first hint of impatience in the policeman’s voice.

‘He’s been very confused lately. His name is Jack Telauga. But he may be calling himself …’ I closed my eyes. ‘He may be calling himself Johnson.’

‘Might that be Doctor Johnson?’

I nodded.

‘If you wouldn’t mind taking a seat, I’ll see where we are with him.’

*

I sat, damp and troubled, among the plastic chairs and the routine gloom of the waiting area. For the first time, I had been forced to acknowledge openly the strange and specific character of Jack’s delusion.

All madness has a touch of death to it. Here, in the Dennis Hill Unit, I’ve come to see that reality is not as robust as we think it is. Of course, there are things that are indifferent to human opinion – gravity, the moondriven motion of the tides, the boiling point of water. But the finer details of reality – the state of a marriage, artistic merit, a person’s true nature – have something delicate and consensual about them. That lurch to the wrong, a noble mind overthrown, even Ron Harbottle sacrificing the good judgement he had lived for: it is more than disquieting. Each time someone opts out of our collective reality, it weakens a little.

To me, Johnson’s recognition of that is part of his acute modernity as a moralist. I think he saw the relation between individual and collective delusion: the threat of madness to the human mind and the body politic. He knew that it was a small step from religious mania to religious wars. Madness is part of that turn away from the real that Johnson was so vigilant in confronting wherever he found it – not because of his confidence in reason, but because he knew from his own experience how fragile the rule of reason is.

No one more embodies the illuminating potency of reason. Johnson was devastating in his capacity to sniff out the fake in its different guises, to know what the real is, or the réal, if you follow Hunter’s comical etymology. But this very power was riddled with its opposites: melancholy and uncertainty; fear of his own loosening grip on the nature of reality.

*

As they fetched Jack up from the cells, he walked like a criminal being led to the scaffold: unsteadily, and with his eyes fixed on the floor. When I said his name, he looked up and his face brightened with relief.

‘Dr Slopen? How come you here?’ His voice was hoarse and he was wearing unfamiliar clothing.

‘Let’s get you home,’ I said. Jack took my arm. His grip seemed frail and desperate.

The police were only too happy to release him with a caution.

‘We’d prefer not to charge him, anyway,’ said the shaven-headed policeman. ‘In cases like these’ – he looked pityingly at Jack – ‘the law is a blunt instrument.’

He explained that Jack had made his way into 17 Gough Square, ignored the clerk at the ticket counter and, when challenged by an attendant, had become confrontational. A guide who was showing a group around the house at the time had been so alarmed by his manner that she fetched a policeman.